2001-12-07 13:19:38 +00:00
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< html >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< head >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< title > Binary Logarithm Template</ title >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
</ head >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< body bgcolor = "white" text = "black" >
2004-07-22 08:57:45 +00:00
2004-10-05 15:45:52 +00:00
< h1 >< img src = "../../../boost.png" alt = "boost.png (6897 bytes)"
2001-12-07 13:19:38 +00:00
align = "middle" width = "277" height = "86" > Binary Logarithm Template</ h1 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< p > The class template in < cite >< a href = "../../../boost/integer/static_log2.hpp" > < boost/integer/static_log2.hpp> </ a ></ cite > determines the position of the highest bit in a given value. This facility is useful for solving generic programming problems.</ p >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< h2 >< a name = "contents" > Contents</ a ></ h2 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< ul >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#contents" > Contents</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#synopsis" > Synopsis</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#usage" > Usage</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#example" > Example</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#demo" > Demonstration Program</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#rationale" > Rationale</ a ></ li >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< li >< a href = "#credits" > Credits</ a ></ li >
2004-07-22 08:57:45 +00:00
< li >< a href = "#whatsnew" >< b > What's new</ b ></ a ></ li >
2001-12-07 13:19:38 +00:00
</ ul >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< h2 >< a name = "synopsis" > Synopsis</ a ></ h2 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< blockquote >< pre >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
namespace boost
{
2004-07-22 08:57:45 +00:00
typedef < em > implementation-defined</ em > static_log2_argument_type;
typedef < em > implementation-defined</ em > static_log2_result_type;
2001-12-07 13:19:38 +00:00
2004-07-22 08:57:45 +00:00
template < static_log2_argument_type arg >
struct static_log2
{
static const static_log2_result_type value = < em > implementation-defined</ em > ;
};
template < >
struct static_log2< 0 >
{
2001-12-07 13:19:38 +00:00
// The logarithm of zero is undefined.
2004-07-22 08:57:45 +00:00
};
2001-12-07 13:19:38 +00:00
} // namespace boost
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
</ pre ></ blockquote >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< h2 >< a name = "usage" > Usage</ a ></ h2 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< p > The < code > boost::static_log2</ code > class template takes one template
2004-07-22 08:57:45 +00:00
parameter, a value of type < code > static_log2_argument_type</ code > . The template
only defines one member, < code > value</ code > , which gives the truncated
base-two logarithm of the template argument.</ p >
2001-12-07 13:19:38 +00:00
< p > Since the logarithm of zero, for any base, is undefined, there is a
2004-07-22 08:57:45 +00:00
specialization of < code > static_log2</ code > for a template argument
2001-12-07 13:19:38 +00:00
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 >
2004-07-22 08:57:45 +00:00
< p > Note: < ul >
< li >< code > static_log2_argument_type</ code > is an < i > unsigned integer
type</ i > (C++ standard, 3.9.1p3).</ li >
< li >< code > static_log2_result_type</ code > is an < i > integer type</ i >
(C++ standard, 3.9.1p7).</ li >
</ ul >
2001-12-07 13:19:38 +00:00
< h2 >< a name = "example" > Example</ a ></ h2 >
2004-07-22 08:57:45 +00:00
< blockquote >< pre >
#include "boost/integer/static_log2.hpp"
template < boost::static_log2_argument_type value >
2001-12-07 13:19:38 +00:00
bool is_it_what()
{
2004-07-22 08:57:45 +00:00
typedef boost::static_log2< value> lb_type;
2001-12-07 13:19:38 +00:00
int temp = lb_type::value;
//...
return (temp % 2) != 0;
}
//...
int main()
{
bool temp = is_it_what< 2000> ();
//...
2004-07-22 08:57:45 +00:00
# if 0
2001-12-07 13:19:38 +00:00
temp = is_it_what< 0> (); // would give an error
2004-07-22 08:57:45 +00:00
# endif
2001-12-07 13:19:38 +00:00
//...
temp = is_it_what< 24> ();
//...
}
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
</ pre ></ blockquote >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< h2 >< a name = "demo" > Demonstration Program</ a ></ h2 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< p > The program < a href = "../test/static_log2_test.cpp" > static_log2_test.cpp</ a >
is a simplistic demonstration of the results from instantiating various
examples of the binary logarithm class template.</ p >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< h2 >< a name = "rationale" > Rationale</ a ></ h2 >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
< p > The base-two (binary) logarithm, abbreviated < dfn > lb</ dfn > , function
2004-07-22 08:57:45 +00:00
is occasionally used to give order-estimates of computer algorithms.
2001-12-07 13:19:38 +00:00
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 (< i > i.e.</ i >
at compile-time) available.</ p >
2004-07-22 08:57:45 +00:00
< h2 >< a name = "whatsnew" > Changes from previous versions:</ a ></ h2 >
< ul >
< li >< i > New in version 1.32.0:</ i >< br >< br >
The argument type and the result type of < code > boost::static_log2</ code >
are now typedef'd. Formerly, they were hardcoded as < code > unsigned long</ code >
and < code > int</ code > respectively. Please, use the provided typedefs in new
code (and update old code as soon as possible).
</ li >
</ ul >
2001-12-07 13:19:38 +00:00
< h2 >< a name = "credits" > Credits</ a ></ h2 >
2004-07-22 08:57:45 +00:00
< p > The original version of the Boost binary logarithm class template was
2008-02-10 14:56:22 +00:00
written by < a href = "http://www.boost.org/people/daryle_walker.html" > Daryle Walker</ a >
2004-07-22 08:57:45 +00:00
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 >
2001-12-07 13:19:38 +00:00
< hr >
2004-07-22 08:57:45 +00:00
< p > Revised July 19, 2004</ p >
< p > © Copyright Daryle Walker 2001.< br >
© Copyright Gennaro Prota 2004.</ 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" >
http://www.boost.org/LICENSE_1_0.txt</ a > )
< br >
2001-12-07 13:19:38 +00:00
</ body >
2004-07-22 08:57:45 +00:00
2001-12-07 13:19:38 +00:00
</ html >
2004-07-22 08:57:45 +00:00
