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<a name="boost_multiprecision.ref.internals"></a><a class="link" href="internals.html" title="Internal Support Code">Internal Support
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<p>
There are some traits classes which authors of new backends should be aware
of:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">multiprecision</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">detail</span><span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_explicitly_convertible</span><span class="special">;</span>
<span class="special">}}}</span>
</pre>
<p>
Inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">true</span><span class="special">></span></code> if type <code class="computeroutput"><span class="identifier">From</span></code>
has an explicit conversion from <code class="computeroutput"><span class="identifier">To</span></code>.
</p>
<p>
For compilers that support C++11 SFINAE-expressions this trait should "just
work". Otherwise it inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">>::</span><span class="identifier">type</span></code>, and will need to be specialised for
Backends that have constructors marked as <code class="computeroutput"><span class="keyword">explicit</span></code>.
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">To</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_lossy_conversion</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
Member <code class="computeroutput"><span class="identifier">value</span></code> is true if the
conversion from <code class="computeroutput"><span class="identifier">From</span></code> to
<code class="computeroutput"><span class="identifier">To</span></code> would result in a loss
of precision, and <code class="computeroutput"><span class="keyword">false</span></code> otherwise.
</p>
<p>
The default version of this trait simply checks whether the <span class="emphasis"><em>kind</em></span>
of conversion (for example from a floating point to an integer type) is inherently
lossy. Note that if either of the types <code class="computeroutput"><span class="identifier">From</span></code>
or <code class="computeroutput"><span class="identifier">To</span></code> are of an unknown number
category (because <code class="computeroutput"><span class="identifier">number_category</span></code>
is not specialised for that type) then this trait will be <code class="computeroutput"><span class="keyword">true</span></code>.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_restricted_conversion</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
Member <code class="computeroutput"><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code> if <code class="computeroutput"><span class="identifier">From</span></code>
is only explicitly convertible to <code class="computeroutput"><span class="identifier">To</span></code>
and not implicitly convertible, or if <code class="computeroutput"><span class="identifier">is_lossy_conversion</span><span class="special"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">>::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
Otherwise <code class="computeroutput"><span class="keyword">false</span></code>.
</p>
<p>
Note that while this trait is the ultimate arbiter of which constructors
are marked as <code class="computeroutput"><span class="keyword">explicit</span></code> in class
<code class="computeroutput"><span class="identifier">number</span></code>, authors of backend
types should generally specialise one of the traits above, rather than this
one directly.
</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="identifier">is_signed_number</span><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="identifier">is_unsigned_number</span><span class="special">;</span>
</pre>
<p>
These two traits inherit from either <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
or <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>, by default types are assumed to
be signed unless <code class="computeroutput"><span class="identifier">is_unsigned_number</span></code>
is specialized for that type.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<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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