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24</div>
25<div class="section boost_functional_overloadedfunction_tutorial">
26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="boost_functional_overloadedfunction.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
28</h2></div></div></div>
29<div class="toc"><dl>
30<dt><span class="section"><a href="tutorial.html#boost_functional_overloadedfunction.tutorial.overloading">Overloading</a></span></dt>
31<dt><span class="section"><a href="tutorial.html#boost_functional_overloadedfunction.tutorial.without_function_types">Without
32 Function Types</a></span></dt>
33</dl></div>
34<p>
35 This section explains how to use this library.
36 </p>
37<div class="section boost_functional_overloadedfunction_tutorial_overloading">
38<div class="titlepage"><div><div><h3 class="title">
39<a name="boost_functional_overloadedfunction.tutorial.overloading"></a><a class="link" href="tutorial.html#boost_functional_overloadedfunction.tutorial.overloading" title="Overloading">Overloading</a>
40</h3></div></div></div>
41<p>
42 Consider the following functions which have distinct signatures:
43 </p>
44<p>
45</p>
46<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>
47 <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span> <span class="special">}</span>
48
49<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>
50<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>
51
52<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>
53<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>
54</pre>
55<p>
56 </p>
57<p>
58 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>
59 provides a <code class="computeroutput"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>
60 class template that creates a single overloaded function object that can
61 be used to call the specified functions instead of using the separate function
62 names (see also <a href="../../../test/functor.cpp" target="_top"><code class="literal">functor.cpp</code></a>
63 and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
64 </p>
65<p>
66</p>
67<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</span><span class="special">&lt;</span>
68 <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>
69 <span class="special">,</span> <span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span>
70 <span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span>
71<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>
72
73<span class="comment">// All calls via single `identity` function.</span>
74<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>
75<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>
76<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>
77</pre>
78<p>
79 </p>
80<p>
81 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
82 the following syntax (this is <a href="http://www.boost.org/libs/function" target="_top">Boost.Function</a>'s
83 preferred syntax):
84 </p>
85<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>
86</pre>
87<p>
88 Then the relative function pointers, function references, or <a href="http://en.wikipedia.org/wiki/Polymorphism_(computer_science)" target="_top">monomorphic
89 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>
90 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>
91 is passed as a function pointer (the function address is automatically taken
92 from the function name by the compiler), <code class="computeroutput"><span class="identifier">identity_i</span></code>
93 as a function reference, and <code class="computeroutput"><span class="identifier">identity_d</span></code>
94 as a function object.
95 </p>
96<p>
97 All specified function types must have distinct parameters from one another
98 (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
99 to specify at least two function types (because there is nothing to overload
100 between one or zero functions).
101 </p>
102</div>
103<div class="section boost_functional_overloadedfunction_tutorial_without_function_types">
104<div class="titlepage"><div><div><h3 class="title">
105<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
106 Function Types</a>
107</h3></div></div></div>
108<p>
109 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>
110 function template which allows to create the overloaded function object without
111 explicitly specifying the function types. The function types are automatically
112 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>
113 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>.
114 </p>
115<p>
116 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>
117 function template can be useful when used together with <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>'s
118 <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>
119 and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
120 </p>
121<p>
122</p>
123<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>
124 <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>
125
126<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>
127<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>
128<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>
129</pre>
130<p>
131 </p>
132<p>
133 Note how the overloaded function object <code class="computeroutput"><span class="identifier">identity</span></code>
134 has been created specifying only the functions <code class="computeroutput"><span class="identifier">identity_s</span></code>,
135 <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
136 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>
137 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;)</span></code>,
138 <code class="computeroutput"><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span></code>, and
139 <code class="computeroutput"><span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span></code> as
140 required instead by <code class="computeroutput"><a class="link" href="../boost/overloaded_function.html" title="Class template overloaded_function">boost::overloaded_function</a></code>.
141 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>
142 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>.
143 </p>
144<p>
145 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>
146 can be useful is when the overloaded function object is passed to a function
147 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>
148 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>
149 and <a href="../../../test/identity.hpp" target="_top"><code class="literal">identity.hpp</code></a>):
150 </p>
151<p>
152</p>
153<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>
154<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>
155 <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>
156 <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>
157 <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>
158<span class="special">}</span>
159</pre>
160<p>
161 </p>
162<p>
163</p>
164<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>
165</pre>
166<p>
167 </p>
168<p>
169 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>
170 uses <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
171 to automatically deduce some of the function types. In order to compile code
172 in <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
173 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
174 <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a> headers
175 should be included (see <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
176 for more information). For the above examples, it is sufficient to include
177 the <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
178 header that registers <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
179 (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>
180 for <a href="http://www.boost.org/doc/libs/typeof" target="_top">Boost.Typeof</a>
181 emulation):
182 </p>
183<p>
184</p>
185<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>
186</pre>
187<p>
188 </p>
189</div>
190<div class="footnotes">
191<br><hr width="100" align="left">
192<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>
193 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
194 C++ compiler is usually able to automatically promote a function name to
195 a function pointer in a context where a function pointer is expected even
196 if the function name is not prefixed by <code class="computeroutput"><span class="special">&amp;</span></code>).
197 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>.
198 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
199 how they lack of both <code class="computeroutput"><span class="special">*</span></code> and
200 <code class="computeroutput"><span class="special">&amp;</span></code> when compared to function
201 pointers and function references). Finally, monomorphic function objects
202 are instances of classes with a non-template call operator of the form
203 <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>.
204 Unfortunately, it is not possible to support polymorphic function objects
205 (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>).
206 </p></div>
207<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>
208 Note that in C++ the function result type is not used for overload resolution
209 (to avoid making the overload resolution context dependent). Therefore,
210 at least one of the function parameters must be distinct for each specified
211 function type.
212 </p></div>
213</div>
214</div>
215<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
216<td align="left"></td>
217<td align="right"><div class="copyright-footer">Copyright &#169; 2011, 2012 Lorenzo Caminiti<p>
218 Distributed under the Boost Software License, Version 1.0 (see accompanying
219 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>)
220 </p>
221</div></td>
222</tr></table>
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