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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.category.transform"></a><a class="link" href="transform.html" title="Type Traits that Transform One Type to Another"> Type Traits that |
| Transform One Type to Another</a> |
| </h3></div></div></div> |
| <p> |
| The following templates transform one type to another, based upon some well-defined |
| rule. Each template has a single member called <code class="computeroutput"><span class="identifier">type</span></code> |
| that is the result of applying the transformation to the template argument |
| <code class="computeroutput"><span class="identifier">T</span></code>. |
| </p> |
| <p> |
| <span class="bold"><strong>Synopsis:</strong></span> |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_const.html" title="add_const">add_const</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_cv.html" title="add_cv">add_cv</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_lvalue_reference.html" title="add_lvalue_reference">add_lvalue_reference</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_pointer.html" title="add_pointer">add_pointer</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_reference.html" title="add_reference">add_reference</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_rvalue_reference.html" title="add_rvalue_reference">add_rvalue_reference</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/add_volatile.html" title="add_volatile">add_volatile</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">bool</span> <span class="identifier">B</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/conditional.html" title="conditional">conditional</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/common_type.html" title="common_type">common_type</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/decay.html" title="decay">decay</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/floating_point_promotion.html" title="floating_point_promotion">floating_point_promotion</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/integral_promotion.html" title="integral_promotion">integral_promotion</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/make_signed.html" title="make_signed">make_signed</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/make_unsigned.html" title="make_unsigned">make_unsigned</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/promote.html" title="promote">promote</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_all_extents.html" title="remove_all_extents">remove_all_extents</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_const.html" title="remove_const">remove_const</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_cv.html" title="remove_cv">remove_cv</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_extent.html" title="remove_extent">remove_extent</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_pointer.html" title="remove_pointer">remove_pointer</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_reference.html" title="remove_reference">remove_reference</a><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <a class="link" href="../reference/remove_volatile.html" title="remove_volatile">remove_volatile</a><span class="special">;</span> |
| </pre> |
| <a name="boost_typetraits.category.transform.broken_compiler_workarounds_"></a><h5> |
| <a name="id1042354"></a> |
| <a class="link" href="transform.html#boost_typetraits.category.transform.broken_compiler_workarounds_">Broken |
| Compiler Workarounds:</a> |
| </h5> |
| <p> |
| For all of these templates support for partial specialization of class templates |
| is required to correctly implement the transformation. On the other hand, |
| practice shows that many of the templates from this category are very useful, |
| and often essential for implementing some generic libraries. Lack of these |
| templates is often one of the major limiting factors in porting those libraries |
| to compilers that do not yet support this language feature. As some of these |
| compilers are going to be around for a while, and at least one of them is |
| very wide-spread, it was decided that the library should provide workarounds |
| where possible. |
| </p> |
| <p> |
| The basic idea behind the workaround is to manually define full specializations |
| of all type transformation templates for all fundamental types, and all their |
| 1st and 2nd rank cv-[un]qualified derivative pointer types, and to provide |
| a user-level macro that will define all the explicit specializations needed |
| for any user-defined type T. |
| </p> |
| <p> |
| The first part guarantees the successful compilation of something like this: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">volatile</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">volatile</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span><span class="special">*,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span><span class="special">*&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="special">...</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">*</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">*</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="keyword">char</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">*</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">*</span> <span class="keyword">const</span> <span class="keyword">volatile</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| </pre> |
| <p> |
| and the second part provides the library's users with a mechanism to make |
| the above code work not only for <code class="computeroutput"><span class="keyword">char</span></code>, |
| <code class="computeroutput"><span class="keyword">int</span></code> or other built-in type, |
| but for their own types as well: |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">myspace</span><span class="special">{</span> |
| <span class="keyword">struct</span> <span class="identifier">MyClass</span> <span class="special">{};</span> |
| <span class="special">}</span> |
| <span class="comment">// declare this at global scope: |
| </span><span class="identifier">BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION</span><span class="special">(</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">MyClass</span><span class="special">)</span> |
| <span class="comment">// transformations on myspace::MyClass now work: |
| </span><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">MyClass</span><span class="special">&>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">remove_const</span><span class="special"><</span><span class="identifier">myspace</span><span class="special">::</span><span class="identifier">MyClass</span> <span class="keyword">const</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span> |
| <span class="comment">// etc. |
| </span></pre> |
| <p> |
| Note that the macro BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION evaluates |
| to nothing on those compilers that <span class="bold"><strong>do</strong></span> support |
| partial specialization. |
| </p> |
| </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 © 2000, 2006 Adobe Systems Inc, David Abrahams, |
| Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat |
| Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten Ottosen, Robert |
| Ramey and Jeremy Siek<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>) |
| </p> |
| </div></td> |
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