| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Using Any</title> |
| <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> |
| <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> |
| <link rel="up" href="../boost_typeerasure.html" title="Chapter 33. Boost.TypeErasure"> |
| <link rel="prev" href="concept.html" title="Concepts in Depth"> |
| <link rel="next" href="examples.html" title="Examples"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> |
| <td align="center"><a href="../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="concept.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="examples.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <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 class="toc"> |
| <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">any</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">any</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">any</code> can be converted |
| to another <code class="computeroutput">any</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">any</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">any</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">any</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">any</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">any</code> |
| arguments. |
| </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 © 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> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="concept.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_typeerasure.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="examples.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
