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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_typeerasure.any"></a><a class="link" href="any.html" title="Using Any">Using Any</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="any.html#boost_typeerasure.any.construction">Construction</a></span></dt>
<dt><span class="section"><a href="any.html#boost_typeerasure.any.conversions">Conversions</a></span></dt>
<dt><span class="section"><a href="any.html#boost_typeerasure.any.references">References</a></span></dt>
<dt><span class="section"><a href="any.html#boost_typeerasure.any.limit">Syntax Limitations</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.any.construction"></a><a class="link" href="any.html#boost_typeerasure.any.construction" title="Construction">Construction</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/construction.cpp" target="_top">construction.cpp</a>)
</p>
<p>
The library provides the <code class="computeroutput"><a class="link" href="../boost/type_erasure/constructible.html" title="Struct template constructible">constructible</a></code>
concept to allow an <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
to capture constructors. The single template argument should be a function
signature. The return type must be a placeholder specifying the type to be
constructed. The arguments are the arguments of the constructor.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</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">_a</span><span class="special">>,</span>
<span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_b</span><span class="special">>,</span>
<span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_c</span><span class="special">>,</span>
<span class="identifier">constructible</span><span class="special"><</span><span class="identifier">_a</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">_b</span><span class="special">&,</span> <span class="keyword">const</span> <span class="identifier">_c</span><span class="special">&)></span>
<span class="special">></span> <span class="identifier">construct</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">_a</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="keyword">double</span><span class="special">></span> <span class="special">>,</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">_b</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">>,</span>
<span class="identifier">mpl</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">_c</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span>
<span class="special">></span> <span class="identifier">types</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_b</span><span class="special">></span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">(</span><span class="number">10</span><span class="special">),</span> <span class="identifier">make_binding</span><span class="special"><</span><span class="identifier">types</span><span class="special">>());</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_c</span><span class="special">></span> <span class="identifier">val</span><span class="special">(</span><span class="number">2.5</span><span class="special">,</span> <span class="identifier">make_binding</span><span class="special"><</span><span class="identifier">types</span><span class="special">>());</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span> <span class="identifier">v</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="identifier">val</span><span class="special">);</span>
<span class="comment">// v holds std::vector<double>(10, 2.5);</span>
</pre>
<p>
</p>
<p>
Now, suppose that we want a default constructor? We can't have the default
constructor of <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
call the default constructor of the contained type, because it would have
no way of knowing what the contained type is. So, we'll need to pass the
placeholder binding information explicitly.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</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">constructible</span><span class="special"><</span><span class="identifier">_self</span><span class="special">()></span>
<span class="special">></span> <span class="identifier">construct</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</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">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</span><span class="special">></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span> <span class="comment">// y == ""</span>
</pre>
<p>
</p>
<p>
This method is not restricted to the default constructor. If the constructor
takes arguments, they can be passed after the bindings.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</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">constructible</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">size_t</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)></span>
<span class="special">></span> <span class="identifier">construct</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</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">string</span><span class="special">(</span><span class="string">"Test"</span><span class="special">));</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">construct</span><span class="special">></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">binding_of</span><span class="special">(</span><span class="identifier">x</span><span class="special">),</span> <span class="number">5</span><span class="special">,</span> <span class="char">'A'</span><span class="special">);</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.any.conversions"></a><a class="link" href="any.html#boost_typeerasure.any.conversions" title="Conversions">Conversions</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/convert.cpp" target="_top">convert.cpp</a>)
</p>
<p>
An <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> can be converted
to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> as long
as the conversion is an "upcast."
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <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">ostreamable</span><span class="special"><></span>
<span class="special">></span>
<span class="special">></span> <span class="identifier">any_printable</span><span class="special">;</span>
<span class="keyword">typedef</span> <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="special">></span>
<span class="special">></span> <span class="identifier">common_any</span><span class="special">;</span>
<span class="identifier">any_printable</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
<span class="identifier">common_any</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
This conversion is okay because the requirements of <code class="computeroutput"><span class="identifier">common_any</span></code>
are a subset of the requirements of <code class="computeroutput"><span class="identifier">any_printable</span></code>.
Conversion in the other direction is illegal.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">common_any</span> <span class="identifier">x</span><span class="special">(</span><span class="number">10</span><span class="special">);</span>
<span class="identifier">any_printable</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// error</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.any.references"></a><a class="link" href="any.html#boost_typeerasure.any.references" title="References">References</a>
</h3></div></div></div>
<p>
(For the source of the examples in this section see <a href="../../../libs/type_erasure/example/references.cpp" target="_top">references.cpp</a>)
</p>
<p>
To capture by reference, we simply add a reference to the <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">typeid_</span><span class="special"><>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">x</span><span class="special">(</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="special">=</span> <span class="number">5</span><span class="special">;</span> <span class="comment">// now i is 5</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>
<code class="computeroutput"><span class="identifier">_self</span></code> is the default <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>, so it is
easiest to use <code class="computeroutput"><span class="identifier">_self</span><span class="special">&</span></code>.
We could use another <code class="computeroutput"><a class="link" href="../boost/type_erasure/placeholder.html" title="Struct placeholder">placeholder</a></code>
instead. <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code><code class="computeroutput"><span class="special"><</span></code><code class="computeroutput"><a class="link" href="../boost/type_erasure/typeid_.html" title="Struct template typeid_">typeid_</a></code><code class="computeroutput"><span class="special"><</span><span class="identifier">_a</span><span class="special">>,</span> <span class="identifier">_a</span><span class="special">&></span></code> has exactly the same behavior.
</p></td></tr>
</table></div>
<p>
References cannot be rebound. Just like a built-in C++ reference, once you've
initialized it you can't change it to point to something else.
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">j</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">typeid_</span><span class="special"><>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">j</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="comment">// error</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>
As with any other operation, <code class="computeroutput"><span class="identifier">x</span>
<span class="special">=</span> <span class="identifier">y</span></code>
for references acts on <code class="computeroutput"><span class="identifier">i</span></code>
and <code class="computeroutput"><span class="identifier">j</span></code>. Assignment like
this is legal if <code class="computeroutput"><a class="link" href="../boost/type_erasure/assignable.html" title="Struct template assignable">assignable</a></code><code class="computeroutput"><span class="special"><></span></code> is in the Concept, but <code class="computeroutput"><span class="identifier">x</span></code> would still hold a reference to <code class="computeroutput"><span class="identifier">i</span></code>.
</p></td></tr>
</table></div>
<p>
A reference can be bound to another <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</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">incrementable</span><span class="special"><></span>
<span class="special">></span> <span class="identifier">requirements</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</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="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
<span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// x is now 11</span>
</pre>
<p>
</p>
<p>
If a reference is used after the underlying object goes out of scope or is
reset, the behavior is undefined.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</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">incrementable</span><span class="special"><>,</span>
<span class="identifier">relaxed</span>
<span class="special">></span> <span class="identifier">requirements</span><span class="special">;</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</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="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
<span class="identifier">x</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span>
<span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// undefined behavior.</span>
</pre>
<p>
</p>
<p>
This only applies when a reference is constructed from a value. If a reference
is constructed from another reference, the new reference does not depend
on the old one.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</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="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="special">></span> <span class="identifier">p</span><span class="special">(</span>
<span class="keyword">new</span> <span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&>(</span><span class="identifier">x</span><span class="special">));</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">requirements</span><span class="special">,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">y</span><span class="special">(*</span><span class="identifier">p</span><span class="special">);</span> <span class="comment">// equivalent to y(x);</span>
<span class="identifier">p</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span>
<span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// okay</span>
</pre>
<p>
</p>
<p>
Both const and non-const references are supported.
</p>
<p>
</p>
<pre class="programlisting"><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">any</span><span class="special"><</span><span class="identifier">incrementable</span><span class="special"><>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">any</span><span class="special"><</span><span class="identifier">incrementable</span><span class="special"><>,</span> <span class="keyword">const</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
A reference to non-const can be converted to a reference to const, but not
the other way around. Naturally, we can't apply mutating operations to a
const reference.
</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">incrementable</span><span class="special"><>,</span> <span class="identifier">_self</span><span class="special">&></span> <span class="identifier">z</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span> <span class="comment">// error</span>
<span class="special">++</span><span class="identifier">y</span><span class="special">;</span> <span class="comment">// error</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typeerasure.any.limit"></a><a class="link" href="any.html#boost_typeerasure.any.limit" title="Syntax Limitations">Syntax Limitations</a>
</h3></div></div></div>
<p>
In most cases using an any has the same syntax as using the underlying object.
However, there are a few cases where this is not possible to implement. An
<code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> reference is proxy
and cannot be used in contexts where a real reference is required. In particular,
<code class="computeroutput"><a class="link" href="../boost/type_erasure/forward_iterator.html" title="Struct template forward_iterator">forward_iterator</a></code>
does not create a conforming ForwardIterator (unless the value_type is fixed.)
Another difference is that all operations which do not take at least one
<code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code> argument have to
be passed the type information explicitly. Static member functions and constructors
can fall in this category. All this means that generic algorithms might not
work when applied to <code class="computeroutput"><a class="link" href="../boost/type_erasure/any.html" title="Class template any">any</a></code>
arguments.
</p>
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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>)
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