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- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_functional_overloadedfunction.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
- </h2></div></div></div>
- <div class="toc"><dl class="toc">
- <dt><span class="section"><a href="tutorial.html#boost_functional_overloadedfunction.tutorial.overloading">Overloading</a></span></dt>
- <dt><span class="section"><a href="tutorial.html#boost_functional_overloadedfunction.tutorial.without_function_types">Without
- Function Types</a></span></dt>
- </dl></div>
- <p>
- This section explains how to use this library.
- </p>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_functional_overloadedfunction.tutorial.overloading"></a><a class="link" href="tutorial.html#boost_functional_overloadedfunction.tutorial.overloading" title="Overloading">Overloading</a>
- </h3></div></div></div>
- <p>
- Consider the following functions which have distinct signatures:
- </p>
- <p>
- </p>
- <pre class="programlisting"><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">identity_s</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">x</span><span class="special">)</span> <span class="comment">// Function (as pointer).</span>
- <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> <span class="special">}</span>
- <span class="keyword">int</span> <span class="identifier">identity_i_impl</span><span class="special">(</span><span class="keyword">int</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="special">}</span>
- <span class="keyword">int</span> <span class="special">(&</span><span class="identifier">identity_i</span><span class="special">)(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">identity_i_impl</span><span class="special">;</span> <span class="comment">// Function reference.</span>
- <span class="keyword">double</span> <span class="identifier">identity_d_impl</span><span class="special">(</span><span class="keyword">double</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="special">}</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)></span> <span class="identifier">identity_d</span> <span class="special">=</span> <span class="identifier">identity_d_impl</span><span class="special">;</span> <span class="comment">// Functor.</span>
- </pre>
- <p>
- </p>
- <p>
- This library header <code class="computeroutput">boost/functional/overloaded_function.hpp</code>
- provides a <code class="computeroutput">boost::overloaded_function</code>
- class template that creates a single overloaded function object that can
- be used to call the specified functions instead of using the separate function
- names (see also <a href="../../../test/functor.cpp" target="_top"><code class="literal">functor.cpp</code></a>
- and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</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="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="special">,</span> <span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span>
- <span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span>
- <span class="special">></span> <span class="identifier">identity</span><span class="special">(</span><span class="identifier">identity_s</span><span class="special">,</span> <span class="identifier">identity_i</span><span class="special">,</span> <span class="identifier">identity_d</span><span class="special">);</span>
- <span class="comment">// All calls via single `identity` function.</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="string">"abc"</span><span class="special">)</span> <span class="special">==</span> <span class="string">"abc"</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">1.23</span><span class="special">)</span> <span class="special">==</span> <span class="number">1.23</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- <p>
- Note how each function type is passed as a template parameter of <code class="computeroutput">boost::overloaded_function</code> using
- the following syntax (this is <a href="http://www.boost.org/libs/function" target="_top">Boost.Function</a>'s
- preferred syntax):
- </p>
- <pre class="programlisting"><span class="emphasis"><em>result-type</em></span> <span class="special">(</span><span class="emphasis"><em>argument1-type</em></span><span class="special">,</span> <span class="emphasis"><em>argument2-type</em></span><span class="special">,</span> <span class="special">...)</span>
- </pre>
- <p>
- Then the relative function pointers, function references, or <a href="http://en.wikipedia.org/wiki/Polymorphism_(computer_science)" target="_top">monomorphic
- function</a> objects are passed to the <code class="computeroutput">boost::overloaded_function</code>
- constructor matching the order of the specified template parameters. <a href="#ftn.boost_functional_overloadedfunction.tutorial.overloading.f0" class="footnote" name="boost_functional_overloadedfunction.tutorial.overloading.f0"><sup class="footnote">[2]</sup></a> In the above example, <code class="computeroutput"><span class="identifier">identity_s</span></code>
- is passed as a function pointer (the function address is automatically taken
- from the function name by the compiler), <code class="computeroutput"><span class="identifier">identity_i</span></code>
- as a function reference, and <code class="computeroutput"><span class="identifier">identity_d</span></code>
- as a function object.
- </p>
- <p>
- All specified function types must have distinct parameters from one another
- (so the overloaded calls can be resolved by this library). <a href="#ftn.boost_functional_overloadedfunction.tutorial.overloading.f1" class="footnote" name="boost_functional_overloadedfunction.tutorial.overloading.f1"><sup class="footnote">[3]</sup></a> In order to create an overloaded function object, it is necessary
- to specify at least two function types (because there is nothing to overload
- between one or zero functions).
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_functional_overloadedfunction.tutorial.without_function_types"></a><a class="link" href="tutorial.html#boost_functional_overloadedfunction.tutorial.without_function_types" title="Without Function Types">Without
- Function Types</a>
- </h3></div></div></div>
- <p>
- For convenience, this library also provides the <code class="computeroutput">boost::make_overloaded_function</code>
- function template which allows to create the overloaded function object without
- explicitly specifying the function types. The function types are automatically
- deduced from the specified functions and the appropriate <code class="computeroutput">boost::overloaded_function</code>
- instantiation is returned by <code class="computeroutput">boost::make_overloaded_function</code>.
- </p>
- <p>
- The <code class="computeroutput">boost::make_overloaded_function</code>
- function template can be useful when used together with <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>'s
- <code class="computeroutput"><span class="identifier">BOOST_AUTO</span></code> (or C++11 <code class="computeroutput"><span class="keyword">auto</span></code>). For example (see also <a href="../../../test/make_decl.cpp" target="_top"><code class="literal">make_decl.cpp</code></a>
- and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_AUTO</span><span class="special">(</span><span class="identifier">identity</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_overloaded_function</span><span class="special">(</span>
- <span class="identifier">identity_s</span><span class="special">,</span> <span class="identifier">identity_i</span><span class="special">,</span> <span class="identifier">identity_d</span><span class="special">));</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="string">"abc"</span><span class="special">)</span> <span class="special">==</span> <span class="string">"abc"</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">1.23</span><span class="special">)</span> <span class="special">==</span> <span class="number">1.23</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- <p>
- Note how the overloaded function object <code class="computeroutput"><span class="identifier">identity</span></code>
- has been created specifying only the functions <code class="computeroutput"><span class="identifier">identity_s</span></code>,
- <code class="computeroutput"><span class="identifier">identity_i</span></code>, <code class="computeroutput"><span class="identifier">identity_d</span></code> and without specifying the function
- types <code class="computeroutput"><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="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>,
- <code class="computeroutput"><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span></code>, and
- <code class="computeroutput"><span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span></code> as
- required instead by <code class="computeroutput">boost::overloaded_function</code>.
- Therefore, <code class="computeroutput">boost::make_overloaded_function</code>
- provides a more concise syntax in this context when compared with <code class="computeroutput">boost::overloaded_function</code>.
- </p>
- <p>
- Another case where <code class="computeroutput">boost::make_overloaded_function</code>
- can be useful is when the overloaded function object is passed to a function
- template which can hold the specific <code class="computeroutput">boost::overloaded_function</code>
- type using a template parameter. For example (see also <a href="../../../test/make_call.cpp" target="_top"><code class="literal">make_call.cpp</code></a>
- and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">></span>
- <span class="keyword">void</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">identity</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="string">"abc"</span><span class="special">)</span> <span class="special">==</span> <span class="string">"abc"</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span>
- <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">identity</span><span class="special">(</span><span class="number">1.23</span><span class="special">)</span> <span class="special">==</span> <span class="number">1.23</span><span class="special">);</span>
- <span class="special">}</span>
- </pre>
- <p>
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="identifier">check</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_overloaded_function</span><span class="special">(</span><span class="identifier">identity_s</span><span class="special">,</span> <span class="identifier">identity_i</span><span class="special">,</span> <span class="identifier">identity_d</span><span class="special">));</span>
- </pre>
- <p>
- </p>
- <p>
- The library implementation of <code class="computeroutput">boost::make_overloaded_function</code>
- uses <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
- to automatically deduce some of the function types. In order to compile code
- in <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
- emulation mode, all types should be properly registered using <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span></code> and <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span></code>, or appropriate
- <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a> headers
- should be included (see <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
- for more information). For the above examples, it is sufficient to include
- the <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
- header that registers <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
- (this library does not require to register <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span></code>
- for <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
- emulation):
- </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">typeof</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">hpp</span><span class="special">></span> <span class="comment">// No need to register `boost::function`.</span>
- </pre>
- <p>
- </p>
- </div>
- <div class="footnotes">
- <br><hr style="width:100; text-align:left;margin-left: 0">
- <div id="ftn.boost_functional_overloadedfunction.tutorial.overloading.f0" class="footnote"><p><a href="#boost_functional_overloadedfunction.tutorial.overloading.f0" class="para"><sup class="para">[2] </sup></a>
- Function pointers are of the form <code class="literal"><span class="emphasis"><em>result-type </em></span></code><code class="computeroutput"><span class="special">(*)(</span></code><code class="literal"><span class="emphasis"><em>argument1-type</em></span></code><code class="computeroutput"><span class="special">,</span> <span class="special">...)</span></code> (the
- C++ compiler is usually able to automatically promote a function name to
- a function pointer in a context where a function pointer is expected even
- if the function name is not prefixed by <code class="computeroutput"><span class="special">&</span></code>).
- Function references are of the form <code class="literal"><span class="emphasis"><em>result-type </em></span></code><code class="computeroutput"><span class="special">(&)(</span></code><code class="literal"><span class="emphasis"><em>argument1-type</em></span></code><code class="computeroutput"><span class="special">,</span> <span class="special">...)</span></code>.
- Function types are of the form <code class="literal"><span class="emphasis"><em>result-type </em></span></code><code class="computeroutput"><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>argument1-type</em></span></code><code class="computeroutput"><span class="special">,</span> <span class="special">...)</span></code> (note
- how they lack of both <code class="computeroutput"><span class="special">*</span></code> and
- <code class="computeroutput"><span class="special">&</span></code> when compared to function
- pointers and function references). Finally, monomorphic function objects
- are instances of classes with a non-template call operator of the form
- <code class="literal"><span class="emphasis"><em>result-type </em></span></code><code class="computeroutput"><span class="keyword">operator</span><span class="special">()(</span></code><code class="literal"><span class="emphasis"><em>argument1-type</em></span></code><code class="computeroutput"><span class="special">,</span> <span class="special">...)</span></code>.
- Unfortunately, it is not possible to support polymorphic function objects
- (see <a href="http://lists.boost.org/Archives/boost/2012/03/191744.php" target="_top">http://lists.boost.org/Archives/boost/2012/03/191744.php</a>).
- </p></div>
- <div id="ftn.boost_functional_overloadedfunction.tutorial.overloading.f1" class="footnote"><p><a href="#boost_functional_overloadedfunction.tutorial.overloading.f1" class="para"><sup class="para">[3] </sup></a>
- Note that in C++ the function result type is not used for overload resolution
- (to avoid making the overload resolution context dependent). Therefore,
- at least one of the function parameters must be distinct for each specified
- function type.
- </p></div>
- </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, 2012 Lorenzo Caminiti<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>
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