156 lines
5.7 KiB
HTML
156 lines
5.7 KiB
HTML
<html>
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<head>
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<title>local_iteration.html</title>
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<link rel="stylesheet" type="text/css" href="../styles.css">
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</head>
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<body>
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<h4>Local Iteration</h4>
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<div>
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Local iteration is a simple vertical repetition construct.
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It expands a macro with each number in a user-specified range.
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Each expansion is on a separate line.
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</div>
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<h4>Tutorial</h4>
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<div>
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This mechanism requires two pieces of information to operate:
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a range to iterate over and a macro to expand on each iteration.
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This information is obtained by the mechanism through two <i>named external arguments</i>.
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These arguments are specified as user-defined macros named <b>BOOST_PP_LOCAL_LIMITS</b> and <b>BOOST_PP_LOCAL_MACRO</b>.
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</div>
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<div>
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<b>BOOST_PP_LOCAL_LIMITS</b> specifies a range of values to iterate over.
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It <i>must</i> expand to a <i>tuple</i> containing two elements--a lower and upper bound.
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Both the upper and lower bounds must be numeric values in the range of <i>0</i> to <b>BOOST_PP_LIMIT_ITERATION</b>.
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For example, if the user wishes a macro to be expanded with numbers ranging from <i>0</i> to <i>10</i>,
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<b>BOOST_PP_LOCAL_LIMITS</b> would be defined like this:
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</div>
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<div class="code"><pre>
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#define BOOST_PP_LOCAL_LIMITS (0, 10)
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</pre></div>
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<div>
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Note that there is whitespace after the name of the macro.
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The macro <i>does not</i> take <i>two</i> arguments.
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In the case above, if there was no whitespace, a preprocessing error would occur because <i>0</i> and <i>10</i> are invalid identifiers.
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</div>
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<div>
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Both the upper and lower bounds specified in the <b>BOOST_PP_LOCAL_LIMITS</b> macro are <i>evaluated parameters</i>.
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This implies that they can include simple arithmetic or logical expressions.
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For instance, the above definition could easily have been written like this:
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</div>
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<div class="code"><pre>
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#define N() 5
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#define BOOST_PP_LOCAL_LIMITS (0, N() + 5)
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</pre></div>
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<div>
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Because of this, if the whitespace after the macro name is elided, it is possible for the definition to be syntactically valid:
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</div>
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<div class="code"><pre>
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#define A 0
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#define B 10
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#define BOOST_PP_LOCAL_LIMITS(A, B)
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// note: no whitespace ^
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</pre></div>
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<div>
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If this happens, an error will occur inside the mechanism when it attempts to use this macro.
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The error messages that result may be obscure, so always remember to include the whitespace.
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A <i>correct</i> version of the above looks like this:
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</div>
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<div class="code"><pre>
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#define A 0
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#define B 10
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#define BOOST_PP_LOCAL_LIMITS (A, B)
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// note: has whitespace ^
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</pre></div>
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<div>
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<b>BOOST_PP_LOCAL_MACRO</b> is the macro that is expanded by the mechanism.
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This macro is expanded on each iteration with the current number of the iteration.
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It must defined as a unary macro <i>or</i> result in a macro that can be called with one argument:
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</div>
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<div class="code"><pre>
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#define BOOST_PP_LOCAL_MACRO(n) \
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template<> struct sample<n> { }; \
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/**/
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</pre></div>
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<div>
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...or...
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</div>
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<div class="code"><pre>
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#define SAMPLE(n) \
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template<> struct sample<n> { }; \
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/**/
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#define BOOST_PP_LOCAL_MACRO SAMPLE
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</pre></div>
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<div>
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Once these two macros are defined, the local iteration is initiated by <i>including</i> <b>BOOST_PP_LOCAL_ITERATE</b>().
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</div>
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<div class="code"><pre>
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??=include BOOST_PP_LOCAL_ITERATE()
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</pre></div>
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<div>
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(The <code>??=</code> token is a trigraph for <code>#</code>.
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I use the trigraph to make it clear that I am <i>including</i> a file rather than defining or expanding a macro, but it is not necessary.
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Even the digraph version, <code>%:</code>, could be used.
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Some compilers do not readily accept trigraphs and digraphs, so keep that in mind.
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Other than that, use whichever one you prefer.)
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</div>
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<div>
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In order to repeat the <code>sample</code> specialization, the pieces must be put together....
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</div>
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<div class="code"><pre>
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#define BOOST_PP_LOCAL_MACRO(n) \
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template<> struct sample<n> { }; \
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/**/
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#define BOOST_PP_LOCAL_LIMITS (0, 10)
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??=include BOOST_PP_LOCAL_ITERATE()
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</pre></div>
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<div>
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This will result in a specialization of <code>sample</code> for each number in the range of <i>0</i> to <i>10</i>.
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The output will look something like this:
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</div>
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<div class="code"><pre>
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template<> struct sample<0> { };
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template<> struct sample<1> { };
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template<> struct sample<2> { };
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// ...
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template<> struct sample<10> { };
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</pre></div>
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<div>
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After the local-iteration is complete, both <b>BOOST_PP_LOCAL_LIMITS</b> and <b>BOOST_PP_LOCAL_MACRO</b> are automatically undefined.
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If the values need to be retained for a future local-iteration, they must be defined indirectly:
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</div>
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<div class="code"><pre>
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#define LIMITS (0, 10)
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#define SAMPLE(n) \
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template<> struct sample<n> { }; \
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/**/
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#define BOOST_PP_LOCAL_LIMITS LIMITS
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#define BOOST_PP_LOCAL_MACRO(n) SAMPLE(n)
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??=include BOOST_PP_LOCAL_ITERATE()
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</pre></div>
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<h4>See Also</h4>
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<ul>
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<li><a href="../ref/local_iterate.html">BOOST_PP_LOCAL_ITERATE</a></li>
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<li><a href="../ref/local_limits.html">BOOST_PP_LOCAL_LIMITS</a></li>
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<li><a href="../ref/local_macro.html">BOOST_PP_LOCAL_MACRO</a></li>
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</ul>
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<div class="sig">- Paul Mensonides</div>
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<hr size="1">
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<div style="margin-left: 0px;">
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<i><EFBFBD> Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
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</br><i><EFBFBD> Copyright Paul Mensonides 2002</i>
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</div>
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<div style="margin-left: 0px;">
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<p><small>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
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</div>
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</body>
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</html> |