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