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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_localfunction.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_">GCC
Lambdas (without C++11)</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.constant_blocks">Constant
Blocks</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.scope_exits">Scope Exits</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.boost_phoenix_functions">Boost.Phoenix
Functions</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.closures">Closures</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.gcc_nested_functions">GCC
Nested Functions</a></span></dt>
<dt><span class="section"><a href="examples.html#boost_localfunction.examples.n_papers">N-Papers</a></span></dt>
</dl></div>
<p>
This section lists some examples that use this library.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.gcc_lambdas__without_c__11_"></a><a class="link" href="examples.html#boost_localfunction.examples.gcc_lambdas__without_c__11_" title="GCC Lambdas (without C++11)">GCC
Lambdas (without C++11)</a>
</h3></div></div></div>
<p>
Combing local functions with the non-standard <a href="http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html" target="_top">statement
expression</a> extension of the GCC compiler, it is possible to implement
lambda functions for GCC compilers even without <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
support.
</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>
This code only works on compilers that support GCC statement expression
extension or that support <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>.
</p></td></tr>
</table></div>
<p>
For example (see also <a href="../../../example/gcc_lambda.cpp" target="_top"><code class="literal">gcc_lambda.cpp</code></a>
and <a href="../../../example/gcc_cxx11_lambda.cpp" target="_top"><code class="literal">gcc_cxx11_lambda.cpp</code></a>):
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
With Local Functions (GCC only)
</p>
</th>
<th>
<p>
C++11 Lambdas
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</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">1</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="keyword">int</span><span class="special">*</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">3</span><span class="special">;</span>
<span class="keyword">int</span><span class="special">*</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</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">GCC_LAMBDA</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">val</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">,</span> <span class="keyword">return</span> <span class="keyword">bool</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">num</span> <span class="special">==</span> <span class="identifier">val</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">GCC_LAMBDA_END</span>
<span class="special">);</span>
</pre>
<p>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</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">1</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="keyword">int</span><span class="special">*</span> <span class="identifier">end</span> <span class="special">=</span> <span class="identifier">nums</span> <span class="special">+</span> <span class="number">3</span><span class="special">;</span>
<span class="keyword">int</span><span class="special">*</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</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="special">[</span><span class="identifier">val</span><span class="special">](</span><span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">-&gt;</span> <span class="keyword">bool</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">num</span> <span class="special">==</span> <span class="identifier">val</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">);</span>
</pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<p>
The GCC lambda function macros are implemented using local functions (see
also <a href="../../../example/gcc_lambda.hpp" target="_top"><code class="literal">gcc_lambda.hpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor"># define</span> <span class="identifier">GCC_LAMBDA_</span><span class="special">(</span><span class="identifier">binds</span><span class="special">,</span> <span class="identifier">params</span><span class="special">,</span> <span class="identifier">results</span><span class="special">)</span> <span class="special">\</span>
<span class="special">({</span> <span class="comment">/* open statement expression (GCC extension only) */</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_LIST_ENUM</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_APPEND</span><span class="special">(</span><span class="identifier">binds</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_LIST_APPEND</span><span class="special">(</span><span class="identifier">params</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_IS_NIL</span><span class="special">(</span><span class="identifier">results</span><span class="special">),</span> <span class="special">\</span>
<span class="special">(</span><span class="keyword">return</span> <span class="keyword">void</span><span class="special">,</span> <span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="comment">/* default for lambdas */</span> <span class="special">\</span>
<span class="special">,</span> <span class="special">\</span>
<span class="identifier">results</span> <span class="special">\</span>
<span class="special">)\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">))</span> <span class="special">\</span>
<span class="special">)</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">GCC_LAMBDA_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">gcc_lambda_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">gcc_lambda_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">);</span> <span class="special">\</span>
<span class="special">})</span> <span class="comment">/* close statement expression (GCC extension only) */</span>
</pre>
<p>
</p>
<p>
This is possible because GCC statement expressions allow to use declaration
statements within expressions and therefore to declare a local function within
an expression. The macros automatically detect if the compiler supports
<a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> in which case the implementation uses native lambdas
instead of local functions in GCC statement expressions. However, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> do not support constant binding so it is best to
only use <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span>
<span class="identifier">variable</span></code> (same as <code class="computeroutput"><span class="special">=</span><span class="identifier">variable</span></code>
for <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>) and <code class="computeroutput"><span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">variable</span></code>
(same as <code class="computeroutput"><span class="special">&amp;</span><span class="identifier">variable</span></code>
for <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>') because these have the exact same semantic between
the local function and the native lambda implementations. Furthermore, local
functions allow to bind data members directly while <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> require to access data members via binding the object
<code class="computeroutput"><span class="keyword">this</span></code>. Unfortunately, the short-hand
binds <code class="computeroutput"><span class="special">&amp;</span></code> and <code class="computeroutput"><span class="special">=</span></code> of <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> (which automatically bind all variables in scope
either by reference or value) are not supported by these GCC lambda function
macros because they are not supported by local functions. Finally, the result
type <code class="computeroutput"><span class="keyword">return</span> </code><code class="literal"><span class="emphasis"><em>result-type</em></span></code>
is optional and it is assumed <code class="computeroutput"><span class="keyword">void</span></code>
when it is not specified (same as with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>).
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.constant_blocks"></a><a class="link" href="examples.html#boost_localfunction.examples.constant_blocks" title="Constant Blocks">Constant
Blocks</a>
</h3></div></div></div>
<p>
It is possible to use local functions to check assertions between variables
that are made constant within the asserted expressions. This is advantageous
because assertions are not supposed to change the state of the program and
ideally the compiler will not compile assertions that modify variables.
</p>
<p>
For example, consider the following assertion where by mistake we programmed
<code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>
instead of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code>:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Mistakenly `=` instead of `==`.</span>
</pre>
<p>
Ideally this code will not compile instead this example not only compiles
but the assertion even passes the run-time check and no error is generated
at all. The <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2004/n1613.pdf" target="_top">[N1613]</a>
paper introduces the concept of a <span class="emphasis"><em>const-block</em></span> which
could be used to wrap the assertion above and catch the programming error
at compile-time. Similarly, the following code will generate a compile-time
error when <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>
is mistakenly used instead of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> because both <code class="computeroutput"><span class="identifier">x</span></code>
and <code class="computeroutput"><span class="identifier">y</span></code> are made constants
(using local functions) within the block of code performing the assertion
(see also <a href="../../../example/const_block_error.cpp" target="_top"><code class="literal">const_block_error.cpp</code></a>):
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
With Local Functions
</p>
</th>
<th>
<p>
N1613 Const-Blocks
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="identifier">CONST_BLOCK</span><span class="special">(</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="comment">// Constant block.</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Compiler error.</span>
<span class="special">}</span> <span class="identifier">CONST_BLOCK_END</span>
</pre>
<p>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="keyword">const</span> <span class="special">{</span> <span class="comment">// Constant block.</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">);</span> <span class="comment">// Compiler error.</span>
<span class="special">}</span>
</pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<p>
The constant block macros are implemented using local functions (see also
<a href="../../../example/const_block.hpp" target="_top"><code class="literal">const_block.hpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">CONST_BLOCK_</span><span class="special">(</span><span class="identifier">variables</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="special">\</span>
<span class="identifier">BOOST_PP_IIF</span><span class="special">(</span><span class="identifier">BOOST_PP_LIST_IS_NIL</span><span class="special">(</span><span class="identifier">variables</span><span class="special">),</span> <span class="special">\</span>
<span class="keyword">void</span> <span class="identifier">BOOST_PP_TUPLE_EAT</span><span class="special">(</span><span class="number">3</span><span class="special">)</span> <span class="special">\</span>
<span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_LIST_FOR_EACH_I</span> <span class="special">\</span>
<span class="special">)(</span><span class="identifier">CONST_BLOCK_BIND_</span><span class="special">,</span> <span class="special">~,</span> <span class="identifier">variables</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">CONST_BLOCK_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">const_block_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">const_block_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">)();</span> <span class="comment">/* call local function immediately */</span>
</pre>
<p>
</p>
<p>
The constant block macros are implemented using a local function which binds
by constant reference <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span></code>
all the specified variables (so the variables are constant within the code
block but they do not need to be <code class="computeroutput"><span class="identifier">CopyConstructible</span></code>
and no extra copy is performed). The local function executes the <code class="computeroutput"><span class="identifier">assert</span></code> instruction in its body and it is
called immediately after it is defined. More in general, constant blocks
can be used to evaluate any instruction (not just assertions) within a block
were all specified variables are constant.
</p>
<p>
Unfortunately, constant blocks cannot be implemented with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> because these do not support constant binding. Variables
bound by value using <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> (<code class="computeroutput"><span class="identifier">variable</span></code>,
<code class="computeroutput"><span class="special">=</span><span class="identifier">variable</span></code>,
and <code class="computeroutput"><span class="special">=</span></code>) are constant but they
are required to be <code class="computeroutput"><span class="identifier">CopyConstructible</span></code>
and they introduce potentially expensive copy operations. <a href="#ftn.boost_localfunction.examples.constant_blocks.f0" class="footnote" name="boost_localfunction.examples.constant_blocks.f0"><sup class="footnote">[29]</sup></a> Of course it is always possible to introduce extra constant variables
and bind these variables to the <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> but the constant block code will then have to manage
the declaration and initialization of these extra variables plus it will
have to use the extra variable names instead of the original variable names:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
<span class="keyword">const</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">x</span><span class="special">)&amp;</span> <span class="identifier">const_x</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">;</span> <span class="comment">// Constant so cannot be modified</span>
<span class="keyword">const</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">y</span><span class="special">)&amp;</span> <span class="identifier">const_y</span> <span class="special">=</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// and reference so no copy.</span>
<span class="special">[&amp;</span><span class="identifier">const_x</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">const_y</span><span class="special">]()</span> <span class="special">{</span> <span class="comment">// Lambda functions (C++11 only).</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">const_x</span> <span class="special">=</span> <span class="identifier">const_y</span><span class="special">);</span> <span class="comment">// Unfortunately, `const_` names.</span>
<span class="special">}();</span>
</pre>
<p>
</p>
<p>
In many cases the use of an extra constant variable <code class="computeroutput"><span class="identifier">const_x</span></code>
can be acceptable but in other cases it might be preferable to maintain the
same variable name <code class="computeroutput"><span class="identifier">x</span></code> within
the function body.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.scope_exits"></a><a class="link" href="examples.html#boost_localfunction.examples.scope_exits" title="Scope Exits">Scope Exits</a>
</h3></div></div></div>
<p>
Scope exits allow to execute arbitrary code at the exit of the enclosing
scope and they are provided by the <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>
library.
</p>
<p>
For curiosity, here we show how to re-implement scope exits using local functions.
One small advantage of scope exits that use local functions is that they
support constant binding. <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>
does not directly support constant binding (however, it is always possible
to introduce an extra <code class="computeroutput"><span class="keyword">const</span></code>
local variable, assign it to the value to bind, and then bind the <code class="computeroutput"><span class="keyword">const</span></code> variable so to effectively have constant
binding with <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>
as well).
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
In general, the authors recommend to use <a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>
instead of the code listed by this example whenever possible (because
<a href="http://www.boost.org/libs/scope_exit" target="_top">Boost.ScopeExit</a>
is a library deliberately designed to support the scope exit construct).
</p></td></tr>
</table></div>
<p>
The following example binds <code class="computeroutput"><span class="identifier">p</span></code>
by constant reference so this variable cannot be modified within the scope
exit body but it is not copied and it will present the value it has at the
exit of the enclosing scope and not at the scope exit declaration (see also
<a href="../../../example/scope_exit.cpp" target="_top"><code class="literal">scope_exit.cpp</code></a>):
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
With Local Functions
</p>
</th>
<th>
<p>
Boost.ScopeExit
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">person</span><span class="special">&amp;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span>
<span class="identifier">person</span><span class="special">::</span><span class="identifier">evolution_t</span> <span class="identifier">checkpoint</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">;</span>
<span class="identifier">SCOPE_EXIT</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">checkpoint</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&amp;</span> <span class="identifier">p</span><span class="special">,</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">checkpoint</span> <span class="special">==</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-&gt;</span><span class="identifier">persons_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span>
<span class="special">}</span> <span class="identifier">SCOPE_EXIT_END</span>
</pre>
<p>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">person</span><span class="special">&amp;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">persons_</span><span class="special">.</span><span class="identifier">back</span><span class="special">();</span>
<span class="identifier">person</span><span class="special">::</span><span class="identifier">evolution_t</span> <span class="identifier">checkpoint</span> <span class="special">=</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">;</span>
<span class="identifier">BOOST_SCOPE_EXIT</span><span class="special">(</span><span class="identifier">checkpoint</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Or extra variable `const_p`.</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">checkpoint</span> <span class="special">==</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">evolution_</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-&gt;</span><span class="identifier">persons_</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span>
<span class="special">}</span> <span class="identifier">BOOST_SCOPE_EXIT_END</span>
</pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<p>
The scope exit macros are implemented by passing a local function when constructing
an object of the following class (see also <a href="../../../example/scope_exit.hpp" target="_top"><code class="literal">scope_exit.hpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">scope_exit</span> <span class="special">{</span>
<span class="identifier">scope_exit</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)&gt;</span> <span class="identifier">f</span><span class="special">):</span> <span class="identifier">f_</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span> <span class="special">{}</span>
<span class="special">~</span><span class="identifier">scope_exit</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">f_</span><span class="special">();</span> <span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)&gt;</span> <span class="identifier">f_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor"># define</span> <span class="identifier">SCOPE_EXIT</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="identifier">__VA_ARGS__</span><span class="special">)</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">SCOPE_EXIT_END_</span><span class="special">(</span><span class="identifier">id</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_func_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">))</span> <span class="special">\</span>
<span class="identifier">scope_exit</span> <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">)(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">scope_exit_func_</span><span class="special">,</span> <span class="identifier">id</span><span class="special">));</span>
</pre>
<p>
</p>
<p>
A local variable within the enclosing scope is used to hold the object so
the destructor will be invoked at the exit of the enclosing scope and it
will in turn call the local function executing the scope exit instructions.
The scope exit local function has no parameter and <code class="computeroutput"><span class="keyword">void</span></code>
result type but it supports binding and constant binding.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.boost_phoenix_functions"></a><a class="link" href="examples.html#boost_localfunction.examples.boost_phoenix_functions" title="Boost.Phoenix Functions">Boost.Phoenix
Functions</a>
</h3></div></div></div>
<p>
Local functions can be used to create <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>
functions. For example (see also <a href="../../../example/phoenix_factorial_local.cpp" target="_top"><code class="literal">phoenix_factorial_local.cpp</code></a>
and <a href="../../../example/phoenix_factorial.cpp" target="_top"><code class="literal">phoenix_factorial.cpp</code></a>):
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Local Functions
</p>
</th>
<th>
<p>
Global Functor
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-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="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">arg_names</span><span class="special">::</span><span class="identifier">arg1</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Unfortunately, monomorphic.</span>
<span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special">&lt;=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="identifier">factorial_impl</span><span class="special">(</span><span class="identifier">n</span> <span class="special">-</span> <span class="number">1</span><span class="special">);</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="identifier">factorial_impl</span><span class="special">)</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)&gt;</span> <span class="special">&gt;</span>
<span class="identifier">factorial</span><span class="special">(</span><span class="identifier">factorial_impl</span><span class="special">);</span> <span class="comment">// Phoenix function from local function.</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Call.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Lazy call.</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">struct</span> <span class="identifier">factorial_impl</span> <span class="special">{</span> <span class="comment">// Phoenix function from global functor.</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">)&gt;</span> <span class="special">:</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span><span class="special">&amp;)&gt;</span> <span class="special">{};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">result</span><span class="special">&lt;</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">&amp;)&gt;</span> <span class="special">{</span> <span class="keyword">typedef</span> <span class="identifier">Arg</span> <span class="identifier">type</span><span class="special">;</span> <span class="special">};</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">&gt;</span> <span class="comment">// Polymorphic.</span>
<span class="identifier">Arg</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special">&lt;=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)(</span><span class="identifier">n</span> <span class="special">-</span> <span class="number">1</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<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="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">arg_names</span><span class="special">::</span><span class="identifier">arg1</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">function</span><span class="special">&lt;</span><span class="identifier">factorial_impl</span><span class="special">&gt;</span> <span class="identifier">factorial</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">4</span><span class="special">;</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">i</span><span class="special">)()</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Call.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">==</span> <span class="number">24</span><span class="special">);</span> <span class="comment">// Lazy call.</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<p>
This is presented here mainly as a curiosity because <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>
functions created from local functions have the important limitation that
they cannot be polymorphic. <a href="#ftn.boost_localfunction.examples.boost_phoenix_functions.f0" class="footnote" name="boost_localfunction.examples.boost_phoenix_functions.f0"><sup class="footnote">[30]</sup></a> Therefore, in many cases creating the <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>
function from global functors (possibly with the help of <a href="http://www.boost.org/libs/phoenix" target="_top">Boost.Phoenix</a>
adaptor macros) might be a more useful.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.closures"></a><a class="link" href="examples.html#boost_localfunction.examples.closures" title="Closures">Closures</a>
</h3></div></div></div>
<p>
The following are examples of <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)" target="_top">closures</a>
that illustrate how to return local functions to the calling scope (note
how extra care is taken in order to ensure that all bound variables remain
valid at the calling scope):
</p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
<p>
Files
</p>
</th></tr></thead>
<tbody>
<tr><td>
<p>
<a href="../../../test/return_inc.cpp" target="_top"><code class="literal">return_inc.cpp</code></a>
</p>
</td></tr>
<tr><td>
<p>
<a href="../../../test/return_this.cpp" target="_top"><code class="literal">return_this.cpp</code></a>
</p>
</td></tr>
<tr><td>
<p>
<a href="../../../test/return_setget.cpp" target="_top"><code class="literal">return_setget.cpp</code></a>
</p>
</td></tr>
<tr><td>
<p>
<a href="../../../test/return_derivative.cpp" target="_top"><code class="literal">return_derivative.cpp</code></a>
</p>
</td></tr>
</tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.gcc_nested_functions"></a><a class="link" href="examples.html#boost_localfunction.examples.gcc_nested_functions" title="GCC Nested Functions">GCC
Nested Functions</a>
</h3></div></div></div>
<p>
The GCC C compiler supports local functions as a non-standard extension under
the name of <a href="http://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html" target="_top">nested
functions</a>. Note that nested functions are exclusively a C extension
of the GCC compiler (they are not supported for C++ not even by the GCC compiler,
and they are not part of any C or C++ standard, nor they are supported by
other compilers like MSVC).
</p>
<p>
The following examples are taken form the GCC nested function documentation
and programmed using local functions:
</p>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
<p>
Files
</p>
</th></tr></thead>
<tbody>
<tr><td>
<p>
<a href="../../../example/gcc_square.cpp" target="_top"><code class="literal">gcc_square.cpp</code></a>
</p>
</td></tr>
<tr><td>
<p>
<a href="../../../example/gcc_access.cpp" target="_top"><code class="literal">gcc_access.cpp</code></a>
</p>
</td></tr>
<tr><td>
<p>
<a href="../../../example/gcc_store.cpp" target="_top"><code class="literal">gcc_store.cpp</code></a>
</p>
</td></tr>
</tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.examples.n_papers"></a><a class="link" href="examples.html#boost_localfunction.examples.n_papers" title="N-Papers">N-Papers</a>
</h3></div></div></div>
<p>
The following examples are taken from different C++ "N-papers"
and programmed using local functions:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Files
</p>
</th>
<th>
<p>
Notes
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a href="../../../example/n2550_find_if.cpp" target="_top"><code class="literal">n2550_find_if.cpp</code></a>
</p>
</td>
<td>
<p>
This example is adapted from <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2550.pdf" target="_top">[N2550]</a>
(<a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>): It passes a local function to the STL
algorithm <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a href="../../../example/n2529_this.cpp" target="_top"><code class="literal">n2529_this.cpp</code></a>
</p>
</td>
<td>
<p>
This example is adapted from <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2529.pdf" target="_top">[N2529]</a>
(<a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>): It binds the object in scope <code class="computeroutput"><span class="keyword">this</span></code> to a local function.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.boost_localfunction.examples.constant_blocks.f0" class="footnote"><p><a href="#boost_localfunction.examples.constant_blocks.f0" class="para"><sup class="para">[29] </sup></a>
Ideally, <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> would allow to bind variables also using <code class="computeroutput"><span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">variable</span></code> (constant reference) and <code class="computeroutput"><span class="keyword">const</span><span class="special">&amp;</span></code>
(all variables by constant reference).
</p></div>
<div id="ftn.boost_localfunction.examples.boost_phoenix_functions.f0" class="footnote"><p><a href="#boost_localfunction.examples.boost_phoenix_functions.f0" class="para"><sup class="para">[30] </sup></a>
<span class="bold"><strong>Rationale.</strong></span> Local functions can only be
monomorphic because they are implemented using local classes and local
classes cannot be templates in C++ (not even in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>).
</p></div>
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Distributed under the Boost Software License, Version 1.0 (see accompanying
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