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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_multiprecision.map.faq"></a><a class="link" href="faq.html" title="FAQ">FAQ</a>
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</h3></div></div></div>
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<div class="variablelist">
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<p class="title"><b></b></p>
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<dl class="variablelist">
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<dt><span class="term">Why do I get compiler errors when passing a <code class="computeroutput"><span class="identifier">number</span></code>
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to a template function?</span></dt>
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<dd><p>
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Most likely you are actually passing an expression template type to
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the function and template-argument-deduction deduces the "wrong"
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type. Try casting the arguments involving expressions to the actual
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number type, or as a last resort turning off expression template support
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in the number type you are using.
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</p></dd>
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<dt><span class="term">When is expression template support a performance gain?</span></dt>
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<dd><p>
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As a general rule, expression template support adds a small runtime
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overhead creating and unpacking the expression templates, but greatly
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reduces the number of temporaries created. So it's most effective in
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improving performance when the cost of creating a temporary is high:
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for example when creating a temporary involves a memory allocation.
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It is least effective (and may even be a dis-optimisation) when temporaries
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are cheap: for example if the number type is basically a thin wrapper
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around a native arithmetic type. In addition, since the library makes
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extensive use of thin inline wrapper functions, turning on compiler
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optimization is essential to achieving high performance.
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</p></dd>
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<dt><span class="term">Do expression templates reorder operations?</span></dt>
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<dd><p>
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Yes they do, sometimes quite radically so, if this is a concern then
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they should be turned off for the number type you are using.
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</p></dd>
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<dt><span class="term">I can't construct my number type from <span class="emphasis"><em>some other type</em></span>,
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but the docs indicate that the conversion should be allowed, what's up?</span></dt>
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<dd><p>
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Some conversions are <span class="emphasis"><em>explicit</em></span>, that includes construction
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from a string, or constructing from any type that may result in loss
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of precision (for example constructing an integer type from a float).
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</p></dd>
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<dt><span class="term">Why do I get an exception thrown (or the program crash due to an
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uncaught exception) when using the bitwise operators on a checked <code class="computeroutput"><span class="identifier">cpp_int</span></code>?</span></dt>
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<dd><p>
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Bitwise operations on negative values (or indeed any signed integer
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type) are unspecified by the standard. As a result any attempt to carry
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out a bitwise operation on a negative checked-integer will result in
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a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">range_error</span></code> being thrown.
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</p></dd>
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<dt><span class="term">Why do I get compiler errors when trying to use the complement operator?</span></dt>
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<dd><p>
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Use of the complement operator on signed types is problematic as the
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result is unspecified by the standard, and is further complicated by
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the fact that most extended precision integer types use a sign-magnitude
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representation rather than the 2's complement one favored by most native
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integer types. As a result the complement operator is deliberately
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disabled for checked <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s.
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Unchecked <code class="computeroutput"><span class="identifier">cpp_int</span></code>'s
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give the same valued result as a 2's complement type would, but not
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the same bit-pattern.
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</p></dd>
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<dt><span class="term">Why can't I negate an unsigned type?</span></dt>
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<dd><p>
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The unary negation operator is deliberately disabled for unsigned integer
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types as its use would almost always be a programming error.
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</p></dd>
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<dt><span class="term">Why doesn't the library use proto?</span></dt>
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<dd><p>
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A very early version of the library did use proto, but compile times
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became too slow for the library to be usable. Since the library only
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required a tiny fraction of what proto has to offer anyway, a lightweight
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expression template mechanism was used instead. Compile times are still
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too slow...
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</p></dd>
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<dt><span class="term">Why not abstract out addition/multiplication algorithms?</span></dt>
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<dd><p>
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This was deemed not to be practical: these algorithms are intimately
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tied to the actual data representation used.
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</p></dd>
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</dl>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div></td>
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