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- <a name="phoenix.modules.scope.lambda"></a><a class="link" href="lambda.html" title="lambda">lambda</a>
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- <pre class="programlisting"><span class="preprocessor">#include</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">scope</span><span class="special">/</span><span class="identifier">lambda</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- </pre>
- <p>
- A lot of times, you'd want to write a lazy function that accepts one or
- more functions (higher order functions). STL algorithms come to mind, for
- example. Consider a lazy version of <code class="computeroutput"><span class="identifier">stl</span><span class="special">::</span><span class="identifier">for_each</span></code>:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">for_each_impl</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">F</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">void</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">C</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">F</span><span class="special">></span>
- <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">C</span><span class="special">&</span> <span class="identifier">c</span><span class="special">,</span> <span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</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">function</span><span class="special"><</span><span class="identifier">for_each_impl</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">for_each</span> <span class="special">=</span> <span class="identifier">for_each_impl</span><span class="special">();</span>
- </pre>
- <p>
- Notice that the function accepts another function, <code class="computeroutput"><span class="identifier">f</span></code>
- as an argument. The scope of this function, <code class="computeroutput"><span class="identifier">f</span></code>,
- is limited within the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. When <code class="computeroutput"><span class="identifier">f</span></code>
- is called inside <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>, it exists in a new scope, along
- with new arguments and, possibly, local variables. This new scope is not
- at all related to the outer scopes beyond the <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>.
- </p>
- <p>
- Simple syntax:
- </p>
- <pre class="programlisting"><span class="identifier">lambda</span>
- <span class="special">[</span>
- <span class="identifier">lambda</span><span class="special">-</span><span class="identifier">body</span>
- <span class="special">]</span>
- </pre>
- <p>
- Like <code class="computeroutput"><span class="identifier">let</span></code>, local variables
- may be declared, allowing 1..N local variable declarations (where N ==
- <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>):
- </p>
- <pre class="programlisting"><span class="identifier">lambda</span><span class="special">(</span><span class="identifier">local</span><span class="special">-</span><span class="identifier">declarations</span><span class="special">)</span>
- <span class="special">[</span>
- <span class="identifier">lambda</span><span class="special">-</span><span class="identifier">body</span>
- <span class="special">]</span>
- </pre>
- <p>
- The same restrictions apply with regard to scope and visibility. The RHS
- (right hand side lambda-expression) of each local-declaration cannot refer
- to any LHS local-id. The local-ids are not in scope yet; they will be in
- scope only in the lambda-body:
- </p>
- <pre class="programlisting"><span class="identifier">lambda</span><span class="special">(</span>
- <span class="identifier">_a</span> <span class="special">=</span> <span class="number">1</span>
- <span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="identifier">_a</span> <span class="comment">// Error: _a is not in scope yet
- </span><span class="special">)</span>
- </pre>
- <p>
- See <a class="link" href="let.html#phoenix.modules.scope.let.visibility"><code class="computeroutput"><span class="identifier">let</span></code> Visibility</a> for more information.
- </p>
- <p>
- Example: Using our lazy <code class="computeroutput"><span class="identifier">for_each</span></code>
- let's print all the elements in a container:
- </p>
- <pre class="programlisting"><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">lambda</span><span class="special">[</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">arg1</span><span class="special">])</span>
- </pre>
- <p>
- As far as the arguments are concerned (arg1..argN), the scope in which
- the lambda-body exists is totally new. The left <code class="computeroutput"><span class="identifier">arg1</span></code>
- refers to the argument passed to <code class="computeroutput"><span class="identifier">for_each</span></code>
- (a container). The right <code class="computeroutput"><span class="identifier">arg1</span></code>
- refers to the argument passed by <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>
- when we finally get to call <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> in our <code class="computeroutput"><span class="identifier">for_each_impl</span></code>
- above (a container element).
- </p>
- <p>
- Yet, we may wish to get information from outer scopes. While we do not
- have access to arguments in outer scopes, what we still have is access
- to local variables from outer scopes. We may only be able to pass argument
- related information from outer <code class="computeroutput"><span class="identifier">lambda</span></code>
- scopes through the local variables.
- </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>
- This is a crucial difference between <code class="computeroutput"><span class="identifier">let</span></code>
- and <code class="computeroutput"><span class="identifier">lambda</span></code>: <code class="computeroutput"><span class="identifier">let</span></code> does not introduce new arguments;
- <code class="computeroutput"><span class="identifier">lambda</span></code> does.
- </p></td></tr>
- </table></div>
- <p>
- Another example: Using our lazy <code class="computeroutput"><span class="identifier">for_each</span></code>,
- and a lazy <code class="computeroutput"><span class="identifier">push_back</span></code>:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">push_back_impl</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">void</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">C</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
- <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">C</span><span class="special">&</span> <span class="identifier">c</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="identifier">c</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="special">};</span>
- <span class="identifier">function</span><span class="special"><</span><span class="identifier">push_back_impl</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">push_back</span> <span class="special">=</span> <span class="identifier">push_back_impl</span><span class="special">();</span>
- </pre>
- <p>
- write a lambda expression that accepts:
- </p>
- <div class="orderedlist"><ol class="orderedlist" type="1">
- <li class="listitem">
- a 2-dimensional container (e.g. <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span></code>)
- </li>
- <li class="listitem">
- a container element (e.g. <code class="computeroutput"><span class="keyword">int</span></code>)
- </li>
- </ol></div>
- <p>
- and pushes-back the element to each of the <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
- </p>
- <p>
- Solution:
- </p>
- <pre class="programlisting"><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span>
- <span class="identifier">lambda</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg2</span><span class="special">)</span>
- <span class="special">[</span>
- <span class="identifier">push_back</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">_a</span><span class="special">)</span>
- <span class="special">]</span>
- <span class="special">)</span>
- </pre>
- <p>
- Since we do not have access to the arguments of the outer scopes beyond
- the lambda-body, we introduce a local variable <code class="computeroutput"><span class="identifier">_a</span></code>
- that captures the second outer argument: <code class="computeroutput"><span class="identifier">arg2</span></code>.
- Hence: _a = arg2. This local variable is visible inside the lambda scope.
- </p>
- <p>
- (See <a href="../../../../../example/lambda.cpp" target="_top">lambda.cpp</a>)
- </p>
- </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 © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
- </p>
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