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<title>Type Traits that Transform One Type to Another</title>
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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_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_volatile.html" title="add_volatile">add_volatile</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="id915484"></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>
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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>)
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