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2009-11-25 12:38:09 +00:00
< title > Compile time log2 Calculation</ title >
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< div class = "titlepage" >< div >< div >< h2 class = "title" style = "clear: both" >
< a name = "boost_integer.log2" ></ a >< a class = "link" href = "log2.html" title = "Compile time log2 Calculation" > Compile time log2 Calculation</ a >
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< div class = "toc" >< dl >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.synopsis" > Synopsis</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.usage" > Usage</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.example" > Example</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.demonstration_program" > Demonstration
Program</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.rationale" > Rationale</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "log2.html#boost_integer.log2.credits" > Credits</ a ></ span ></ dt >
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< p >
The class template in < a href = "../../../../../boost/integer/static_log2.hpp" target = "_top" > < boost/integer/static_log2.hpp> </ a >
determines the position of the highest bit in a given value. This facility
is useful for solving generic programming problems.
</ p >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.synopsis" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.synopsis" title = "Synopsis" > Synopsis</ a >
</ h3 ></ div ></ div ></ div >
< pre class = "programlisting" >< span class = "keyword" > namespace</ span > < span class = "identifier" > boost</ span >
< span class = "special" > {</ span >
< span class = "keyword" > typedef</ span > < span class = "emphasis" >< em > implementation-defined</ em ></ span > < span class = "identifier" > static_log2_argument_type</ span >< span class = "special" > ;</ span >
< span class = "keyword" > typedef</ span > < span class = "emphasis" >< em > implementation-defined</ em ></ span > < span class = "identifier" > static_log2_result_type</ span >< span class = "special" > ;</ span >
< span class = "keyword" > template</ span > < span class = "special" > < </ span >< span class = "identifier" > static_log2_argument_type</ span > < span class = "identifier" > arg</ span >< span class = "special" > > </ span >
< span class = "keyword" > struct</ span > < span class = "identifier" > static_log2</ span >
< span class = "special" > {</ span >
< span class = "keyword" > static</ span > < span class = "keyword" > const</ span > < span class = "identifier" > static_log2_result_type</ span > < span class = "identifier" > value</ span > < span class = "special" > =</ span > < span class = "emphasis" >< em > implementation-defined</ em ></ span >< span class = "special" > ;</ span >
< span class = "special" > };</ span >
< span class = "keyword" > template</ span > < span class = "special" > < </ span > < span class = "special" > > </ span >
< span class = "keyword" > struct</ span > < span class = "identifier" > static_log2</ span >< span class = "special" > < </ span > < span class = "number" > 0</ span > < span class = "special" > > </ span >
< span class = "special" > {</ span >
< span class = "comment" > // The logarithm of zero is undefined.
</ span > < span class = "special" > };</ span >
< span class = "special" > }</ span > < span class = "comment" > // namespace boost
</ span ></ pre >
</ div >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.usage" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.usage" title = "Usage" > Usage</ a >
</ h3 ></ div ></ div ></ div >
< p >
The < code class = "literal" > boost::static_log2</ code > class template takes one template
parameter, a value of type < code class = "literal" > static_log2_argument_type</ code > .
The template only defines one member, < code class = "literal" > value</ code > , which gives
the truncated base-two logarithm of the template argument.
</ p >
< p >
Since the logarithm of zero, for any base, is undefined, there is a specialization
of < code class = "literal" > static_log2</ code > for a template argument of zero. This specialization
has no members, so an attempt to use the base-two logarithm of zero results
in a compile-time error.
</ p >
< p >
Note:
</ p >
< div class = "itemizedlist" >< ul type = "disc" >
< li >
< code class = "literal" > static_log2_argument_type</ code > is an < span class = "emphasis" >< em > unsigned integer
type</ em ></ span > (C++ standard, 3.9.1p3).
</ li >
< li >
< code class = "literal" > static_log2_result_type</ code > is an < span class = "emphasis" >< em > integer type</ em ></ span >
(C++ standard, 3.9.1p7).
</ li >
</ ul ></ div >
</ div >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.example" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.example" title = "Example" > Example</ a >
</ h3 ></ div ></ div ></ div >
< pre class = "programlisting" >< span class = "preprocessor" > #include</ span > < span class = "string" > "boost/integer/static_log2.hpp"</ span >
< span class = "keyword" > template</ span > < span class = "special" > < </ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > static_log2_argument_type</ span > < span class = "identifier" > value</ span > < span class = "special" > > </ span >
< span class = "keyword" > bool</ span > < span class = "identifier" > is_it_what</ span >< span class = "special" > ()</ span >
< span class = "special" > {</ span >
< span class = "keyword" > typedef</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > static_log2</ span >< span class = "special" > < </ span >< span class = "identifier" > value</ span >< span class = "special" > > </ span > < span class = "identifier" > lb_type</ span >< span class = "special" > ;</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > temp</ span > < span class = "special" > =</ span > < span class = "identifier" > lb_type</ span >< span class = "special" > ::</ span >< span class = "identifier" > value</ span >< span class = "special" > ;</ span >
< span class = "comment" > //...
</ span > < span class = "keyword" > return</ span > < span class = "special" > (</ span >< span class = "identifier" > temp</ span > < span class = "special" > %</ span > < span class = "number" > 2</ span >< span class = "special" > )</ span > < span class = "special" > !=</ span > < span class = "number" > 0</ span >< span class = "special" > ;</ span >
< span class = "special" > }</ span >
< span class = "comment" > //...
</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > main</ span >< span class = "special" > ()</ span >
< span class = "special" > {</ span >
< span class = "keyword" > bool</ span > < span class = "identifier" > temp</ span > < span class = "special" > =</ span > < span class = "identifier" > is_it_what</ span >< span class = "special" > < </ span >< span class = "number" > 2000</ span >< span class = "special" > > ();</ span >
< span class = "comment" > //...
</ span >< span class = "preprocessor" > # if</ span > < span class = "number" > 0</ span >
< span class = "identifier" > temp</ span > < span class = "special" > =</ span > < span class = "identifier" > is_it_what</ span >< span class = "special" > < </ span >< span class = "number" > 0</ span >< span class = "special" > > ();</ span > < span class = "comment" > // would give an error
</ span >< span class = "preprocessor" > # endif</ span >
< span class = "comment" > //...
</ span > < span class = "identifier" > temp</ span > < span class = "special" > =</ span > < span class = "identifier" > is_it_what</ span >< span class = "special" > < </ span >< span class = "number" > 24</ span >< span class = "special" > > ();</ span >
< span class = "comment" > //...
</ span >< span class = "special" > }</ span >
</ pre >
</ div >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.demonstration_program" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.demonstration_program" title = "Demonstration Program" > Demonstration
Program</ a >
</ h3 ></ div ></ div ></ div >
< p >
The program < a href = "../../../test/static_log2_test.cpp" target = "_top" > static_log2_test.cpp</ a >
is a simplistic demonstration of the results from instantiating various examples
of the binary logarithm class template.
</ p >
</ div >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.rationale" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.rationale" title = "Rationale" > Rationale</ a >
</ h3 ></ div ></ div ></ div >
< p >
The base-two (binary) logarithm, abbreviated lb, function is occasionally
used to give order-estimates of computer algorithms. The truncated logarithm
can be considered the highest power-of-two in a value, which corresponds
to the value's highest set bit (for binary integers). Sometimes the highest-bit
position could be used in generic programming, which requires the position
to be statically (< span class = "emphasis" >< em > i.e.</ em ></ span > at compile-time) available.
</ p >
</ div >
< div class = "section" lang = "en" >
< div class = "titlepage" >< div >< div >< h3 class = "title" >
< a name = "boost_integer.log2.credits" ></ a >< a class = "link" href = "log2.html#boost_integer.log2.credits" title = "Credits" > Credits</ a >
</ h3 ></ div ></ div ></ div >
< p >
The original version of the Boost binary logarithm class template was written
by < a href = "http://www.boost.org/people/daryle_walker.html" target = "_top" > Daryle Walker</ a >
and then enhanced by Giovanni Bajo with support for compilers without partial
template specialization. The current version was suggested, together with
a reference implementation, by Vesa Karvonen. Gennaro Prota wrote the actual
source file.
</ p >
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2009-11-27 11:13:02 +00:00
< td align = "right" >< div class = "copyright-footer" > Copyright © 2001 -2009 Beman Dawes, Daryle Walker, Gennaro Prota,
2009-11-25 12:38:09 +00:00
John Maddock< 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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