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26 <div class=
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29 </h3></div></div></div>
31 There are some traits classes which authors of new backends should be aware
34 <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>
36 <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>
37 <span class=
"keyword">struct
</span> <span class=
"identifier">is_explicitly_convertible
</span><span class=
"special">;
</span>
39 <span class=
"special">}}}
</span>
42 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>
43 has an explicit conversion from
<code class=
"computeroutput"><span class=
"identifier">To
</span></code>.
46 For compilers that support C++
11 SFINAE-expressions this trait should
"just
47 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
48 Backends that have constructors marked as
<code class=
"computeroutput"><span class=
"keyword">explicit
</span></code>.
50 <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>
51 <span class=
"keyword">struct
</span> <span class=
"identifier">is_lossy_conversion
</span>
52 <span class=
"special">{
</span>
53 <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>
54 <span class=
"special">};
</span>
57 Member
<code class=
"computeroutput"><span class=
"identifier">value
</span></code> is true if the
58 conversion from
<code class=
"computeroutput"><span class=
"identifier">From
</span></code> to
59 <code class=
"computeroutput"><span class=
"identifier">To
</span></code> would result in a loss
60 of precision, and
<code class=
"computeroutput"><span class=
"keyword">false
</span></code> otherwise.
63 The default version of this trait simply checks whether the
<span class=
"emphasis"><em>kind
</em></span>
64 of conversion (for example from a floating-point to an integer type) is inherently
65 lossy. Note that if either of the types
<code class=
"computeroutput"><span class=
"identifier">From
</span></code>
66 or
<code class=
"computeroutput"><span class=
"identifier">To
</span></code> are of an unknown number
67 category (because
<code class=
"computeroutput"><span class=
"identifier">number_category
</span></code>
68 is not specialised for that type) then this trait will be
<code class=
"computeroutput"><span class=
"keyword">true
</span></code>.
70 <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>
71 <span class=
"keyword">struct
</span> <span class=
"identifier">is_restricted_conversion
</span>
72 <span class=
"special">{
</span>
73 <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>
74 <span class=
"special">};
</span>
77 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>
78 is only explicitly convertible to
<code class=
"computeroutput"><span class=
"identifier">To
</span></code>
79 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>.
80 Otherwise
<code class=
"computeroutput"><span class=
"keyword">false
</span></code>.
83 Note that while this trait is the ultimate arbiter of which constructors
84 are marked as
<code class=
"computeroutput"><span class=
"keyword">explicit
</span></code> in class
85 <code class=
"computeroutput"><span class=
"identifier">number
</span></code>, authors of backend
86 types should generally specialise one of the traits above, rather than this
89 <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>
90 <span class=
"identifier">is_signed_number
</span><span class=
"special">;
</span>
91 <span class=
"keyword">template
</span> <span class=
"special"><</span><span class=
"keyword">class
</span> <span class=
"identifier">T
</span><span class=
"special">></span>
92 <span class=
"identifier">is_unsigned_number
</span><span class=
"special">;
</span>
95 These two traits inherit from either
<code class=
"computeroutput"><span class=
"identifier">mpl
</span><span class=
"special">::
</span><span class=
"identifier">true_
</span></code>
96 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
97 be signed unless
<code class=
"computeroutput"><span class=
"identifier">is_unsigned_number
</span></code>
98 is specialized for that type.
101 <table xmlns:
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"http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width=
"100%"><tr>
102 <td align=
"left"></td>
103 <td align=
"right"><div class=
"copyright-footer">Copyright
© 2002-
2013 John Maddock and Christopher Kormanyos
<p>
104 Distributed under the Boost Software License, Version
1.0. (See accompanying
105 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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