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| <h4>Incompatibilities</h4> |
| <div> |
| There are several incompatibilities with the previous Boost release (1.28). |
| These fall into roughly three categories: |
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| <ul> |
| <li>the horizontal repetition primitives based on <b>BOOST_PP_REPEAT</b></li> |
| <li>the reentrancy syntax</li> |
| <li><i>list</i> folding</li> |
| </ul> |
| <h4>Repetition Targets</h4> |
| <div> |
| First, and probably the most commonly used, is the target macros passed into <b>BOOST_PP_REPEAT</b> and the horizontal repetition contructs that use <b>BOOST_PP_REPEAT</b>. |
| This includes all of the <b>BOOST_PP_REPEAT_</b>* primitives and all of the <b>BOOST_PP_ENUM_</b>* primitives that require a target macro. |
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| The incompatiblity is trivial, but it will require that the source be updated. |
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| These target macros must now except a <i>third</i> parameter. |
| This extra parameter becomes the <i>first</i> parameter in every target macro. |
| It represents the next repetition dimension and brings <b>BOOST_PP_REPEAT</b> inline with rest of the library. |
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| So, what once was: |
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| <div class="code"> |
| #define <i>macro</i>(<i>n</i>, <i>data</i>) ...<br> |
| <b>BOOST_PP_REPEAT</b>(<i>5</i>, <i>macro</i>, <i>data</i>) |
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| ...is now: |
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| <div class="code"> |
| #define <i>macro</i>(<i>z</i>, <i>n</i>, <i>data</i>) ...<br> |
| <b>BOOST_PP_REPEAT</b>(<i>5</i>, <i>macro</i>, <i>data</i>) |
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| <div> |
| This parameter can be used for highly efficient reentrance into the <b>BOOST_PP_REPEAT</b> mechanism. |
| However, it is not necessary to use it as the library can automatically detect the next available repetition dimension. |
| </div> |
| <h4>Dimensional Ordering</h4> |
| <div> |
| Because of this detection, however, it is unsafe to use <b>BOOST_PP_REPEAT_1ST</b>, <b>BOOST_PP_REPEAT_2ND</b>, and <b>BOOST_PP_REPEAT_3RD</b> out of order. |
| These macros bypass the <i>automatic-recursion</i> mechanism, and the <i>automatic-recursion</i> mechanism relies on macros being used in the proper order. |
| To clarify, if you use these bypass macros, the outer-most repetition <i>must</i> be <b>BOOST_PP_REPEAT_1ST</b>, then <b>BOOST_PP_REPEAT_2ND</b>, and finally <b>BOOST_PP_REPEAT_3RD</b>. |
| Any other usage is not supported by the library. |
| Sometimes it may work, and other times it won't. |
| </div> |
| <h4>Reentrancy Syntax</h4> |
| <div> |
| <i>Automatic-recursion</i> brings with it another issue as well. Previously, the reentrancy syntax for <b>BOOST_PP_WHILE</b> (and similarly for <b>BOOST_PP_FOR</b>) was: |
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| <div class="code"> |
| <b>BOOST_PP_WHILE</b> ## <i>d</i>(<i>pred</i>, <i>op</i>, <i>state</i>) |
| </div> |
| <div> |
| ...or: |
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| <div class="code"> |
| <b>BOOST_PP_CAT</b>(<b>BOOST_PP_WHILE</b>, <i>d</i>)(<i>pred</i>, <i>op</i>, <i>state</i>) |
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| <div> |
| Under the <i>automatic-recursion</i> model, the <b>BOOST_PP_CAT</b> version breaks. |
| This is because <b>BOOST_PP_CAT</b> allows its arguments to expand prior to concatenation, |
| and <b>BOOST_PP_WHILE</b> is a macro that expands without arguments. |
| The library makes it appear that it takes three parameters, but that is the trick of <i>automatic-recursion</i>. |
| It works similarly to the following: |
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| <div class="code"> |
| #define A(x, y) ...<br> |
| #define B A<br> |
| // ...<br> |
| B(2, 3) |
| </div> |
| <div> |
| The syntax makes it look like the <i>B</i> macro above takes two arguments, but it doesn't. |
| The <i>automatic-recursion</i> mechanism works in this fashion, except that the "<i>B</i>" macro deduces the next available "<i>A</i>" macro. |
| </div> |
| <div> |
| Because some preprocessors are still slow, direct reentrancy (sans <i>automatic-recursion</i>) is still necessary in non-trivial cases. |
| Consequently, the library uses a new syntax to handle reentrancy: |
| </div> |
| <div class="code"> |
| <b>BOOST_PP_FOR_</b> ## <i>r</i>(<i>state</i>, <i>pred</i>, <i>op</i>, <i>macro</i>)<br> |
| <b>BOOST_PP_REPEAT_</b> ## <i>z</i>(<i>count</i>, <i>macro</i>, <i>data</i>)<br> |
| <b>BOOST_PP_WHILE_</b> ## <i>d</i>(<i>pred</i>, <i>op</i>, <i>state</i>) |
| </div> |
| <h4>Folding</h4> |
| <div> |
| Previously, the <b>BOOST_PP_LIST_FOLD_RIGHT</b> macros' arguments were the reverse of <b>BOOST_PP_LIST_FOLD_LEFT</b>. |
| Also, the accumulation macro passed into <b>BOOST_PP_LIST_FOLD_RIGHT</b> was called with reversed parameters as well. |
| This discrepancy has been eliminated. |
| </div> |
| <div> |
| To illustrate, <b>BOOST_PP_LIST_FOLD_RIGHT</b> used to be used like this: |
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| <div class="code"> |
| #define <i>macro</i>(<i>d</i>, <u><i>elem</i></u>, <u><i>state</i></u>)<br> |
| <b>BOOST_PP_LIST_FOLD_RIGHT</b>(<i>macro</i>, <u><i>list</i></u>, <u><i>state</i></u>) |
| </div> |
| <div> |
| This signature has been replaced by... |
| </div> |
| <div class="code"> |
| #define <i>macro</i>(<i>d</i>, <u><i>state</i></u>, <u><i>elem</i></u>)<br> |
| <b>BOOST_PP_LIST_FOLD_RIGHT</b>(<i>macro</i>, <u><i>state</i></u>, <u><i>list</i></u>) |
| </div> |
| <h4>Summary</h4> |
| <div> |
| The library has many new features not present in the 1.28 release, and this list does not attempt to enumerate them. |
| This is simply a list of things that <i>must</i> change for code to be compatible with this new release. |
| </div> |
| <h4>See Also</h4> |
| <ul> |
| <li><a href="../ref/for.html">BOOST_PP_FOR</a></li> |
| <li><a href="../ref/list_fold_right.html">BOOST_PP_LIST_FOLD_RIGHT</a></li> |
| <li><a href="../ref/repeat.html">BOOST_PP_REPEAT</a></li> |
| <li><a href="../ref/while.html">BOOST_PP_WHILE</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> |
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