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25<div class="section">
26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="boost_localfunction.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
28</h2></div></div></div>
29<div class="toc"><dl class="toc">
30<dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.local_functions">Local Functions</a></span></dt>
31<dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.Binding">Binding Variables</a></span></dt>
32<dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.binding_the_object__this_">Binding
33 the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt>
34<dt><span class="section"><a href="tutorial.html#boost_localfunction.tutorial.templates">Templates</a></span></dt>
35</dl></div>
36<p>
37 This section illustrates basic usage of this library.
38 </p>
39<div class="section">
40<div class="titlepage"><div><div><h3 class="title">
41<a name="boost_localfunction.tutorial.local_functions"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.local_functions" title="Local Functions">Local Functions</a>
42</h3></div></div></div>
43<p>
44 Local functions are defined using macros from the header file <code class="computeroutput"><a class="link" href="../reference.html#header.boost.local_function_hpp" title="Header &lt;boost/local_function.hpp&gt;">boost/local_function.hpp</a></code>. The
45 macros must be used from within a declarative context (this is a limitation
46 with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
47 lambda functions</a> which can instead be declared also within expressions):
48 </p>
49<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">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span> <span class="comment">// This library header.</span>
50
51<span class="special">...</span>
52<span class="special">{</span> <span class="comment">// Some declarative context.</span>
53 <span class="special">...</span>
54 <span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
55 <span class="emphasis"><em>body-code</em></span>
56 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="emphasis"><em>name</em></span><span class="special">)</span>
57 <span class="special">...</span>
58<span class="special">}</span>
59</pre>
60<p>
61 The code expanded by the macros declares a function object (or <a href="http://en.wikipedia.org/wiki/Functor" target="_top">functor</a>)
62 with the local function name specified by <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code>.
63 <a href="#ftn.boost_localfunction.tutorial.local_functions.f0" class="footnote" name="boost_localfunction.tutorial.local_functions.f0"><sup class="footnote">[5]</sup></a> The usual C++ scope visibility rules apply to local functions
64 for which a local function is visible only within the enclosing scope in
65 which it is declared.
66 </p>
67<p>
68 The local function result type is specified just before the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
69 macro.
70 </p>
71<p>
72 The local function body is specified using the usual C++ statement syntax
73 in a code block <code class="computeroutput"><span class="special">{</span> <span class="special">...</span>
74 <span class="special">}</span></code> between the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
75 and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code>
76 macros. The body is specified outside any of the macros so eventual compiler
77 error messages and related line numbers retain their usual meaning and format.
78 <a href="#ftn.boost_localfunction.tutorial.local_functions.f1" class="footnote" name="boost_localfunction.tutorial.local_functions.f1"><sup class="footnote">[6]</sup></a>
79 </p>
80<p>
81 The local function parameters are passed to the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
82 macro as a comma-separated list of tokens (see the <a class="link" href="no_variadic_macros.html" title="Annex: No Variadic Macros">No
83 Variadic Macros</a> section for compilers that do not support variadic
84 macros):
85 </p>
86<pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><code class="literal"><span class="emphasis"><em>parameter-type1 parameter-name1</em></span></code><span class="special">,</span> <code class="literal"><span class="emphasis"><em>parameter-type2 parameter-name2, ...</em></span></code><span class="special">)</span>
87</pre>
88<p>
89 The maximum number of parameters that can be passed to a local function is
90 controlled at compile-time by the configuration macro <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_CONFIG_ARITY_MAX.html" title="Macro BOOST_LOCAL_FUNCTION_CONFIG_ARITY_MAX">BOOST_LOCAL_FUNCTION_CONFIG_ARITY_MAX</a></code>.
91 For example, let's program a local function named <code class="computeroutput"><span class="identifier">add</span></code>
92 that adds together two integers <code class="computeroutput"><span class="identifier">x</span></code>
93 and <code class="computeroutput"><span class="identifier">y</span></code> (see also <a href="../../../test/add_params_only.cpp" target="_top"><code class="literal">add_params_only.cpp</code></a>):
94 </p>
95<p>
96</p>
97<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Local function.</span>
98 <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
99<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
100
101<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span> <span class="comment">// Local function call.</span>
102</pre>
103<p>
104 </p>
105<p>
106 If the local function has no parameter, it is possible to pass <code class="computeroutput"><span class="keyword">void</span></code> to the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
107 macro (similarly to the C++ syntax that allows to use <code class="literal"><span class="emphasis"><em>result-type
108 function-name</em></span></code><code class="computeroutput"><span class="special">(</span><span class="keyword">void</span><span class="special">)</span></code> to declare
109 a function with no parameter): <a href="#ftn.boost_localfunction.tutorial.local_functions.f2" class="footnote" name="boost_localfunction.tutorial.local_functions.f2"><sup class="footnote">[7]</sup></a>
110 </p>
111<pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="comment">// No parameter.</span>
112</pre>
113<p>
114 For example, let's program a local function that always returns <code class="computeroutput"><span class="number">10</span></code> (see also <a href="../../../test/ten_void.cpp" target="_top"><code class="literal">ten_void.cpp</code></a>):
115 </p>
116<p>
117</p>
118<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// No parameter.</span>
119 <span class="keyword">return</span> <span class="number">10</span><span class="special">;</span>
120<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">ten</span><span class="special">)</span>
121
122<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">ten</span><span class="special">()</span> <span class="special">==</span> <span class="number">10</span><span class="special">);</span>
123</pre>
124<p>
125 </p>
126</div>
127<div class="section">
128<div class="titlepage"><div><div><h3 class="title">
129<a name="boost_localfunction.tutorial.Binding"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.Binding" title="Binding Variables">Binding Variables</a>
130</h3></div></div></div>
131<p>
132 Variables in scope (local variables, enclosing function parameters, data
133 members, etc) can be bound to a local function declaration. Only bound variables,
134 static variables, global variables, functions, and enumerations from the
135 enclosing scope are accessible from within the local function body. The types
136 of bound variables are deduced automatically by this library using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>. <a href="#ftn.boost_localfunction.tutorial.Binding.f0" class="footnote" name="boost_localfunction.tutorial.Binding.f0"><sup class="footnote">[8]</sup></a>
137 </p>
138<p>
139 This library introduces the new "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
140 <a href="#ftn.boost_localfunction.tutorial.Binding.f1" class="footnote" name="boost_localfunction.tutorial.Binding.f1"><sup class="footnote">[9]</sup></a> which is used in place of the parameter type to specify the name
141 of a variable in scope to bind (therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
142 cannot be used as a local function parameter type). A variable can be bound
143 by value:
144 </p>
145<pre class="programlisting"><span class="identifier">bind</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by value.</span>
146</pre>
147<p>
148 Or by reference prefixing the variable name with <code class="computeroutput"><span class="special">&amp;</span></code>:
149 </p>
150<pre class="programlisting"><span class="identifier">bind</span><span class="special">&amp;</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by reference.</span>
151</pre>
152<p>
153 Furthermore, the "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
154 can be prefixed by <code class="computeroutput"><span class="keyword">const</span></code> to
155 bind the variable by constant value:
156 </p>
157<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value.</span>
158</pre>
159<p>
160 Or by constant reference:
161 </p>
162<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value.</span>
163</pre>
164<p>
165 Note that when <code class="computeroutput"><span class="keyword">const</span></code> is used,
166 it must always precede <code class="computeroutput"><span class="identifier">bind</span></code>.
167 <a href="#ftn.boost_localfunction.tutorial.Binding.f2" class="footnote" name="boost_localfunction.tutorial.Binding.f2"><sup class="footnote">[10]</sup></a>
168 </p>
169<p>
170 If a variable is bound by value, then a copy of the variable value is taken
171 at the point of the local function declaration. If a variable is bound by
172 reference instead, the variable will refer to the value it has at the point
173 of the local function call. Furthermore, it is the programmers' responsibility
174 to ensure that variables bound by reference survive the existence scope of
175 the local function otherwise the bound references will be invalid when the
176 local function is called resulting in undefined behaviour (in other words,
177 the usual care in using C++ references must be taken for variables bound
178 by reference).
179 </p>
180<p>
181 The type of a bound variable is automatically deduced using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
182 and it is the exact same type used to declare such a variable in the enclosing
183 scope with the following notes:
184 </p>
185<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
186<li class="listitem">
187 If a bound variable was declared constant in the enclosing scope, it
188 will always be bound by constant value or constant reference even if
189 <code class="computeroutput"><span class="identifier">bind</span><span class="special">...</span></code>
190 is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">...</span></code>
191 . However, if a bound variable was not declared constant in the enclosing
192 scope then it will not be bound as constant unless constant binding is
193 forced using <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">...</span></code>. (Note that binding by constant reference
194 is not supported by <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
195 lambda functions</a> but it is supported by this library.) <a href="#ftn.boost_localfunction.tutorial.Binding.f3" class="footnote" name="boost_localfunction.tutorial.Binding.f3"><sup class="footnote">[11]</sup></a>
196 </li>
197<li class="listitem">
198 If a bound variable was declared as a reference in the enclosing scope,
199 it will still be bound by value unless it is explicitly bound by reference
200 using <code class="computeroutput"><span class="identifier">bind</span><span class="special">&amp;</span></code>
201 or <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span></code>. <a href="#ftn.boost_localfunction.tutorial.Binding.f4" class="footnote" name="boost_localfunction.tutorial.Binding.f4"><sup class="footnote">[12]</sup></a>
202 </li>
203</ul></div>
204<p>
205 When a variable is bound by value (constant or not), its type must be <a href="http://www.boost.org/doc/libs/release/doc/html/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> (i.e., its must
206 provide a copy constructor). As with passing parameters to usual C++ functions,
207 programmers might want to bind variables of complex types by (possibly constant)
208 reference instead of by value to avoid expensive copy operations when these
209 variables are bound to a local function.
210 </p>
211<p>
212 For example, let's program the local function <code class="computeroutput"><span class="identifier">add</span></code>
213 from the example in the <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
214 section. We bind the local variable <code class="computeroutput"><span class="identifier">factor</span></code>
215 by constant value (because its value should not be modified by the local
216 function), the local variable <code class="computeroutput"><span class="identifier">sum</span></code>
217 by non-constant reference (because its value needs to be updated with the
218 summation result), and program the body to perform the summation (see also
219 <a href="../../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>):
220 </p>
221<p>
222</p>
223<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Some local scope.</span>
224 <span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span> <span class="comment">// Variables in scope to bind.</span>
225
226 <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
227 <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
228 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
229
230 <span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Call the local function.</span>
231 <span class="keyword">int</span> <span class="identifier">nums</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span><span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">};</span>
232 <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span> <span class="comment">// Pass it to an algorithm.</span>
233
234 <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">60</span><span class="special">);</span> <span class="comment">// Assert final summation value.</span>
235 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
236<span class="special">}</span>
237</pre>
238<p>
239 </p>
240</div>
241<div class="section">
242<div class="titlepage"><div><div><h3 class="title">
243<a name="boost_localfunction.tutorial.binding_the_object__this_"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.binding_the_object__this_" title="Binding the Object this">Binding
244 the Object <code class="computeroutput"><span class="keyword">this</span></code></a>
245</h3></div></div></div>
246<p>
247 It is also possible to bind the object <code class="computeroutput"><span class="keyword">this</span></code>
248 when it is in scope (e.g., from an enclosing non-static member function).
249 This is done by using the special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
250 (instead of <code class="computeroutput"><span class="keyword">this</span></code>) as the name
251 of the variable to bind in the local function declaration and also to access
252 the object within the local function body. <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f0" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f0"><sup class="footnote">[13]</sup></a>
253 </p>
254<div class="warning"><table border="0" summary="Warning">
255<tr>
256<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
257<th align="left">Warning</th>
258</tr>
259<tr><td align="left" valign="top"><p>
260 The library will generate a compile-time error if <code class="computeroutput"><span class="keyword">this</span></code>
261 is mistakenly used instead of <code class="computeroutput"><span class="identifier">this_</span></code>
262 to bind the object in the local function declaration. However, mistakenly
263 using <code class="computeroutput"><span class="keyword">this</span></code> instead of <code class="computeroutput"><span class="identifier">this_</span></code> to access the object within the
264 local function body will leads to undefined behaviour and it will not necessarily
265 generate a compile-time error. <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f1" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f1"><sup class="footnote">[14]</sup></a> Programmers are ultimately responsible to make sure that <code class="computeroutput"><span class="keyword">this</span></code> is never used within a local function.
266 </p></td></tr>
267</table></div>
268<p>
269 The object <code class="computeroutput"><span class="keyword">this</span></code> can be bound
270 by value:
271 </p>
272<pre class="programlisting"><span class="identifier">bind</span> <span class="identifier">this_</span> <span class="comment">// Bind the object `this` by value.</span>
273</pre>
274<p>
275 In this case the local function will be able to modify the object when the
276 enclosing scope is not a constant member and it will not be able to modify
277 the object when the enclosing scope is a constant member. Otherwise, the
278 object <code class="computeroutput"><span class="keyword">this</span></code> can be bound by
279 constant value:
280 </p>
281<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">this_</span> <span class="comment">// Bind the object `this` by constant value.</span>
282</pre>
283<p>
284 In this case the local function will never be able to modify the object (regardless
285 of whether the enclosing scope is a constant member or not).
286 </p>
287<p>
288 Note that the object <code class="computeroutput"><span class="keyword">this</span></code> can
289 never be bound by reference because C++ does not allow to obtain a reference
290 to <code class="computeroutput"><span class="keyword">this</span></code> (the library will generate
291 a compile-time error if programmers try to use <code class="computeroutput"><span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">this_</span></code>
292 or <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">this_</span></code>).
293 Note that <code class="computeroutput"><span class="keyword">this</span></code> is a pointer
294 so the pointed object is never copied even if <code class="computeroutput"><span class="keyword">this</span></code>
295 is bound by value (also it is not possible to directly bind <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> because
296 <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
297 is an expression and not a variable name).
298 </p>
299<p>
300 For example, let's program a local function <code class="computeroutput"><span class="identifier">add</span></code>
301 similar to the one in the example from the <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a>
302 section but using a member function to illustrate how to bind the object
303 <code class="computeroutput"><span class="keyword">this</span></code> (see also <a href="../../../test/add_this.cpp" target="_top"><code class="literal">add_this.cpp</code></a>):
304 </p>
305<p>
306</p>
307<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">adder</span> <span class="special">{</span>
308 <span class="identifier">adder</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">sum_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
309
310 <span class="keyword">int</span> <span class="identifier">sum</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;&amp;</span> <span class="identifier">nums</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">)</span> <span class="special">{</span>
311
312 <span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
313 <span class="identifier">this_</span><span class="special">-&gt;</span><span class="identifier">sum_</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span> <span class="comment">// Use `this_` instead of `this`.</span>
314 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
315
316 <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">nums</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">add</span><span class="special">);</span>
317 <span class="keyword">return</span> <span class="identifier">sum_</span><span class="special">;</span>
318 <span class="special">}</span>
319
320<span class="keyword">private</span><span class="special">:</span>
321 <span class="keyword">int</span> <span class="identifier">sum_</span><span class="special">;</span>
322<span class="special">};</span>
323</pre>
324<p>
325 </p>
326<p>
327 Note that the local function has access to all class members via the bound
328 object <code class="computeroutput"><span class="identifier">this_</span></code> regardless of
329 their access level (<code class="computeroutput"><span class="keyword">public</span></code>,
330 <code class="computeroutput"><span class="keyword">protected</span></code>, or <code class="computeroutput"><span class="keyword">private</span></code>). <a href="#ftn.boost_localfunction.tutorial.binding_the_object__this_.f2" class="footnote" name="boost_localfunction.tutorial.binding_the_object__this_.f2"><sup class="footnote">[15]</sup></a> Specifically, in the example above the local function updates
331 the private data member <code class="computeroutput"><span class="identifier">sum_</span></code>.
332 </p>
333</div>
334<div class="section">
335<div class="titlepage"><div><div><h3 class="title">
336<a name="boost_localfunction.tutorial.templates"></a><a class="link" href="tutorial.html#boost_localfunction.tutorial.templates" title="Templates">Templates</a>
337</h3></div></div></div>
338<p>
339 When local functions are programmed within templates, they need to be declared
340 using the special macros <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>
341 and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_NAME_TPL">BOOST_LOCAL_FUNCTION_NAME_TPL</a></code>:
342 <a href="#ftn.boost_localfunction.tutorial.templates.f0" class="footnote" name="boost_localfunction.tutorial.templates.f0"><sup class="footnote">[16]</sup></a>
343 </p>
344<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">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span> <span class="comment">// This library header.</span>
345
346<span class="special">...</span>
347<span class="special">{</span> <span class="comment">// Some declarative context within a template.</span>
348 <span class="special">...</span>
349 <span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION_TPL</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
350 <span class="emphasis"><em>body-code</em></span>
351 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="emphasis"><em>name</em></span><span class="special">)</span>
352 <span class="special">...</span>
353<span class="special">}</span>
354</pre>
355<p>
356 The <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>
357 and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_NAME_TPL">BOOST_LOCAL_FUNCTION_NAME_TPL</a></code>
358 macros have the exact same syntax of the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
359 and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code>
360 macros that we have seen so far.
361 </p>
362<p>
363 For example, let's program a local function similar to the one from the
364 <a class="link" href="../index.html#boost_localfunction.introduction" title="Introduction">Introduction</a> section
365 but within a template (see also <a href="../../../test/add_template.cpp" target="_top"><code class="literal">add_template.cpp</code></a>):
366 </p>
367<p>
368</p>
369<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
370<span class="identifier">T</span> <span class="identifier">total</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
371 <span class="identifier">T</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="identifier">T</span><span class="special">(),</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
372
373 <span class="comment">// Must use the `..._TPL` macros within templates.</span>
374 <span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_TPL</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">sum</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
375 <span class="keyword">return</span> <span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
376 <span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
377
378 <span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
379 <span class="identifier">T</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="identifier">nums</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
380 <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">,</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">2</span><span class="special">,</span> <span class="identifier">add</span><span class="special">);</span>
381
382 <span class="keyword">return</span> <span class="identifier">sum</span><span class="special">;</span>
383<span class="special">}</span>
384</pre>
385<p>
386 </p>
387</div>
388<div class="footnotes">
389<br><hr style="width:100; text-align:left;margin-left: 0">
390<div id="ftn.boost_localfunction.tutorial.local_functions.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f0" class="para"><sup class="para">[5] </sup></a>
391 <span class="bold"><strong>Rationale.</strong></span> The local function name must
392 be passed to the macro <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code>
393 ending the function definition so this macro can declare a local variable
394 with the local function name to hold the local function object. Therefore
395 the local function name cannot be specified within the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
396 and it must appear instead after the local function body (even if that
397 differs from the usual C++ function declaration syntax).
398 </p></div>
399<div id="ftn.boost_localfunction.tutorial.local_functions.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f1" class="para"><sup class="para">[6] </sup></a>
400 <span class="bold"><strong>Rationale.</strong></span> If the local function body
401 were instead passed as a macro parameter, it would be expanded on a single
402 line of code (because macros always expand as a single line of code). Therefore,
403 eventual compiler error line numbers would all report the same value and
404 would no longer be useful to pinpoint errors.
405 </p></div>
406<div id="ftn.boost_localfunction.tutorial.local_functions.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.local_functions.f2" class="para"><sup class="para">[7] </sup></a>
407 <span class="bold"><strong>Rationale.</strong></span> The <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
408 standard does not allow to pass empty parameters to a macro so the macro
409 cannot be invoked as <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">()</span></code>. On <a href="http://www.open-std.org/jtc1/sc22/wg14/www/projects#9899" target="_top">C99</a>
410 compilers with properly implemented empty macro parameter support, it would
411 be possible to allow <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">()</span></code> but this is already not the case for
412 MSVC so this syntax is never allowed to ensure better portability.
413 </p></div>
414<div id="ftn.boost_localfunction.tutorial.Binding.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f0" class="para"><sup class="para">[8] </sup></a>
415 <span class="bold"><strong>Rationale.</strong></span> By binding a variable in scope,
416 the local function declaration is specifying that such a variable should
417 be accessible within the local function body regardless of its type. Semantically,
418 this binding should be seen as an "extension" of the scope of
419 the bound variable from the enclosing scope to the scope of the local function
420 body. Therefore, contrary to the semantic of passing a function parameter,
421 the semantic of binding a variable does not depend on the variable type
422 but just on the variable name: "The variable in scope named <span class="emphasis"><em>x</em></span>
423 should be accessible within the local function named <span class="emphasis"><em>f</em></span>".
424 For example, this reduces maintenance because if a bound variable type
425 is changed, the local function declaration does not have to change.
426 </p></div>
427<div id="ftn.boost_localfunction.tutorial.Binding.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f1" class="para"><sup class="para">[9] </sup></a>
428 Obviously, the token <code class="computeroutput"><span class="identifier">bind</span></code>
429 is not a keyword of the C++ language. This library parses the token <code class="computeroutput"><span class="identifier">bind</span></code> during macro expansion using preprocessor
430 meta-programming (see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
431 section). Therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
432 can be considered a new "keyword" only at the preprocessor meta-programming
433 level within the syntax defined by the macros of this library (thus it
434 is referred to as a "keyword" only within quotes).
435 </p></div>
436<div id="ftn.boost_localfunction.tutorial.Binding.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f2" class="para"><sup class="para">[10] </sup></a>
437 <span class="bold"><strong>Rationale.</strong></span> The library macros could have
438 been implemented to accept both syntaxes <code class="computeroutput"><span class="keyword">const</span>
439 <span class="identifier">bind</span> <span class="special">...</span></code>
440 and <code class="computeroutput"><span class="identifier">bind</span> <span class="keyword">const</span>
441 <span class="special">...</span></code> equivalently. However, handling
442 both syntaxes would have complicated the macro implementation without adding
443 any feature so only one syntax <code class="computeroutput"><span class="keyword">const</span>
444 <span class="identifier">bind</span> <span class="special">...</span></code>
445 is supported.
446 </p></div>
447<div id="ftn.boost_localfunction.tutorial.Binding.f3" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f3" class="para"><sup class="para">[11] </sup></a>
448 An historical note: Constant binding of variables in scope was the
449 main use case that originally motivated the authors in developing this
450 library. The authors needed to locally create a chuck of code to assert
451 some correctness conditions while these assertions were not supposed
452 to modify any of the variables they were using (see the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a>
453 library). This was achieved by binding by constant reference <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span></code> the variables needed by the assertions
454 and then by programming the local function body to check the assertions.
455 This way if any of the assertions mistakenly changes a bound variable
456 (for example confusing the operator <code class="computeroutput"><span class="special">==</span></code>
457 with <code class="computeroutput"><span class="special">=</span></code>), the compiler
458 correctly generates an error because the bound variable is of <code class="computeroutput"><span class="keyword">const</span></code> type within the local function
459 body (see also <span class="emphasis"><em>constant blocks</em></span> in the <a class="link" href="examples.html" title="Examples">Examples</a>
460 section).
461 </p></div>
462<div id="ftn.boost_localfunction.tutorial.Binding.f4" class="footnote"><p><a href="#boost_localfunction.tutorial.Binding.f4" class="para"><sup class="para">[12] </sup></a>
463 <span class="bold"><strong>Rationale.</strong></span> Variables originally declared
464 as references are bound by value unless <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span><span class="special">&amp;</span></code>
465 is used so that references can be bound by both value <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span></code>
466 and reference <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span> <span class="identifier">bind</span><span class="special">&amp;</span></code> (this is the same binding semantic
467 adopted by <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>).
468 However, variables originally declared as constants should never loose
469 their <code class="computeroutput"><span class="keyword">const</span></code> qualifier
470 (to prevent their modification not just in the enclosing scope but
471 also in the local scope) thus they are always bound by constant even
472 if <code class="computeroutput"><span class="identifier">bind</span><span class="special">[&amp;]</span></code>
473 is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">[&amp;]</span></code>.
474 </p></div>
475<div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f0" class="para"><sup class="para">[13] </sup></a>
476 <span class="bold"><strong>Rationale.</strong></span> The special name <code class="computeroutput"><span class="identifier">this_</span></code> was chosen following <a href="http://lists.boost.org/Archives/boost/2011/04/179729.php" target="_top">Boost
477 practise</a> to postfix with an underscore identifiers that are named
478 after keywords (the C++ keyword <code class="computeroutput"><span class="keyword">this</span></code>
479 in this case). The special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
480 is needed because <code class="computeroutput"><span class="keyword">this</span></code> is
481 a reserved C++ keyword so it cannot be used as the name of the internal
482 parameter that passes the bound object to the local function body. It would
483 have been possible to use <code class="computeroutput"><span class="keyword">this</span></code>
484 (instead of <code class="computeroutput"><span class="identifier">this_</span></code>) within
485 the local function body either at the expenses of copying the bound object
486 (which would introduce run-time overhead and also the stringent requirement
487 that the bound object must have a deep copy constructor) or by relying
488 on an <a href="http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/d3a86f27277f713b" target="_top">undefined
489 behaviour of <code class="computeroutput"><span class="keyword">static_cast</span></code></a>
490 (which might not work on all platforms at the cost of portability).
491 </p></div>
492<div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f1" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f1" class="para"><sup class="para">[14] </sup></a>
493 <span class="bold"><strong>Rationale.</strong></span> The local function body cannot
494 be a static member function of the local functor object in order to support
495 recursion (because the local function name is specified by the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code>
496 macro only after the body so it must be made available via a functor
497 data member named after the local function and local classes cannot have
498 static data members in C++) and nesting (because the argument binding
499 variable must be declared as a data member so it is visible in a local
500 function nested within the body member function) -- see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
501 section. Therefore, from within the local function body the variable
502 <code class="computeroutput"><span class="keyword">this</span></code> is visible but it refers
503 to the local functor and not to the bound object.
504 </p></div>
505<div id="ftn.boost_localfunction.tutorial.binding_the_object__this_.f2" class="footnote"><p><a href="#boost_localfunction.tutorial.binding_the_object__this_.f2" class="para"><sup class="para">[15] </sup></a>
506 <span class="bold"><strong>Rationale.</strong></span> This is possible because of
507 the fix to C++ <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#45" target="_top">defect
508 45</a> that made inner and local types able to access all outer class
509 members regardless of their access level.
510 </p></div>
511<div id="ftn.boost_localfunction.tutorial.templates.f0" class="footnote"><p><a href="#boost_localfunction.tutorial.templates.f0" class="para"><sup class="para">[16] </sup></a>
512 <span class="bold"><strong>Rationale.</strong></span> Within templates, this library
513 needs to use <code class="computeroutput"><span class="keyword">typename</span></code> to explicitly
514 indicate that some expressions evaluate to a type. Because <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
515 does not allow to use <code class="computeroutput"><span class="keyword">typename</span></code>
516 outside templates, the special <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code> macros are used to indicate that
517 the enclosing scope is a template so this library can safely use <code class="computeroutput"><span class="keyword">typename</span></code> to resolve expression type ambiguities.
518 <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> and
519 other compilers might compile local functions within templates even when
520 the <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code>
521 macros are not used. However, it is recommended to always use the <code class="computeroutput"><span class="special">...</span><span class="identifier">_TPL</span></code>
522 macros within templates to maximize portability.
523 </p></div>
524</div>
525</div>
526<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
527<td align="left"></td>
528<td align="right"><div class="copyright-footer">Copyright &#169; 2009-2012 Lorenzo
529 Caminiti<p>
530 Distributed under the Boost Software License, Version 1.0 (see accompanying
531 file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
532 </p>
533</div></td>
534</tr></table>
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