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<html>
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<title>Basic Usage</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_typeerasure.basic"></a><a class="link" href="basic.html" title="Basic Usage">Basic Usage</a>
</h2></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/basic.cpp" target="_top">basic.cpp</a>)
</p>
<p>
The main class in the library is <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
An <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> can store objects
that meet whatever requirements we specify. These requirements are passed to
<code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> as an MPL sequence.
</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>
The MPL sequence combines multiple concepts. In the rare case when we only
want a single concept, it doesn't need to be wrapped in an MPL sequence.
</p></td></tr>
</table></div>
<p>
</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">copy_constructible</span><span class="special"><>,</span> <span class="identifier">typeid_</span><span class="special"><>,</span> <span class="identifier">relaxed</span><span class="special">></span> <span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">any_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// i == 10</span>
</pre>
<p>
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost/type_erasure/copy_constructible.html" title="Struct template copy_constructible">copy_constructible</a></code>
is a builtin concept that allows us to copy and destroy the object. <code class="computeroutput"><a class="link" href="../boost/type_erasure/typeid_.html" title="Struct template typeid_">typeid_</a></code> provides run-time type
information so that we can use <code class="computeroutput"><a class="link" href="../boost/type_erasure/any_cast_idp219443872.html" title="Function any_cast">any_cast</a></code>.
<code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code> enables various
useful defaults. Without <code class="computeroutput"><a class="link" href="../boost/type_erasure/relaxed.html" title="Struct relaxed">relaxed</a></code>,
<code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> supports <span class="emphasis"><em>exactly</em></span>
what you specify and nothing else. In particular, it allows default construction
and assignment of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
</p>
<p>
Now, this example doesn't do very much. <code class="computeroutput"><span class="identifier">x</span></code>
is approximately equivalent to a <a href="../../../libs/any/index.html" target="_top">boost::any</a>.
We can make it more interesting by adding some operators, such as <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> and
<code class="computeroutput"><span class="keyword">operator</span><span class="special"><<</span></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span>
<span class="identifier">copy_constructible</span><span class="special"><>,</span>
<span class="identifier">typeid_</span><span class="special"><>,</span>
<span class="identifier">incrementable</span><span class="special"><>,</span>
<span class="identifier">ostreamable</span><span class="special"><></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
<span class="special">++</span><span class="identifier">x</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="identifier">x</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">// prints 11</span>
</pre>
<p>
</p>
<p>
The library provides concepts for most C++ operators, but this obviously won't
cover all use cases; we often need to define our own requirements. Let's take
the <code class="computeroutput"><span class="identifier">push_back</span></code> member, defined
by several STL containers.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_MEMBER</span><span class="special">((</span><span class="identifier">has_push_back</span><span class="special">),</span> <span class="identifier">push_back</span><span class="special">,</span> <span class="number">1</span><span class="special">)</span>
<span class="keyword">void</span> <span class="identifier">append_many</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_push_back</span><span class="special"><</span><span class="keyword">void</span><span class="special">(</span><span class="keyword">int</span><span class="special">)>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">container</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</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="number">10</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
<span class="identifier">container</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
We use the macro <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_MEMBER.html" title="Macro BOOST_TYPE_ERASURE_MEMBER">BOOST_TYPE_ERASURE_MEMBER</a></code>
to define a concept called <code class="computeroutput"><span class="identifier">has_push_back</span></code>.
The second parameter is the name of the member function and the last macro
parameter indicates the number of arguments which is <code class="computeroutput"><span class="number">1</span></code>
since <code class="computeroutput"><span class="identifier">push_back</span></code> is unary. When
we use <code class="computeroutput"><span class="identifier">has_push_back</span></code>, we have
to tell it the signature of the function, <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span></code>.
This means that the type we store in the any has to have a member that looks
like:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">push_back</span><span class="special">(</span><span class="keyword">int</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
Thus, we could call <code class="computeroutput"><span class="identifier">append_many</span></code>
with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>,
or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> (because
<code class="computeroutput"><span class="keyword">int</span></code> is convertible to <code class="computeroutput"><span class="keyword">long</span></code>), but not <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
</p>
<p>
Also, note that <code class="computeroutput"><span class="identifier">append_many</span></code>
has to operate directly on its argument. It cannot make a copy. To handle this
we use <code class="computeroutput"><span class="identifier">_self</span><span class="special">&</span></code>
as the second argument of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
<code class="computeroutput"><span class="identifier">_self</span></code> is a <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>.
By using <code class="computeroutput"><span class="identifier">_self</span><span class="special">&</span></code>,
we indicate that the <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
stores a reference to an external object instead of allocating its own object.
</p>
<p>
There's actually another <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>
here. The second parameter of <code class="computeroutput"><span class="identifier">has_push_back</span></code>
defaults to <code class="computeroutput"><span class="identifier">_self</span></code>. If we wanted
to define a const member function, we would have to change it to <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">_self</span></code>,
as shown below.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_MEMBER</span><span class="special">((</span><span class="identifier">has_empty</span><span class="special">),</span> <span class="identifier">empty</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
<span class="keyword">bool</span> <span class="identifier">is_empty</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_empty</span><span class="special"><</span><span class="keyword">bool</span><span class="special">(),</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">>,</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">x</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">empty</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
For free functions, we can use the macro <code class="computeroutput"><a class="link" href="../BOOST_TYPE_ERASURE_FREE.html" title="Macro BOOST_TYPE_ERASURE_FREE">BOOST_TYPE_ERASURE_FREE</a></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">BOOST_TYPE_ERASURE_FREE</span><span class="special">((</span><span class="identifier">has_getline</span><span class="special">),</span> <span class="identifier">getline</span><span class="special">,</span> <span class="number">2</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">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">read_lines</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_getline</span><span class="special"><</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">_self</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">_self</span><span class="special">&></span> <span class="identifier">stream</span><span class="special">)</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">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">result</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">tmp</span><span class="special">;</span>
<span class="keyword">while</span><span class="special">(</span><span class="identifier">getline</span><span class="special">(</span><span class="identifier">stream</span><span class="special">,</span> <span class="identifier">tmp</span><span class="special">))</span>
<span class="identifier">result</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">tmp</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
The use of <code class="computeroutput"><span class="identifier">has_getline</span></code> is very
similar to <code class="computeroutput"><span class="identifier">has_push_back</span></code> above.
The difference is that the placeholder <code class="computeroutput"><span class="identifier">_self</span></code>
is passed in the function signature instead of as a separate argument.
</p>
<p>
The <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>
doesn't have to be the first argument. We could just as easily make it the
second argument.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">read_line</span><span class="special">(</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">has_getline</span><span class="special"><</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream</span><span class="special">&,</span> <span class="identifier">_self</span><span class="special">&)>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">str</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">getline</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span><span class="special">,</span> <span class="identifier">str</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Steven Watanabe<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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