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<div class="section boost_functional_overloadedfunction_tutorial">
<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>
<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 boost_functional_overloadedfunction_tutorial_overloading">
<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">&amp;</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">&amp;</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">(&amp;</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">&lt;</span><span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)&gt;</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"><a class="link" href="../reference.html#header.boost.functional.overloaded_function_hpp" title="Header &lt;boost/functional/overloaded_function.hpp&gt;">boost/functional/overloaded_function.hpp</a></code>
provides a <code class="computeroutput"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></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">&lt;</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</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">&amp;)</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">&gt;</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"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></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"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>
constructor matching the order of the specified template parameters. <sup>[<a name="boost_functional_overloadedfunction.tutorial.overloading.f0" href="#ftn.boost_functional_overloadedfunction.tutorial.overloading.f0" class="footnote">2</a>]</sup> 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). <sup>[<a name="boost_functional_overloadedfunction.tutorial.overloading.f1" href="#ftn.boost_functional_overloadedfunction.tutorial.overloading.f1" class="footnote">3</a>]</sup> 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 boost_functional_overloadedfunction_tutorial_without_function_types">
<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"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></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"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>
instantiation is returned by <code class="computeroutput"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></code>.
</p>
<p>
The <code class="computeroutput"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></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">&amp;</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">&amp;)</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"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>.
Therefore, <code class="computeroutput"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></code>
provides a more concise syntax in this context when compared with <code class="computeroutput"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>.
</p>
<p>
Another case where <code class="computeroutput"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></code>
can be useful is when the overloaded function object is passed to a function
template which can hold the specific <code class="computeroutput"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></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">&lt;</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">&gt;</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"><a class="link" href="../boost/make_overloaded_function.html" title="Function template make_overloaded_function">boost::make_overloaded_function</a></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">&lt;</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">&gt;</span> <span class="comment">// No need to register `boost::function`.</span>
</pre>
<p>
</p>
</div>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.boost_functional_overloadedfunction.tutorial.overloading.f0" href="#boost_functional_overloadedfunction.tutorial.overloading.f0" class="para">2</a>] </sup>
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">&amp;</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">(&amp;)(</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">&amp;</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 class="footnote"><p><sup>[<a id="ftn.boost_functional_overloadedfunction.tutorial.overloading.f1" href="#boost_functional_overloadedfunction.tutorial.overloading.f1" class="para">3</a>] </sup>
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>
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<td align="right"><div class="copyright-footer">Copyright &#169; 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>
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