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26 <div class=
"titlepage"><div><div><h4 class=
"title">
27 <a name=
"phoenix.modules.scope.let"></a><a class=
"link" href=
"let.html" title=
"let">let
</a>
28 </h4></div></div></div>
29 <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>
32 You declare local variables using the syntax:
34 <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>
35 <span class=
"special">[
</span>
36 <span class=
"identifier">let
</span><span class=
"special">-
</span><span class=
"identifier">body
</span>
37 <span class=
"special">]
</span>
40 <code class=
"computeroutput"><span class=
"identifier">let
</span></code> allows
1..N local variable
41 declarations (where N ==
<code class=
"computeroutput"><span class=
"identifier">BOOST_PHOENIX_LOCAL_LIMIT
</span></code>).
42 Each declaration follows the form:
44 <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>
46 <div class=
"note"><table border=
"0" summary=
"Note">
48 <td rowspan=
"2" align=
"center" valign=
"top" width=
"25"><img alt=
"[Note]" src=
"../../../../../../../doc/src/images/note.png"></td>
49 <th align=
"left">Note
</th>
51 <tr><td align=
"left" valign=
"top"><p>
52 You can set
<code class=
"computeroutput"><span class=
"identifier">BOOST_PHOENIX_LOCAL_LIMIT
</span></code>,
53 the predefined maximum local variable declarations in a let expression.
54 By default,
<code class=
"computeroutput"><span class=
"identifier">BOOST_PHOENIX_LOCAL_LIMIT
</span></code>
55 is set to
<code class=
"computeroutput"><span class=
"identifier">BOOST_PHOENIX_LIMIT
</span></code>.
61 <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>
62 <span class=
"special">[
</span>
63 <span class=
"identifier">_a
</span> <span class=
"special">+
</span> <span class=
"identifier">_b
</span>
64 <span class=
"special">]
</span>
67 <span class=
"bold"><strong>Reference Preservation
</strong></span>
70 The type of the local variable assumes the type of the lambda- expression.
71 Type deduction is reference preserving. For example:
73 <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>
76 <code class=
"computeroutput"><span class=
"identifier">_a
</span></code> assumes the type of
77 <code class=
"computeroutput"><span class=
"identifier">arg1
</span></code>: a reference to an
78 argument, while
<code class=
"computeroutput"><span class=
"identifier">_b
</span></code> has
79 type
<code class=
"computeroutput"><span class=
"keyword">int
</span></code>.
84 <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>
86 <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>
87 <span class=
"special">[
</span>
88 <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>
89 <span class=
"special">]
</span>
90 <span class=
"special">(
</span><span class=
"identifier">i
</span><span class=
"special">);
</span>
92 <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>
95 the output of above is :
0 0
100 <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>
102 <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>
103 <span class=
"special">[
</span>
104 <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>
105 <span class=
"special">]
</span>
106 <span class=
"special">(
</span><span class=
"identifier">i
</span><span class=
"special">);
</span>
108 <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>
114 Reference preservation is necessary because we need to have L-value access
115 to outer lambda-scopes (especially the arguments).
<code class=
"computeroutput"><span class=
"identifier">arg
</span></code>s
116 and
<code class=
"computeroutput"><span class=
"identifier">ref
</span></code>s are L-values.
117 <code class=
"computeroutput"><span class=
"identifier">val
</span></code>s are R-values.
120 <span class=
"bold"><strong>Visibility
</strong></span> <a name=
"phoenix.modules.scope.let.visibility"></a>
123 The scope and lifetimes of the local variables is limited within the let-body.
124 <code class=
"computeroutput"><span class=
"identifier">let
</span></code> blocks can be nested.
125 A local variable may hide an outer local variable. For example:
127 <pre class=
"programlisting"><span class=
"identifier">let
</span><span class=
"special">(
</span><span class=
"identifier">_x
</span> <span class=
"special">=
</span> <span class=
"identifier">_1
</span><span class=
"special">,
</span> <span class=
"identifier">_y
</span> <span class=
"special">=
</span> <span class=
"identifier">_2
</span><span class=
"special">)
</span>
128 <span class=
"special">[
</span>
129 <span class=
"comment">// _x here is an int:
1</span>
131 <span class=
"identifier">let
</span><span class=
"special">(
</span><span class=
"identifier">_x
</span> <span class=
"special">=
</span> <span class=
"identifier">_3
</span><span class=
"special">)
</span> <span class=
"comment">// hides the outer _x
</span>
132 <span class=
"special">[
</span>
133 <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>
134 <span class=
"special">]
</span>
135 <span class=
"special">](
</span><span class=
"number">1</span><span class=
"special">,
</span><span class=
"string">" World"</span><span class=
"special">,
</span><span class=
"string">"Hello,"</span><span class=
"special">);
</span>
138 The actual values of the parameters _1, _2 and _3 are supplied from the
139 bracketed list at the end of the
<code class=
"computeroutput"><span class=
"identifier">let
</span></code>.
142 There is currently a limitation that the inner
<code class=
"computeroutput"><span class=
"identifier">let
</span></code>
143 cannot be supplied with a constant e.g.
<code class=
"computeroutput"><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></code>.
146 The RHS (right hand side lambda-expression) of each local-declaration cannot
147 refer to any LHS local-id. At this point, the local-ids are not in scope
148 yet; they will only be in scope in the let-body. The code below is in error:
150 <pre class=
"programlisting"><span class=
"identifier">let
</span><span class=
"special">(
</span>
151 <span class=
"identifier">_a
</span> <span class=
"special">=
</span> <span class=
"number">1</span>
152 <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>
153 <span class=
"special">)
</span>
154 <span class=
"special">[
</span>
155 <span class=
"comment">// _a and _b's scope starts here
</span>
156 <span class=
"comment">/*. body .*/
</span>
157 <span class=
"special">]
</span>
160 However, if an outer let scope is available, this will be searched. Since
161 the scope of the RHS of a local-declaration is the outer scope enclosing
162 the let, the RHS of a local-declaration can refer to a local variable of
165 <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>
166 <span class=
"special">[
</span>
167 <span class=
"identifier">let
</span><span class=
"special">(
</span>
168 <span class=
"identifier">_a
</span> <span class=
"special">=
</span> <span class=
"identifier">_1
</span>
169 <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>
170 <span class=
"special">)
</span>
171 <span class=
"special">[
</span>
172 <span class=
"comment">/*. body .*/
</span>
173 <span class=
"special">]
</span>
174 <span class=
"special">](
</span><span class=
"number">1</span><span class=
"special">)
</span>
177 <table xmlns:
rev=
"http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width=
"100%"><tr>
178 <td align=
"left"></td>
179 <td align=
"right"><div class=
"copyright-footer">Copyright
© 2002-
2005,
2010,
2014,
2015 Joel de Guzman, Dan Marsden, Thomas
180 Heller, John Fletcher
<p>
181 Distributed under the Boost Software License, Version
1.0. (See accompanying
182 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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