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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_dll.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.plugin_basics">Plugin basics</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.factory_method_in_plugin">Factory
method in plugin</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.searching_for_a_symbol_in_multiple_plugins">Searching
for a symbol in multiple plugins</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.linking_plugin_into_the_executable">Linking
plugin into the executable</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.symbol_shadowing_problem__linux_">Symbol
shadowing problem (Linux)</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.executing_callbacks_on_library_unload">Executing
callbacks on library unload</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.querying_libraries_for_symbols">Querying
libraries for symbols</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.advanced_library_reference_counting">Advanced
library reference counting</a></span></dt>
<dt><span class="section"><a href="tutorial.html#boost_dll.tutorial.importing_a_c_function_from_windows_dll">Importing
a C function from Windows dll</a></span></dt>
</dl></div>
<p>
Tutorial is provided to give you an idea of how to create and use plugins.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.plugin_basics"></a><a class="link" href="tutorial.html#boost_dll.tutorial.plugin_basics" title="Plugin basics">Plugin basics</a>
</h3></div></div></div>
<p>
The first thing to do when creating your own plugins is define the plugin
interface. There is an example of an abstract class that will be our plugin
API:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">my_plugin_api</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">virtual</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">virtual</span> <span class="keyword">float</span> <span class="identifier">calculate</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">float</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">my_plugin_api</span><span class="special">()</span> <span class="special">{}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Now let's make a DLL/DSO library that will holds implementation of plugin
interface and exports it using the <code class="computeroutput"><span class="keyword">extern</span>
<span class="string">"C"</span></code> and <code class="computeroutput">BOOST_SYMBOL_EXPORT</code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">config</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_SYMBOL_EXPORT</span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="keyword">class</span> <span class="identifier">my_plugin_sum</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">my_plugin_api</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">my_plugin_sum</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Constructing my_plugin_sum"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="string">"sum"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">float</span> <span class="identifier">calculate</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">float</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">~</span><span class="identifier">my_plugin_sum</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Destructing my_plugin_sum ;o)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="comment">// Exporting `my_namespace::plugin` variable with alias name `plugin`</span>
<span class="comment">// (Has the same effect as `BOOST_DLL_ALIAS(my_namespace::plugin, plugin)`)</span>
<span class="keyword">extern</span> <span class="string">"C"</span> <span class="identifier">BOOST_SYMBOL_EXPORT</span> <span class="identifier">my_plugin_sum</span> <span class="identifier">plugin</span><span class="special">;</span>
<span class="identifier">my_plugin_sum</span> <span class="identifier">plugin</span><span class="special">;</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
</pre>
<p>
</p>
<p>
Simple application that loads plugin using the <code class="computeroutput"><a class="link" href="../boost/dll/import.html" title="Function import">boost::dll::import</a></code>
and <code class="computeroutput"><a class="link" href="../boost/dll/load_mode/type.html" title="Type type">append_decorations</a></code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for import_alias</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">lib_path</span><span class="special">(</span><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]);</span> <span class="comment">// argv[1] contains path to directory with our plugin library</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="identifier">plugin</span><span class="special">;</span> <span class="comment">// variable to hold a pointer to plugin variable</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Loading the plugin"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">plugin</span> <span class="special">=</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">import</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">>(</span> <span class="comment">// type of imported symbol is located between `<` and `>`</span>
<span class="identifier">lib_path</span> <span class="special">/</span> <span class="string">"my_plugin_sum"</span><span class="special">,</span> <span class="comment">// path to the library and library name</span>
<span class="string">"plugin"</span><span class="special">,</span> <span class="comment">// name of the symbol to import</span>
<span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">append_decorations</span> <span class="comment">// makes `libmy_plugin_sum.so` or `my_plugin_sum.dll` from `my_plugin_sum`</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"plugin->calculate(1.5, 1.5) call: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">calculate</span><span class="special">(</span><span class="number">1.5</span><span class="special">,</span> <span class="number">1.5</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
That application will output:
</p>
<pre class="programlisting">Loading the plugin
Constructing my_plugin_sum
plugin->calculate(1.5, 1.5) call: 3
Destructing my_plugin_sum ;o)
</pre>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial_common/my_plugin_api.hpp" target="_top">example/tutorial_common/my_plugin_api.hpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial1/my_plugin_sum.cpp" target="_top">example/tutorial1/my_plugin_sum.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial1/tutorial1.cpp" target="_top">example/tutorial1/tutorial1.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.factory_method_in_plugin"></a><a class="link" href="tutorial.html#boost_dll.tutorial.factory_method_in_plugin" title="Factory method in plugin">Factory
method in plugin</a>
</h3></div></div></div>
<p>
In previous example we were importing from a plugin a single variable. Let's
make a class that uses our plugin API plugin and holds some state:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">alias</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_DLL_ALIAS </span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="keyword">class</span> <span class="identifier">my_plugin_aggregator</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">my_plugin_api</span> <span class="special">{</span>
<span class="keyword">float</span> <span class="identifier">aggr_</span><span class="special">;</span>
<span class="identifier">my_plugin_aggregator</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">aggr_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="string">"aggregator"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">float</span> <span class="identifier">calculate</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">float</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">aggr_</span> <span class="special">+=</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">aggr_</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// Factory method</span>
<span class="keyword">static</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_aggregator</span><span class="special">></span> <span class="identifier">create</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_aggregator</span><span class="special">>(</span>
<span class="keyword">new</span> <span class="identifier">my_plugin_aggregator</span><span class="special">()</span>
<span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="identifier">BOOST_DLL_ALIAS</span><span class="special">(</span>
<span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">my_plugin_aggregator</span><span class="special">::</span><span class="identifier">create</span><span class="special">,</span> <span class="comment">// <-- this function is exported with...</span>
<span class="identifier">create_plugin</span> <span class="comment">// <-- ...this alias name</span>
<span class="special">)</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
</pre>
<p>
</p>
<p>
As you may see, <code class="computeroutput"><span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">create_plugin</span></code>
is a factory method, that creates instances of <code class="computeroutput"><span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">my_plugin_aggregator</span></code>.
We export that method with the name "create_plugin" using <code class="computeroutput"><a class="link" href="../BOOST_DLL_ALIAS.html" title="Macro BOOST_DLL_ALIAS">BOOST_DLL_ALIAS</a></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for import_alias</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">shared_library_path</span><span class="special">(</span><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]);</span> <span class="comment">// argv[1] contains path to directory with our plugin library</span>
<span class="identifier">shared_library_path</span> <span class="special">/=</span> <span class="string">"my_plugin_aggregator"</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="special">(</span><span class="identifier">pluginapi_create_t</span><span class="special">)();</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">pluginapi_create_t</span><span class="special">></span> <span class="identifier">creator</span><span class="special">;</span>
<span class="identifier">creator</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">import_alias</span><span class="special"><</span><span class="identifier">pluginapi_create_t</span><span class="special">>(</span> <span class="comment">// type of imported symbol must be explicitly specified</span>
<span class="identifier">shared_library_path</span><span class="special">,</span> <span class="comment">// path to library</span>
<span class="string">"create_plugin"</span><span class="special">,</span> <span class="comment">// symbol to import</span>
<span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">append_decorations</span> <span class="comment">// do append extensions and prefixes</span>
<span class="special">);</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="identifier">plugin</span> <span class="special">=</span> <span class="identifier">creator</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"plugin->calculate(1.5, 1.5) call: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">calculate</span><span class="special">(</span><span class="number">1.5</span><span class="special">,</span> <span class="number">1.5</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"plugin->calculate(1.5, 1.5) second call: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">calculate</span><span class="special">(</span><span class="number">1.5</span><span class="special">,</span> <span class="number">1.5</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Plugin Name: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">name</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
In that application we have imported the factory method using <code class="computeroutput"><a class="link" href="../boost/dll/import_alias.html" title="Function import_alias">boost::dll::import_alias</a></code>.
</p>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
Be careful: <code class="computeroutput"><span class="identifier">creator</span></code> variable
holds a reference to the loaded shared library. If this variable goes out
of scope or will be reset, then the <span class="bold"><strong>DLL/DSO will
be unloaded</strong></span> and any attempt to dereference the <code class="computeroutput"><span class="identifier">plugin</span></code> variable will lead to <span class="bold"><strong>undefined behavior</strong></span>.
</p></td></tr>
</table></div>
<p>
Output of the application will be the following:
</p>
<pre class="programlisting">plugin->calculate(1.5, 1.5) call: 3
plugin->calculate(1.5, 1.5) second call: 6
Plugin Name: aggregator
</pre>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial2/my_plugin_aggregator.cpp" target="_top">example/tutorial2/my_plugin_aggregator.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial2/tutorial2.cpp" target="_top">example/tutorial2/tutorial2.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.searching_for_a_symbol_in_multiple_plugins"></a><a class="link" href="tutorial.html#boost_dll.tutorial.searching_for_a_symbol_in_multiple_plugins" title="Searching for a symbol in multiple plugins">Searching
for a symbol in multiple plugins</a>
</h3></div></div></div>
<p>
Consider the situation: we have multiple plugins, but only some of them have
symbols that we need. Let's write a function that search list of plugins
and attempts to find <code class="computeroutput"><span class="string">"create_plugin"</span></code>
method.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for import_alias</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">make_shared</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">search_for_symbols</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span><span class="special">>&</span> <span class="identifier">plugins</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">plugins_found</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">plugins</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Loading plugin: "</span> <span class="special"><<</span> <span class="identifier">plugins</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special"><<</span> <span class="char">'\n'</span><span class="special">;</span>
<span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span> <span class="identifier">lib</span><span class="special">(</span><span class="identifier">plugins</span><span class="special">[</span><span class="identifier">i</span><span class="special">],</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">append_decorations</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(!</span><span class="identifier">lib</span><span class="special">.</span><span class="identifier">has</span><span class="special">(</span><span class="string">"create_plugin"</span><span class="special">))</span> <span class="special">{</span>
<span class="comment">// no such symbol</span>
<span class="keyword">continue</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// library has symbol, importing...</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="special">(</span><span class="identifier">pluginapi_create_t</span><span class="special">)();</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">pluginapi_create_t</span><span class="special">></span> <span class="identifier">creator</span>
<span class="special">=</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">import_alias</span><span class="special"><</span><span class="identifier">pluginapi_create_t</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">lib</span><span class="special">),</span> <span class="string">"create_plugin"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Matching plugin name: "</span> <span class="special"><<</span> <span class="identifier">creator</span><span class="special">()-></span><span class="identifier">name</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">++</span> <span class="identifier">plugins_found</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="identifier">plugins_found</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
If we call that method for all our plugins we'll get the following output:
</p>
<pre class="programlisting">Loading plugin: "/test/libmy_plugin_aggregator.so"
Matching plugin name: aggregator
Loading plugin: "/test/libmy_plugin_sum.so"
Constructing my_plugin_sum
Destructing my_plugin_sum ;o)
</pre>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial3/tutorial3.cpp" target="_top">example/tutorial3/tutorial3.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial2/my_plugin_aggregator.cpp" target="_top">example/tutorial2/my_plugin_aggregator.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial1/my_plugin_sum.cpp" target="_top">example/tutorial1/my_plugin_sum.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.linking_plugin_into_the_executable"></a><a class="link" href="tutorial.html#boost_dll.tutorial.linking_plugin_into_the_executable" title="Linking plugin into the executable">Linking
plugin into the executable</a>
</h3></div></div></div>
<p>
Linking plugin into the executable has the advantages of
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
reducing common size of distribution
</li>
<li class="listitem">
simplification of installation of distribution
</li>
<li class="listitem">
faster plugin load
</li>
</ul></div>
<p>
Let's start from creating a linkable in plugin. Such plugin will have a header,
common for plugin library itself and for the executable:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">alias</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_DLL_ALIAS</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">shared_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="identifier">create_plugin</span><span class="special">();</span> <span class="comment">// Forward declaration</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
<span class="identifier">BOOST_DLL_ALIAS</span><span class="special">(</span>
<span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">create_plugin</span><span class="special">,</span> <span class="comment">// <-- this function is exported with...</span>
<span class="identifier">create_plugin</span> <span class="comment">// <-- ...this alias name</span>
<span class="special">)</span>
</pre>
<p>
</p>
<p>
Main trick here is the alias definition. When linking plugin into the executable,
the alias <span class="bold"><strong>must</strong></span> be instantiated in one of
the source files of the executable. Otherwise the linker will optimize away
our plugin.
</p>
<p>
Here's how the implementation of the plugin looks like:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"static_plugin.hpp"</span> <span class="comment">// this is essential, BOOST_SYMBOL_ALIAS must be seen in this file</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">make_shared</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="keyword">class</span> <span class="identifier">my_plugin_static</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">my_plugin_api</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">my_plugin_static</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Constructing my_plugin_static"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="string">"static"</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">float</span> <span class="identifier">calculate</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">float</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">~</span><span class="identifier">my_plugin_static</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Destructing my_plugin_static"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">></span> <span class="identifier">create_plugin</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special"><</span><span class="identifier">my_plugin_static</span><span class="special">>();</span>
<span class="special">}</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
</pre>
<p>
</p>
<p>
Now if we make a static library from source file and link that static library
with the following code, we'll be able to import symbols from plugin:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">shared_library</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for shared_library</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">runtime_symbol_info</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for program_location()</span>
<span class="preprocessor">#include</span> <span class="string">"static_plugin.hpp"</span> <span class="comment">// without this headers some compilers may optimize out the `create_plugin` symbol</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span> <span class="identifier">self</span><span class="special">(</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">program_location</span><span class="special">());</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Call function"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">>()></span> <span class="identifier">creator</span>
<span class="special">=</span> <span class="identifier">self</span><span class="special">.</span><span class="identifier">get_alias</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_plugin_api</span><span class="special">>()>(</span><span class="string">"create_plugin"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Computed Value: "</span> <span class="special"><<</span> <span class="identifier">creator</span><span class="special">()-></span><span class="identifier">calculate</span><span class="special">(</span><span class="number">2</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Flag '-rdynamic' must be used when linking the plugin into the executable
on Linux OS. Otherwise loading symbols from self <span class="bold"><strong>will
fail</strong></span>.
</p></td></tr>
</table></div>
<p>
Running the program will output the following:
</p>
<pre class="programlisting">Call function
Constructing my_plugin_static
Computed Value: 0
Destructing my_plugin_static
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
If we want to make a traditional plugin that is located in a separate shared
library, all we need to do is remove the <code class="computeroutput"><span class="preprocessor">#include</span>
<span class="string">"static_plugin.hpp"</span></code> line
and replace <code class="computeroutput"><span class="identifier">dll</span><span class="special">::</span><span class="identifier">program_location</span><span class="special">()</span></code>
with plugin location and name.
</p></td></tr>
</table></div>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial4/static_plugin.hpp" target="_top">example/tutorial4/static_plugin.hpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial4/static_plugin.cpp" target="_top">example/tutorial4/static_plugin.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial4/load_self.cpp" target="_top">example/tutorial4/load_self.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.symbol_shadowing_problem__linux_"></a><a class="link" href="tutorial.html#boost_dll.tutorial.symbol_shadowing_problem__linux_" title="Symbol shadowing problem (Linux)">Symbol
shadowing problem (Linux)</a>
</h3></div></div></div>
<p>
Let's make an executable, link a plugin into it and attempt to load all the
existing plugins:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="keyword">class</span> <span class="identifier">plugins_collector</span> <span class="special">{</span>
<span class="comment">// Name => plugin</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">container</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">></span> <span class="identifier">plugins_t</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">plugins_directory_</span><span class="special">;</span>
<span class="identifier">plugins_t</span> <span class="identifier">plugins_</span><span class="special">;</span>
<span class="comment">// loads all plugins in plugins_directory_</span>
<span class="keyword">void</span> <span class="identifier">load_all</span><span class="special">();</span>
<span class="comment">// Gets `my_plugin_api` instance using "create_plugin" or "plugin" imports,</span>
<span class="comment">// stores plugin with its name in the `plugins_` map.</span>
<span class="keyword">void</span> <span class="identifier">insert_plugin</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">)</span> <span class="identifier">lib</span><span class="special">);</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">plugins_collector</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span><span class="special">&</span> <span class="identifier">plugins_directory</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">plugins_directory_</span><span class="special">(</span><span class="identifier">plugins_directory</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">load_all</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">print_plugins</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">count</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="comment">// ... </span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span> <span class="special">{</span>
<span class="identifier">plugins_collector</span> <span class="identifier">plugins</span><span class="special">(</span><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\n\nUnique plugins "</span> <span class="special"><<</span> <span class="identifier">plugins</span><span class="special">.</span><span class="identifier">count</span><span class="special">()</span> <span class="special"><<</span> <span class="string">":\n"</span><span class="special">;</span>
<span class="identifier">plugins</span><span class="special">.</span><span class="identifier">print_plugins</span><span class="special">();</span>
<span class="comment">// ...</span>
</pre>
<p>
</p>
<p>
With the default flags you'll get a very strange output:
</p>
<pre class="programlisting">Loaded (0x180db60):"/libs/dll/test/libmy_plugin_aggregator.so"
Constructing my_plugin_static
Destructing my_plugin_static
...
Unique plugins 2:
(0x180db60): static
(0x180e3b0): sum
Destructing my_plugin_sum ;o)
</pre>
<p>
Why <code class="computeroutput"><span class="identifier">my_plugin_static</span></code> was
constructed while we were loading <code class="computeroutput"><span class="identifier">my_plugin_aggregator</span></code>?
</p>
<p>
That's because function <code class="computeroutput"><span class="identifier">create_plugin</span></code>
from <code class="computeroutput"><span class="identifier">libmy_plugin_aggregator</span><span class="special">.</span><span class="identifier">so</span></code> was
shadowed by the <code class="computeroutput"><span class="identifier">create_plugin</span></code>
function from other plugin. Dynamic linker thought that <code class="computeroutput"><span class="identifier">create_plugin</span></code>
was already loaded and there is no need to load it again.
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
Use "-fvisibility=hidden" flag (at least for plugins) while compiling
for POSIX platforms. This flag makes your code more portable ("-fvisibility=hidden"
is the default behavior under Windows), reduces size of the binaries and
improves binary load time.
</p></td></tr>
</table></div>
<p>
Now if we recompile your example with "-fvisibility=hidden" we'll
get the following output:
</p>
<pre class="programlisting">Loaded (0x2406b60):"/libs/dll/test/libmy_plugin_aggregator.so"
Loaded (0x2407410):"/libs/dll/test/libgetting_started_library.so"
Constructing my_plugin_sum
...
Unique plugins 3:
(0x2406b60): aggregator
(0x7fd1cadce2c8): static
(0x24073b0): sum
Destructing my_plugin_sum ;o)
</pre>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial5/load_all.cpp" target="_top">example/tutorial5/load_all.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial4/static_plugin.cpp" target="_top">example/tutorial4/static_plugin.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial2/my_plugin_aggregator.cpp" target="_top">example/tutorial2/my_plugin_aggregator.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial1/my_plugin_sum.cpp" target="_top">example/tutorial1/my_plugin_sum.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.executing_callbacks_on_library_unload"></a><a class="link" href="tutorial.html#boost_dll.tutorial.executing_callbacks_on_library_unload" title="Executing callbacks on library unload">Executing
callbacks on library unload</a>
</h3></div></div></div>
<p>
Boost.DLL provides no out of the box mechanism for catching library unloads.
However such task could be easily implemented.
</p>
<p>
All you need to do, is write a simple class that stores callbacks and calls
them at destruction:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">alias</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_DLL_ALIAS</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">on_unload</span> <span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span><span class="special">()></span> <span class="identifier">callback_t</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">on_unload</span> <span class="identifier">this_type</span><span class="special">;</span>
<span class="special">~</span><span class="identifier">on_unload</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">callbacks_</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">callback_t</span><span class="special">&</span> <span class="identifier">function</span> <span class="special">=</span> <span class="identifier">callbacks_</span><span class="special">[</span><span class="identifier">i</span><span class="special">];</span>
<span class="identifier">function</span><span class="special">();</span> <span class="comment">// calling the callback</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="comment">// not thread safe</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">add</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">callback_t</span><span class="special">&</span> <span class="identifier">function</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">static</span> <span class="identifier">this_type</span> <span class="identifier">instance</span><span class="special">;</span>
<span class="identifier">instance</span><span class="special">.</span><span class="identifier">callbacks_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">function</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">callback_t</span><span class="special">></span> <span class="identifier">callbacks_</span><span class="special">;</span>
<span class="identifier">on_unload</span><span class="special">()</span> <span class="special">{}</span> <span class="comment">// prohibit construction outside of the `add` function</span>
<span class="special">};</span>
<span class="comment">// Exporting the static "add" function with name "on_unload"</span>
<span class="identifier">BOOST_DLL_ALIAS</span><span class="special">(</span><span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">on_unload</span><span class="special">::</span><span class="identifier">add</span><span class="special">,</span> <span class="identifier">on_unload</span><span class="special">)</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
</pre>
<p>
</p>
<p>
In the example above <code class="computeroutput"><span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">on_unload</span></code>
is a singleton structure that holds a vector of callbacks and calls all the
callbacks at destruction.
</p>
<p>
Now we can load this library and provide a callback:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span><span class="special">()></span> <span class="identifier">callback_t</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">print_unloaded</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"unloaded"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span> <span class="special">{</span>
<span class="comment">// argv[1] contains full path to our plugin library</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">shared_library_path</span> <span class="special">=</span> <span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">];</span>
<span class="comment">// loading library and getting a function from it</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">callback_t</span><span class="special">&)></span> <span class="identifier">on_unload</span>
<span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">import_alias</span><span class="special"><</span><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">callback_t</span><span class="special">&)>(</span>
<span class="identifier">shared_library_path</span><span class="special">,</span> <span class="string">"on_unload"</span>
<span class="special">);</span>
<span class="identifier">on_unload</span><span class="special">(&</span><span class="identifier">print_unloaded</span><span class="special">);</span> <span class="comment">// adding a callback</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Before library unload."</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// Releasing last reference to the library, so that it gets unloaded</span>
<span class="identifier">on_unload</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"After library unload."</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
If we run the example we'll get the following output:
</p>
<pre class="programlisting">Before library unload.
unloaded
After library unload.
</pre>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial6/on_unload_lib.cpp" target="_top">example/tutorial6/on_unload_lib.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial6/tutorial6.cpp" target="_top">example/tutorial6/tutorial6.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.querying_libraries_for_symbols"></a><a class="link" href="tutorial.html#boost_dll.tutorial.querying_libraries_for_symbols" title="Querying libraries for symbols">Querying
libraries for symbols</a>
</h3></div></div></div>
<p>
Situation when we do not know names of functions in plugin could occur. In
that case querying library could be useful.
</p>
<p>
Imagine the situation: we have a project called 'Anna' that is capable of
loading and using plugins that contain functions with signature <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&)</span></code>.
We do not know function names, but wish to find out them somehow.
</p>
<p>
Solution would be pretty simple. Let's agree with plugin developers, that
they can name functions as they like, but all the plugin functions aliases
must be located in section named 'Anna':
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">alias</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_DLL_ALIAS_SECTIONED</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Hello, "</span> <span class="special"><<</span> <span class="identifier">s</span> <span class="special"><<</span> <span class="char">'!'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">BOOST_DLL_ALIAS_SECTIONED</span><span class="special">(</span><span class="identifier">print</span><span class="special">,</span> <span class="identifier">print_hello</span><span class="special">,</span> <span class="identifier">Anna</span><span class="special">)</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">alias</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for BOOST_DLL_ALIAS_SECTIONED</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">print_howdy</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"How're you doing, "</span> <span class="special"><<</span> <span class="identifier">s</span> <span class="special"><<</span> <span class="char">'?'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">print_bored</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Are you bored, "</span> <span class="special"><<</span> <span class="identifier">s</span> <span class="special"><<</span> <span class="char">'?'</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">BOOST_DLL_ALIAS_SECTIONED</span><span class="special">(</span><span class="identifier">print_howdy</span><span class="special">,</span> <span class="identifier">howdy</span><span class="special">,</span> <span class="identifier">Anna</span><span class="special">)</span>
<span class="identifier">BOOST_DLL_ALIAS_SECTIONED</span><span class="special">(</span><span class="identifier">print_bored</span><span class="special">,</span> <span class="identifier">are_you_bored</span><span class="special">,</span> <span class="identifier">Anna</span><span class="special">)</span>
</pre>
<p>
</p>
<p>
Now we can easily get those functions using the <code class="computeroutput"><a class="link" href="../boost/dll/library_info.html" title="Class library_info">boost::dll::library_info</a></code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">shared_library</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">library_info</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">load_and_execute</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">libraries</span><span class="special">[],</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">libs_count</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">username</span> <span class="special">=</span> <span class="string">"User"</span><span class="special">;</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">libs_count</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// Class `library_info` can extract information from a library</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">library_info</span> <span class="identifier">inf</span><span class="special">(</span><span class="identifier">libraries</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span>
<span class="comment">// Getting symbols exported from 'Anna' section</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">exports</span> <span class="special">=</span> <span class="identifier">inf</span><span class="special">.</span><span class="identifier">symbols</span><span class="special">(</span><span class="string">"Anna"</span><span class="special">);</span>
<span class="comment">// Loading library and importing symbols from it</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span> <span class="identifier">lib</span><span class="special">(</span><span class="identifier">libraries</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">j</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">j</span> <span class="special"><</span> <span class="identifier">exports</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">j</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"\nFunction '"</span> <span class="special"><<</span> <span class="identifier">exports</span><span class="special">[</span><span class="identifier">j</span><span class="special">]</span> <span class="special"><<</span> <span class="string">"' prints:\n\t"</span><span class="special">;</span>
<span class="identifier">lib</span><span class="special">.</span><span class="identifier">get_alias</span><span class="special"><</span><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&)>(</span><span class="identifier">exports</span><span class="special">[</span><span class="identifier">j</span><span class="special">])</span> <span class="comment">// importing function</span>
<span class="special">(</span><span class="identifier">username</span><span class="special">);</span> <span class="comment">// calling function</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
If we run the example we'll get the following output:
</p>
<pre class="programlisting">Function 'print_hello' prints:
Hello, User!
Function 'are_you_bored' prints:
Are you bored, User?
Function 'howdy' prints:
How're you doing, User?
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
<code class="computeroutput"><span class="identifier">BOOST_DLL_ALIAS</span></code> macro by
default places all the aliases into the "boostdll" section.
</p></td></tr>
</table></div>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial7/library1.cpp" target="_top">example/tutorial7/library1.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial7/library2.cpp" target="_top">example/tutorial7/library2.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial7/tutorial7.cpp" target="_top">example/tutorial7/tutorial7.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.advanced_library_reference_counting"></a><a class="link" href="tutorial.html#boost_dll.tutorial.advanced_library_reference_counting" title="Advanced library reference counting">Advanced
library reference counting</a>
</h3></div></div></div>
<p>
As noted in documentation to the <code class="computeroutput"><a class="link" href="../boost/dll/import.html" title="Function import">boost::dll::import</a></code>
variables and functions returned from those functions hold a reference to
the shared library. However nested objects and objects that are returned
by <code class="computeroutput"><span class="identifier">import</span><span class="special">*</span></code>
functions do not hold a reference to the shared library. There's no way to
solve this issue on the Boost.DLL library level, but you can take care of
this problem by your own. Here's an example how this could be done.
</p>
<p>
In this example we'll be importing function that constructs an instance of
plugin and binding that instance to shared_library.
</p>
<p>
First of all we need to define a new plugin api:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"../tutorial_common/my_plugin_api.hpp"</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">filesystem</span><span class="special">/</span><span class="identifier">path</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">my_refcounting_api</span><span class="special">:</span> <span class="keyword">public</span> <span class="identifier">my_plugin_api</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// Returns path to shared object that holds a plugin.</span>
<span class="comment">// Must be instantiated in plugin.</span>
<span class="keyword">virtual</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">location</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
This API does not differ much from the previous one. Only one abstract method
was added.
</p>
<p>
Now let's define the plugin:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"refcounting_plugin.hpp"</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">runtime_symbol_info</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for this_line_location()</span>
<span class="keyword">namespace</span> <span class="identifier">my_namespace</span> <span class="special">{</span>
<span class="keyword">class</span> <span class="identifier">my_plugin_refcounting</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">my_refcounting_api</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">// Must be instantiated in plugin</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">location</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">this_line_location</span><span class="special">();</span> <span class="comment">// location of this plugin</span>
<span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="string">"refcounting"</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// ...</span>
<span class="special">};</span>
<span class="special">}</span> <span class="comment">// namespace my_namespace</span>
<span class="comment">// Factory method. Returns *simple pointer*!</span>
<span class="identifier">my_refcounting_api</span><span class="special">*</span> <span class="identifier">create</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">my_namespace</span><span class="special">::</span><span class="identifier">my_plugin_refcounting</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
This plugin does not differ much from our previous examples except the additional
method that calls <code class="computeroutput"><a class="link" href="../boost/dll/this_line_location.html" title="Function this_line_location">boost::dll::this_line_location</a></code>
and <code class="computeroutput"><span class="identifier">create</span><span class="special">()</span></code>
function that returns a simple pointer instead of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span></code>.
</p>
<p>
Now lets make a function that binds a newly created instance of <code class="computeroutput"><span class="identifier">my_refcounting_api</span></code> to a shared library:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">shared_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">make_shared</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">shared_library</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">library_holding_deleter</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">></span> <span class="identifier">lib_</span><span class="special">;</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">my_refcounting_api</span><span class="special">*</span> <span class="identifier">p</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">delete</span> <span class="identifier">p</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="keyword">inline</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span> <span class="identifier">bind</span><span class="special">(</span><span class="identifier">my_refcounting_api</span><span class="special">*</span> <span class="identifier">plugin</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// getting location of the shared library that holds the plugin</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">location</span> <span class="special">=</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">location</span><span class="special">();</span>
<span class="comment">// `make_shared` is an efficient way to create a shared pointer</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">></span> <span class="identifier">lib</span>
<span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_shared</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">>(</span><span class="identifier">location</span><span class="special">);</span>
<span class="identifier">library_holding_deleter</span> <span class="identifier">deleter</span><span class="special">;</span>
<span class="identifier">deleter</span><span class="special">.</span><span class="identifier">lib_</span> <span class="special">=</span> <span class="identifier">lib</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">>(</span>
<span class="identifier">plugin</span><span class="special">,</span> <span class="identifier">deleter</span>
<span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
In <code class="computeroutput"><span class="identifier">bind</span></code> method we call <code class="computeroutput"><span class="identifier">plugin</span><span class="special">-></span><span class="identifier">location</span><span class="special">()</span></code>.
This call results in a call to <code class="computeroutput"><a class="link" href="../boost/dll/this_line_location.html" title="Function this_line_location">boost::dll::this_line_location</a></code>
and returns the plugin location. Then a <code class="computeroutput"><span class="identifier">shared_ptr</span></code>
that holds a <code class="computeroutput"><span class="identifier">shared_library</span></code>
is created using the <code class="computeroutput"><span class="identifier">make_shared</span></code>
call.
</p>
<p>
After that we construct a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span></code> with a <code class="computeroutput"><span class="identifier">library_holding_deleter</span></code>
that keeps an instance of the shared library.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span></code>
instead of <code class="computeroutput"><span class="identifier">my_refcounting_api</span><span class="special">*</span></code> in production code to avoid memory leaks
when <code class="computeroutput"><span class="identifier">plugin</span><span class="special">-></span><span class="identifier">location</span><span class="special">()</span></code>
throws or when some other class rises an exception.
</p></td></tr>
</table></div>
<p>
That's it, now we can get instance of a plugin:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">inline</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span> <span class="identifier">get_plugin</span><span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">path</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">func_name</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">my_refcounting_api</span><span class="special">*(</span><span class="identifier">func_t</span><span class="special">)();</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">func_t</span><span class="special">></span> <span class="identifier">creator</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">import_alias</span><span class="special"><</span><span class="identifier">func_t</span><span class="special">>(</span>
<span class="identifier">path</span><span class="special">,</span>
<span class="identifier">func_name</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">append_decorations</span> <span class="comment">// will be ignored for executable</span>
<span class="special">);</span>
<span class="comment">// `plugin` does not hold a reference to shared library. If `creator` will go out of scope, </span>
<span class="comment">// then `plugin` can not be used.</span>
<span class="identifier">my_refcounting_api</span><span class="special">*</span> <span class="identifier">plugin</span> <span class="special">=</span> <span class="identifier">creator</span><span class="special">();</span>
<span class="comment">// Returned variable holds a reference to </span>
<span class="comment">// shared_library and it is safe to use it.</span>
<span class="keyword">return</span> <span class="identifier">bind</span><span class="special">(</span> <span class="identifier">plugin</span> <span class="special">);</span>
<span class="comment">// `creator` goes out of scope here and will be destroyed.</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Here's how it <code class="computeroutput"><span class="identifier">main</span><span class="special">()</span></code>
function look like:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
</th>
<th>
<p>
Runtime plugin load
</p>
</th>
<th>
<p>
Plugin was linked in
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Code
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"refcounting_api.hpp"</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span> <span class="identifier">plugin</span> <span class="special">=</span> <span class="identifier">get_plugin</span><span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span><span class="special">(</span><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">])</span> <span class="special">/</span> <span class="string">"refcounting_plugin"</span><span class="special">,</span>
<span class="string">"create_refc_plugin"</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Plugin name: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">name</span><span class="special">()</span>
<span class="special"><<</span> <span class="string">", \nlocation: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">location</span><span class="special">()</span>
<span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">runtime_symbol_info</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// program_location()</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="string">"refcounting_plugin.hpp"</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">my_refcounting_api</span><span class="special">></span> <span class="identifier">plugin</span> <span class="special">=</span> <span class="identifier">get_plugin</span><span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">program_location</span><span class="special">(),</span>
<span class="string">"create_refc_plugin"</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Plugin name: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">name</span><span class="special">()</span>
<span class="special"><<</span> <span class="string">", \nlocation: "</span> <span class="special"><<</span> <span class="identifier">plugin</span><span class="special">-></span><span class="identifier">location</span><span class="special">()</span>
<span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</td>
</tr>
<tr>
<td>
<p>
Output
</p>
</td>
<td>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">Plugin name: refcounting,
location: "/libs/dll/librefcounting_plugin.so"</pre>
</td>
<td>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting">Plugin name: refcounting,
location: "/tutorial8_static"</pre>
</td>
</tr>
</tbody>
</table></div>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial8/refcounting_api.hpp" target="_top">example/tutorial8/refcounting_api.hpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial8/refcounting_plugin.hpp" target="_top">example/tutorial8/refcounting_plugin.hpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial8/refcounting_plugin.cpp" target="_top">example/tutorial8/refcounting_plugin.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial8/tutorial8.cpp" target="_top">example/tutorial8/tutorial8.cpp</a>
</li>
<li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial8/tutorial8_static.cpp" target="_top">example/tutorial8/tutorial8_static.cpp</a>
</li>
</ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_dll.tutorial.importing_a_c_function_from_windows_dll"></a><a class="link" href="tutorial.html#boost_dll.tutorial.importing_a_c_function_from_windows_dll" title="Importing a C function from Windows dll">Importing
a C function from Windows dll</a>
</h3></div></div></div>
<p>
This a trivial example, but it has one tricky place. When you are importing
a C function by <span class="bold"><strong>it's name</strong></span> you must use
<code class="computeroutput"><a class="link" href="../boost/dll/import.html" title="Function import">boost::dll::import</a></code>,
not <code class="computeroutput"><a class="link" href="../boost/dll/import_alias.html" title="Function import_alias">boost::dll::import_alias</a></code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">import</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for dll::import</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">/</span><span class="identifier">shared_library</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// for dll::shared_library</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">windows</span><span class="special">.</span><span class="identifier">h</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">dll</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">HANDLE</span><span class="special">(</span><span class="identifier">__stdcall</span> <span class="identifier">GetStdHandle_t</span><span class="special">)(</span><span class="identifier">DWORD</span> <span class="special">);</span> <span class="comment">// function signature with calling convention</span>
<span class="comment">// OPTION #0, requires C++11 compatible compiler that understands GetStdHandle_t signature.</span>
<span class="keyword">auto</span> <span class="identifier">get_std_handle</span> <span class="special">=</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">import</span><span class="special"><</span><span class="identifier">GetStdHandle_t</span><span class="special">>(</span>
<span class="string">"Kernel32.dll"</span><span class="special">,</span>
<span class="string">"GetStdHandle"</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">search_system_folders</span>
<span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"0.0 GetStdHandle() returned "</span> <span class="special"><<</span> <span class="identifier">get_std_handle</span><span class="special">(</span><span class="identifier">STD_OUTPUT_HANDLE</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// You may put the `get_std_handle` into boost::function<>. But boost::function<Signature> can not compile with</span>
<span class="comment">// Signature template parameter that contains calling conventions, so you'll have to remove the calling convention.</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">HANDLE</span><span class="special">(</span><span class="identifier">DWORD</span><span class="special">)></span> <span class="identifier">get_std_handle2</span> <span class="special">=</span> <span class="identifier">get_std_handle</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"0.1 GetStdHandle() returned "</span> <span class="special"><<</span> <span class="identifier">get_std_handle2</span><span class="special">(</span><span class="identifier">STD_OUTPUT_HANDLE</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// OPTION #1, does not require C++11. But without C++11 dll::import<> can not handle calling conventions,</span>
<span class="comment">// so you'll need to hand write the import.</span>
<span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span> <span class="identifier">lib</span><span class="special">(</span><span class="string">"Kernel32.dll"</span><span class="special">,</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">search_system_folders</span><span class="special">);</span>
<span class="identifier">GetStdHandle_t</span><span class="special">&</span> <span class="identifier">func</span> <span class="special">=</span> <span class="identifier">lib</span><span class="special">.</span><span class="identifier">get</span><span class="special"><</span><span class="identifier">GetStdHandle_t</span><span class="special">>(</span><span class="string">"GetStdHandle"</span><span class="special">);</span>
<span class="comment">// Here `func` does not keep a reference to `lib`, you'll have to deal with that on your own.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"1.0 GetStdHandle() returned "</span> <span class="special"><<</span> <span class="identifier">func</span><span class="special">(</span><span class="identifier">STD_OUTPUT_HANDLE</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
<span class="bold"><strong>Full sources:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<a href="https://github.com/apolukhin/Boost.DLL/blob/develop/example/tutorial9/tutorial9.cpp" target="_top">example/tutorial9/tutorial9.cpp</a>
</li></ul></div>
<p>
<a class="link" href="tutorial.html" title="Tutorial">Back to the Top</a>
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2014 Renato Tegon Forti, Antony Polukhin<br>Copyright © 2015, 2016 Antony Polukhin<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</p>
</div></td>
</tr></table>
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