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- </div>
- <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">-></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">&</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>') 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">&</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">&</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">)&</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">)&</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">[&</span><span class="identifier">const_x</span><span class="special">,</span> <span class="special">&</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">&</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">&</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">-></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">&</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">&</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">-></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"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</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="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"><</span><span class="keyword">void</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></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"><=</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"><</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)></span> <span class="special">></span>
- <span class="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"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
- <span class="keyword">template</span><span class="special"><</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">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">)></span> <span class="special">:</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span><span class="special">&)></span> <span class="special">{};</span>
- <span class="keyword">template</span><span class="special"><</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">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span> <span class="special">(</span><span class="identifier">Arg</span><span class="special">&)></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"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></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"><=</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"><</span><span class="identifier">factorial_impl</span><span class="special">></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">&</span> <span class="identifier">variable</span></code> (constant reference) and <code class="computeroutput"><span class="keyword">const</span><span class="special">&</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>
- </div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 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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