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<div class="section boost_localfunction_Tutorial">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_localfunction.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_localfunction.Tutorial.local_functions">Local Functions</a></span></dt>
<dt><span class="section"><a href="Tutorial.html#boost_localfunction.Tutorial.Binding">Binding Variables</a></span></dt>
<dt><span class="section"><a href="Tutorial.html#boost_localfunction.Tutorial.binding_the_object__this_">Binding
the Object <code class="computeroutput"><span class="keyword">this</span></code></a></span></dt>
<dt><span class="section"><a href="Tutorial.html#boost_localfunction.Tutorial.templates">Templates</a></span></dt>
</dl></div>
<p>
This section illustrates basic usages of this library.
</p>
<div class="section boost_localfunction_Tutorial_local_functions">
<div class="titlepage"><div><div><h3 class="title">
<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>
</h3></div></div></div>
<p>
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
macros must be used from within a declarative context (this is a limitation
with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda</a> functions which can instead be declared also within expressions):
</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">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span> <span class="comment">// This library header.</span>
<span class="special">...</span>
<span class="special">{</span> <span class="comment">// Some declarative context.</span>
<span class="special">...</span>
<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>
<span class="emphasis"><em>body-code</em></span>
<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>
<span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
The code expanded by the macros declares a function object (or <a href="http://en.wikipedia.org/wiki/Functor" target="_top">functor</a>)
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>.
<sup>[<a name="boost_localfunction.Tutorial.local_functions.f0" href="#ftn.boost_localfunction.Tutorial.local_functions.f0" class="footnote">5</a>]</sup> The usual C++ scope visibility rules apply to local functions
for which a local function is visible only within the enclosing scope in
which it is declared.
</p>
<p>
The local function body is specified in a code block <code class="computeroutput"><span class="special">{</span>
<span class="special">...</span> <span class="special">}</span></code>
using the usual C++ syntax. The body is specified outside any of the macros
so eventual compiler error messages and related line numbers retain their
usual meaning and format. <sup>[<a name="boost_localfunction.Tutorial.local_functions.f1" href="#ftn.boost_localfunction.Tutorial.local_functions.f1" class="footnote">6</a>]</sup>
</p>
<p>
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>
macro as a comma-separated list of tokens (see the <a class="link" href="No_Variadic_Macros.html" title="Annex: No Variadic Macros">No
Variadic Macros</a> section for compilers that do not support variadic
macros):
</p>
<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>
</pre>
<p>
The maximum number of parameters that can be passed to a local function is
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>.
For example, let's program a local function named <code class="computeroutput"><span class="identifier">add</span></code>
that adds together two integers <code class="computeroutput"><span class="identifier">x</span></code>
and <code class="computeroutput"><span class="identifier">y</span></code> (see also <a href="../../../test/add_params.cpp" target="_top"><code class="literal">add_params.cpp</code></a>):
</p>
<p>
</p>
<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>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<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>
<span class="identifier">BOOST_CHECK</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>
</pre>
<p>
</p>
<p>
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>
macro (similarly to the C syntax that allows to use <code class="literal"><span class="emphasis"><em>result-type
function-name</em></span></code><code class="computeroutput"><span class="special">(</span><span class="keyword">void</span><span class="special">)</span></code> to declare
a function with no parameter): <sup>[<a name="boost_localfunction.Tutorial.local_functions.f2" href="#ftn.boost_localfunction.Tutorial.local_functions.f2" class="footnote">7</a>]</sup>
</p>
<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>
</pre>
<p>
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>):
</p>
<p>
</p>
<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>
<span class="keyword">return</span> <span class="number">10</span><span class="special">;</span>
<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>
<span class="identifier">BOOST_CHECK</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>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_Tutorial_Binding">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.Tutorial.Binding"></a><a class="link" href="Tutorial.html#boost_localfunction.Tutorial.Binding" title="Binding Variables">Binding Variables</a>
</h3></div></div></div>
<p>
Variables in scope (local variables, enclosing function parameters, member
variables, etc) can be bound to a local function declaration. Only bound
variables, static variables, global variables, functions, and enumerations
from the enclosing scope are accessible from within the local function body.
The types of bound variables are deduced automatically by this library.
<sup>[<a name="boost_localfunction.Tutorial.Binding.f0" href="#ftn.boost_localfunction.Tutorial.Binding.f0" class="footnote">8</a>]</sup>
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The library implementation uses <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
to automatically deduce the types of the bound variables. Therefore, in
order to compile code in type-of emulation mode, all types should be properly
registered with <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span></code>
or <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span></code>
macros, or appropriate <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
headers should be included (see the <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
documentation for more detail).
</p></td></tr>
</table></div>
<p>
This library introduces the new "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
<sup>[<a name="boost_localfunction.Tutorial.Binding.f1" href="#ftn.boost_localfunction.Tutorial.Binding.f1" class="footnote">9</a>]</sup> which is used in place of the parameter type to specify the name
of a variable in scope to bind (therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
cannot be used as a local function parameter type). A variable can be bound
by value:
</p>
<pre class="programlisting"><span class="identifier">bind</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by value.</span>
</pre>
<p>
Or by reference prefixing the variable name with <code class="computeroutput"><span class="special">&amp;</span></code>:
</p>
<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>
</pre>
<p>
Furthermore, the "keyword" <code class="computeroutput"><span class="identifier">bind</span></code>
can be prefixed by <code class="computeroutput"><span class="keyword">const</span></code> to
bind the variable by constant value:
</p>
<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>
</pre>
<p>
Or by constant reference:
</p>
<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>
</pre>
<p>
Note that when <code class="computeroutput"><span class="keyword">const</span></code> is used,
it must always precede <code class="computeroutput"><span class="identifier">bind</span></code>.
<sup>[<a name="boost_localfunction.Tutorial.Binding.f2" href="#ftn.boost_localfunction.Tutorial.Binding.f2" class="footnote">10</a>]</sup>
</p>
<p>
If a variable is bound by value, then a copy of the variable value is taken
at the point of the local function declaration. If a variable is bound by
reference instead, the variable will refer to the value it has at the point
of the local function call. Furthermore, it is the programmers' responsibility
to ensure that variables bound by reference survive the existence scope of
the local function otherwise the bound references will be invalid when the
local function is called resulting in undefined behaviour (in other words,
the usual care in using C++ references must be taken for variables bound
by reference).
</p>
<p>
The type of a bound variable is automatically deduced and it is the exact
same type used to declare such a variable in the enclosing scope with the
following notes:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If a bound variable was declared constant in the enclosing scope, it
will always be bound by constant value or constant reference even if
<code class="computeroutput"><span class="identifier">bind</span><span class="special">...</span></code>
is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">...</span></code>
. However, if a bound variable was not declared constant in the enclosing
scope then it will not be bound as constant unless constant binding is
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
is not supported by <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda</a> functions but it is supported by this library.) <sup>[<a name="boost_localfunction.Tutorial.Binding.f3" href="#ftn.boost_localfunction.Tutorial.Binding.f3" class="footnote">11</a>]</sup>
</li>
<li class="listitem">
If a bound variable was declared as a reference in the enclosing scope,
it will still be bound by value unless it is explicitly bound by reference
using <code class="computeroutput"><span class="identifier">bind</span><span class="special">&amp;</span></code>
or <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span></code>. <sup>[<a name="boost_localfunction.Tutorial.Binding.f4" href="#ftn.boost_localfunction.Tutorial.Binding.f4" class="footnote">12</a>]</sup>
</li>
</ul></div>
<p>
When a variable is bound by value (constant or not), its type must be <code class="computeroutput"><span class="identifier">CopyConstructible</span></code> (i.e., its must provide
a copy constructor). As with passing parameters to usual C++ functions, programmers
might want to bind variables of complex types by (possibly constant) reference
instead of by value to avoid expensive copy operations when these variables
are bound to a local function.
</p>
<p>
For example, let's program the local function <code class="computeroutput"><span class="identifier">add</span></code>
from the example in the <a class="link" href="../index.html#boost_localfunction.Introduction" title="Introduction">Introduction</a>
section. We bind the local variable <code class="computeroutput"><span class="identifier">factor</span></code>
by constant value (because its value should not be modified by the local
function), the local variable <code class="computeroutput"><span class="identifier">sum</span></code>
by non-constant reference (because its value needs to be updated with the
summation result), and program the body to perform the summation (see also
<a href="../../../test/add.cpp" target="_top"><code class="literal">add.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="special">{</span> <span class="comment">// Some local scope.</span>
<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>
<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>
<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="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</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="comment">// Call the local function.</span>
<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>
<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>
<span class="identifier">BOOST_CHECK</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>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_Tutorial_binding_the_object__this_">
<div class="titlepage"><div><div><h3 class="title">
<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
the Object <code class="computeroutput"><span class="keyword">this</span></code></a>
</h3></div></div></div>
<p>
It is also possible to bind the object <code class="computeroutput"><span class="keyword">this</span></code>
when it is in scope (e.g., from an enclosing non-static member function).
This is done by using the special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
(instead of <code class="computeroutput"><span class="keyword">this</span></code>) as the name
of the variable to bind in the local function declaration and also to access
the object within the local function body. <sup>[<a name="boost_localfunction.Tutorial.binding_the_object__this_.f0" href="#ftn.boost_localfunction.Tutorial.binding_the_object__this_.f0" class="footnote">13</a>]</sup>
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
The library will generate a compile-time error if <code class="computeroutput"><span class="keyword">this</span></code>
is mistakenly used instead of <code class="computeroutput"><span class="identifier">this_</span></code>
to bind the object in the local function declaration. However, mistakenly
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
local function body will leads to undefined behaviour and it will not necessarily
generate a compile-time error. <sup>[<a name="boost_localfunction.Tutorial.binding_the_object__this_.f1" href="#ftn.boost_localfunction.Tutorial.binding_the_object__this_.f1" class="footnote">14</a>]</sup> Programmers are ultimately responsible to make sure that <code class="computeroutput"><span class="keyword">this</span></code> is never used within a local function.
</p></td></tr>
</table></div>
<p>
The object <code class="computeroutput"><span class="keyword">this</span></code> can be bound
by value:
</p>
<pre class="programlisting"><span class="identifier">bind</span> <span class="identifier">this_</span> <span class="comment">// Bind the object `this` by value.</span>
</pre>
<p>
In this case the local function will be able to modify the object when the
enclosing scope is not a constant member and it will not be able to modify
the object when the enclosing scope is a constant member. Otherwise, the
object <code class="computeroutput"><span class="keyword">this</span></code> can be bound by
constant value:
</p>
<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>
</pre>
<p>
In this case the local function will never be able to modify the object (regardless
of whether the enclosing scope is a constant member or not).
</p>
<p>
Note that the object <code class="computeroutput"><span class="keyword">this</span></code> can
never be bound by reference because C++ does not allow to obtain a reference
to <code class="computeroutput"><span class="keyword">this</span></code> (the library will generate
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>
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>).
Note that <code class="computeroutput"><span class="keyword">this</span></code> is a pointer
so the pointed object is never copied even if <code class="computeroutput"><span class="keyword">this</span></code>
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
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
is an expression and not a variable name).
</p>
<p>
For example, let's program a local function <code class="computeroutput"><span class="identifier">add</span></code>
similar to the one in the example from the <a class="link" href="../index.html#boost_localfunction.Introduction" title="Introduction">Introduction</a>
section but using a member function to illustrate how to bind the object
<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>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">adder</span> <span class="special">{</span>
<span class="identifier">adder</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>
<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>
<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>
<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>
<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>
<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>
<span class="keyword">return</span> <span class="identifier">sum_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">int</span> <span class="identifier">sum_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Note that the local function has access to all class members via the bound
object <code class="computeroutput"><span class="identifier">this_</span></code> regardless of
their access level (<code class="computeroutput"><span class="keyword">public</span></code>,
<code class="computeroutput"><span class="keyword">protected</span></code>, or <code class="computeroutput"><span class="keyword">private</span></code>). <sup>[<a name="boost_localfunction.Tutorial.binding_the_object__this_.f2" href="#ftn.boost_localfunction.Tutorial.binding_the_object__this_.f2" class="footnote">15</a>]</sup> Specifically, in the example above the local function updates
the <code class="computeroutput"><span class="keyword">private</span></code> data member <code class="computeroutput"><span class="identifier">sum_</span></code>.
</p>
</div>
<div class="section boost_localfunction_Tutorial_templates">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.Tutorial.templates"></a><a class="link" href="Tutorial.html#boost_localfunction.Tutorial.templates" title="Templates">Templates</a>
</h3></div></div></div>
<p>
When local functions are programmed within templates, they need to be declared
using the special macro <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>:
<sup>[<a name="boost_localfunction.Tutorial.templates.f0" href="#ftn.boost_localfunction.Tutorial.templates.f0" class="footnote">16</a>]</sup>
</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">local_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span> <span class="comment">// This library header.</span>
<span class="special">...</span>
<span class="special">{</span> <span class="comment">// Some declarative context within a template.</span>
<span class="special">...</span>
<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>
<span class="emphasis"><em>body-code</em></span>
<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>
<span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
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>
macro has 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>
macro that we have seen so far.
</p>
<p>
For example, let's program a local function similar to the one from the
<a class="link" href="../index.html#boost_localfunction.Introduction" title="Introduction">Introduction</a> section
but within a template (see also <a href="../../../test/add_template.cpp" target="_top"><code class="literal">add_template.cpp</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">T</span><span class="special">&gt;</span>
<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>
<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>
<span class="comment">// Using the `..._TPL` macro.</span>
<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>
<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>
<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>
<span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
<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>
<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="keyword">return</span> <span class="identifier">sum</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.local_functions.f0" href="#boost_localfunction.Tutorial.local_functions.f0" class="para">5</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The local function name must
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>
ending the function definition so this macro can declare a local variable
with the local function name to hold the local function object. Therefore
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>
and it must appear instead after the local function body (even if that
differs from the usual C++ function declaration syntax).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.local_functions.f1" href="#boost_localfunction.Tutorial.local_functions.f1" class="para">6</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> If the local function body
were instead passed as a macro parameter, it would be expanded on a single
line of code (because macros always expand as a single line of code). Therefore,
eventual compiler error line numbers would all report the same value and
would no longer be useful to pinpoint errors.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.local_functions.f2" href="#boost_localfunction.Tutorial.local_functions.f2" class="para">7</a>] </sup>
<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>
standard does not allow to pass empty parameters to a macro so the macro
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>
compilers with properly implemented empty macro parameter support, it would
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
MSVC so this syntax is never allowed to ensure better portability.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.Binding.f0" href="#boost_localfunction.Tutorial.Binding.f0" class="para">8</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> By binding a variable in scope,
the local function declaration is specifying that such a variable should
be accessible within the local function body regardless of its type. Semantically,
this binding should be seen as an "extension" of the scope of
the bound variable from the enclosing scope to the scope of the local function
body. Therefore, contrary to the semantic of passing a function parameter,
the semantic of binding a variable does not depend on the variable type
but just on the variable name: "The variable in scope named <span class="emphasis"><em>x</em></span>
should be accessible within the local function named <span class="emphasis"><em>f</em></span>".
For example, this reduces maintenance because if a bound variable type
is changed, the local function declaration does not have to change.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.Binding.f1" href="#boost_localfunction.Tutorial.Binding.f1" class="para">9</a>] </sup>
Obviously, the token <code class="computeroutput"><span class="identifier">bind</span></code>
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
meta-programming (see the <a class="link" href="Implementation.html" title="Annex: Implementation">Implementation</a>
section). Therefore, <code class="computeroutput"><span class="identifier">bind</span></code>
can be considered a new "keyword" only at the preprocessor meta-programming
level within the syntax defined by the macros of this library (thus it
is referred to as a "keyword" only within quotes).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.Binding.f2" href="#boost_localfunction.Tutorial.Binding.f2" class="para">10</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The library macros could have
been implemented to accept both syntaxes <code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">bind</span> <span class="special">...</span></code>
and <code class="computeroutput"><span class="identifier">bind</span> <span class="keyword">const</span>
<span class="special">...</span></code> equivalently. However, handling
both syntaxes would have complicated the macro implementation without adding
any feature so only one syntax (<code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">bind</span> <span class="special">...</span></code>)
is supported.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.Binding.f3" href="#boost_localfunction.Tutorial.Binding.f3" class="para">11</a>] </sup>
An historical note: Constant binding of variables in scope was the
main use case that originally motivated the authors in developing this
library. The authors needed to locally create a chuck of code to assert
some correctness conditions while these assertions were not supposed
to modify any of the variables they were using (see the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a>
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
and then by programming the local function body to check the assertions.
This way if any of the assertions mistakenly changes a bound variable
(for example confusing the operator <code class="computeroutput"><span class="special">==</span></code>
with <code class="computeroutput"><span class="special">=</span></code>), the compiler
correctly generates an error because the bound variable is of <code class="computeroutput"><span class="keyword">const</span></code> type within the local function
body (see also <span class="emphasis"><em>constant blocks</em></span> in the <a class="link" href="Examples.html" title="Examples">Examples</a>
section).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.Binding.f4" href="#boost_localfunction.Tutorial.Binding.f4" class="para">12</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> Variables originally declared
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>
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>
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
adopted by <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>).
On the other hand, variables originally declared as constants should
never loose their <code class="computeroutput"><span class="keyword">const</span></code>
qualifier (to prevent their modification not just in the enclosing
scope but also in the local scope) thus they are always bound by constant
even if <code class="computeroutput"><span class="identifier">bind</span><span class="special">[&amp;]</span></code>
is used instead of <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">[&amp;]</span></code>.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.binding_the_object__this_.f0" href="#boost_localfunction.Tutorial.binding_the_object__this_.f0" class="para">13</a>] </sup>
<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://boost.2283326.n4.nabble.com/local-this-or-this-td3423912.html" target="_top">Boost
practises</a> which postfix with an underscore identifiers that are
named after keywords (the C++ keyword <code class="computeroutput"><span class="keyword">this</span></code>
in this case). The special symbol <code class="computeroutput"><span class="identifier">this_</span></code>
is needed because <code class="computeroutput"><span class="keyword">this</span></code> is
a reserved C++ keyword so it cannot be used as the name of the internal
parameter that passes the bound object to the local function body. It would
have been possible to use <code class="computeroutput"><span class="keyword">this</span></code>
(instead of <code class="computeroutput"><span class="identifier">this_</span></code>) within
the local function body either at the expenses of copying the bound object
(which would introduce run-time overhead and also the stringent requirement
that the bound object must have a deep copy constructor) or by relying
on an <a href="http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/d3a86f27277f713b" target="_top">undefined
behaviour of <code class="computeroutput"><span class="keyword">static_cast</span></code></a>
(which might not work on all platforms at the cost of portability).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.binding_the_object__this_.f1" href="#boost_localfunction.Tutorial.binding_the_object__this_.f1" class="para">14</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The local function body cannot
be a static member function of the local functor object in order to support
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>
macro only after the body so it must be made available via a functor
member variable named after the local function and local classes cannot
have static member variables in C++) and nesting (because the argument
binding variable must be declared as a member variable so it is visible
in a local function nested within the body member function) -- see the
<a class="link" href="Implementation.html" title="Annex: Implementation">Implementation</a>
section. Therefore, from within the local function body the variable
<code class="computeroutput"><span class="keyword">this</span></code> is visible but it refers
to the local functor and not to the bound object.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.binding_the_object__this_.f2" href="#boost_localfunction.Tutorial.binding_the_object__this_.f2" class="para">15</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> This is possible because of
the fix to C++ <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#45" target="_top">defect
45</a> that made inner and local types able to access all outer class
members regardless of their access level.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.Tutorial.templates.f0" href="#boost_localfunction.Tutorial.templates.f0" class="para">16</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> Within templates, this library
needs to use <code class="computeroutput"><span class="keyword">typename</span></code> to explicitly
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>
does not allow to use <code class="computeroutput"><span class="keyword">typename</span></code>
outside templates, the special <code class="computeroutput"><span class="identifier">TPL</span></code>
macro is used to indicate that 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.
</p></div>
</div>
</div>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2009-2012 Lorenzo
Caminiti<p>
Distributed under the Boost Software License, Version 1.0 (see accompanying
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>)
</p>
</div></td>
</tr></table>
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