ceph/src/boost/libs/multiprecision/doc/html/boost_multiprecision/map/faq.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_multiprecision.map.faq"></a><a class="link" href="faq.html" title="FAQ">FAQ</a>
  28 </h3></div></div></div>
  29 <div class="variablelist">
  30 <p class="title"><b></b></p>
  31 <dl class="variablelist">
  32 <dt><span class="term">Why do I get compiler errors when passing a <code class="computeroutput"><span class="identifier">number</span></code>
  33           to a template function?</span></dt>
  34 <dd><p>
  35               Most likely you are actually passing an expression template type to
  36               the function and template-argument-deduction deduces the "wrong"
  37               type. Try casting the arguments involving expressions to the actual
  38               number type, or as a last resort turning off expression template support
  39               in the number type you are using.
  40             </p></dd>
  41 <dt><span class="term">When is expression template support a performance gain?</span></dt>
  42 <dd><p>
  43               As a general rule, expression template support adds a small runtime
  44               overhead creating and unpacking the expression templates, but greatly
  45               reduces the number of temporaries created. So it's most effective in
  46               improving performance when the cost of creating a temporary is high:
  47               for example when creating a temporary involves a memory allocation.
  48               It is least effective (and may even be a dis-optimisation) when temporaries
  49               are cheap: for example if the number type is basically a thin wrapper
  50               around a native arithmetic type. In addition, since the library makes
  51               extensive use of thin inline wrapper functions, turning on compiler
  52               optimization is essential to achieving high performance.
  53             </p></dd>
  54 <dt><span class="term">Do expression templates reorder operations?</span></dt>
  55 <dd><p>
  56               Yes they do, sometimes quite radically so, if this is a concern then
  57               they should be turned off for the number type you are using.
  58             </p></dd>
  59 <dt><span class="term">I can't construct my number type from <span class="emphasis"><em>some other type</em></span>,
  60           but the docs indicate that the conversion should be allowed, what's up?</span></dt>
  61 <dd><p>
  62               Some conversions are <span class="emphasis"><em>explicit</em></span>, that includes construction
  63               from a string, or constructing from any type that may result in loss
  64               of precision (for example constructing an integer type from a float).
  65             </p></dd>
  66 <dt><span class="term">Why do I get an exception thrown (or the program crash due to an
  67           uncaught exception) when using the bitwise operators on a checked <code class="computeroutput"><span class="identifier">cpp_int</span></code>?</span></dt>
  68 <dd><p>
  69               Bitwise operations on negative values (or indeed any signed integer
  70               type) are unspecified by the standard. As a result any attempt to carry
  71               out a bitwise operation on a negative checked-integer will result in
  72               a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">range_error</span></code> being thrown.
  73             </p></dd>
  74 <dt><span class="term">Why do I get compiler errors when trying to use the complement operator?</span></dt>
  75 <dd><p>
  76               Use of the complement operator on signed types is problematic as the
  77               result is unspecified by the standard, and is further complicated by
  78               the fact that most extended precision integer types use a sign-magnitude
  79               representation rather than the 2's complement one favored by most native
  80               integer types. As a result the complement operator is deliberately
  81               disabled for checked <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s.
  82               Unchecked <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s
  83               give the same valued result as a 2's complement type would, but not
  84               the same bit-pattern.
  85             </p></dd>
  86 <dt><span class="term">Why can't I negate an unsigned type?</span></dt>
  87 <dd><p>
  88               The unary negation operator is deliberately disabled for unsigned integer
  89               types as its use would almost always be a programming error.
  90             </p></dd>
  91 <dt><span class="term">Why doesn't the library use proto?</span></dt>
  92 <dd><p>
  93               A very early version of the library did use proto, but compile times
  94               became too slow for the library to be usable. Since the library only
  95               required a tiny fraction of what proto has to offer anyway, a lightweight
  96               expression template mechanism was used instead. Compile times are still
  97               too slow...
  98             </p></dd>
  99 <dt><span class="term">Why not abstract out addition/multiplication algorithms?</span></dt>
 100 <dd><p>
 101               This was deemed not to be practical: these algorithms are intimately
 102               tied to the actual data representation used.
 103             </p></dd>
 104 </dl>
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 107 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 108 <td align="left"></td>
 109 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2013 John Maddock and Christopher Kormanyos<p>
 110         Distributed under the Boost Software License, Version 1.0. (See accompanying
 111         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>)
 112       </p>
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