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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.modules.scope.let"></a><a class="link" href="let.html" title="let">let</a> |
| </h4></div></div></div> |
| <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">let</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| You declare local variables using the syntax: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</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">let</span><span class="special">-</span><span class="identifier">body</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">let</span></code> allows 1..N local variable |
| declarations (where N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>). |
| Each declaration follows the form: |
| </p> |
| <pre class="programlisting"><span class="identifier">local</span><span class="special">-</span><span class="identifier">id</span> <span class="special">=</span> <span class="identifier">lambda</span><span class="special">-</span><span class="identifier">expression</span> |
| </pre> |
| <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> |
| You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>, |
| the predefined maximum local variable declarations in a let expression. |
| By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code> |
| is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>. |
| </p></td></tr> |
| </table></div> |
| <p> |
| Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="number">123</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span> |
| <span class="special">[</span> |
| <span class="identifier">_a</span> <span class="special">+</span> <span class="identifier">_b</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>Reference Preservation</strong></span> |
| </p> |
| <p> |
| The type of the local variable assumes the type of the lambda- expression. |
| Type deduction is reference preserving. For example: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">_a</span></code> assumes the type of |
| <code class="computeroutput"><span class="identifier">arg1</span></code>: a reference to an |
| argument, while <code class="computeroutput"><span class="identifier">_b</span></code> has |
| type <code class="computeroutput"><span class="keyword">int</span></code>. |
| </p> |
| <p> |
| Consider this: |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> |
| |
| <span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">)</span> |
| <span class="special">[</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span> |
| <span class="special">]</span> |
| <span class="special">(</span><span class="identifier">i</span><span class="special">);</span> |
| |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| </pre> |
| <p> |
| the output of above is : 0 0 |
| </p> |
| <p> |
| While with this: |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> |
| |
| <span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">))</span> |
| <span class="special">[</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span> |
| <span class="special">]</span> |
| <span class="special">(</span><span class="identifier">i</span><span class="special">);</span> |
| |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> |
| </pre> |
| <p> |
| the output is : 0 1 |
| </p> |
| <p> |
| Reference preservation is necessary because we need to have L-value access |
| to outer lambda-scopes (especially the arguments). <code class="computeroutput"><span class="identifier">arg</span></code>s |
| and <code class="computeroutput"><span class="identifier">ref</span></code>s are L-values. |
| <code class="computeroutput"><span class="identifier">val</span></code>s are R-values. |
| </p> |
| <p> |
| <span class="bold"><strong>Visibility</strong></span> <a name="phoenix.modules.scope.let.visibility"></a> |
| </p> |
| <p> |
| The scope and lifetimes of the local variables is limited within the let-body. |
| <code class="computeroutput"><span class="identifier">let</span></code> blocks can be nested. |
| A local variable may hide an outer local variable. For example: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</span><span class="special">(</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="string">", World"</span><span class="special">)</span> |
| <span class="special">[</span> |
| <span class="comment">// _x here is an int: 1 |
| </span> |
| <span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="string">"Hello"</span><span class="special">)</span> <span class="comment">// hides the outer _x |
| </span> <span class="special">[</span> |
| <span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_x</span> <span class="special"><<</span> <span class="identifier">_y</span> <span class="comment">// prints "Hello, World" |
| </span> <span class="special">]</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| The RHS (right hand side lambda-expression) of each local-declaration cannot |
| refer to any LHS local-id. At this point, the local-ids are not in scope |
| yet; they will only be in scope in the let-body. The code below is in error: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</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> |
| <span class="special">[</span> |
| <span class="comment">// _a and _b's scope starts here |
| </span> <span class="comment">/*. body .*/</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| However, if an outer let scope is available, this will be searched. Since |
| the scope of the RHS of a local-declaration is the outer scope enclosing |
| the let, the RHS of a local-declaration can refer to a local variable of |
| an outer scope: |
| </p> |
| <pre class="programlisting"><span class="identifier">let</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="special">[</span> |
| <span class="identifier">let</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">// Ok. _a refers to the outer _a |
| </span> <span class="special">)</span> |
| <span class="special">[</span> |
| <span class="comment">/*. body .*/</span> |
| <span class="special">]</span> |
| <span class="special">]</span> |
| </pre> |
| </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>) |
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