Binary Logarithm TemplateBinary Logarithm TemplateThe class template in <boost/integer/static_log2.hpp> determines the position of the highest bit in a given value. This facility is useful for solving generic programming problems.
+ +
+
namespace boost
{
-template < unsigned long Value >
-struct static_log2
-{
- static const int value = implementation_defined;
-};
+ typedef implementation-defined static_log2_argument_type;
+ typedef implementation-defined static_log2_result_type;
-template < >
-struct static_log2< 0ul >
-{
+ template < static_log2_argument_type arg >
+ struct static_log2
+ {
+ static const static_log2_result_type value = implementation-defined;
+ };
+
+
+ template < >
+ struct static_log2< 0 >
+ {
// The logarithm of zero is undefined.
-};
+ };
+
} // namespace boost
+
The boost::static_log2 class template takes one template
-parameter, a value of type unsigned long. The template
-only defines one member, value, that returns the truncated
-base-two logarithm of the template parameter.
static_log2_argument_type. The template
+only defines one member, value, which gives the truncated
+base-two logarithm of the template argument.
Since the logarithm of zero, for any base, is undefined, there is a
-specialization of static_log2 for a template parameter
+specialization of static_log2 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.
Note:
static_log2_argument_type is an unsigned integer
+ type (C++ standard, 3.9.1p3).static_log2_result_type is an integer type
+ (C++ standard, 3.9.1p7).-#include <boost/integer/static_log2.hpp> -template < unsigned long Value > + ++ ++ +#include "boost/integer/static_log2.hpp" + + +template < boost::static_log2_argument_type value > bool is_it_what() { - typedef boost::static_log2<Value> lb_type; + typedef boost::static_log2<value> lb_type; int temp = lb_type::value; //... - return (temp % 2) != 0; } @@ -77,47 +129,87 @@ int main() { bool temp = is_it_what<2000>(); //... - - #if 0 +# if 0 temp = is_it_what<0>(); // would give an error - #endif +# endif //... - temp = is_it_what<24>(); //... } +Demonstration Program
+ +The program static_log2_test.cpp is a simplistic demonstration of the results from instantiating various examples of the binary logarithm class template.
+ +Rationale
+ +The base-two (binary) logarithm, abbreviated lb, function -is occasionally used to give order-estimates of computer algorithms. +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 (i.e. at compile-time) available.
+ + +Changes from previous versions:
+ + + ++
+ + +- New in version 1.32.0:
+
+ +The argument type and the result type ofboost::static_log2+are now typedef'd. Formerly, they were hardcoded asunsigned long+andintrespectively. Please, use the provided typedefs in new +code (and update old code as soon as possible). +Credits
-The author of the Boost binary logarithm class template is Daryle Walker. Giovanni Bajo -added support for compilers without partial template specialization.
+ + +The original version of the Boost binary logarithm class template was +written by Daryle Walker +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. +
-Revised May 14, 2002
-© Copyright Daryle Walker 2001. Permission to copy, use, -modify, sell and distribute this document is granted provided this -copyright notice appears in all copies. This document is provided -"as is" without express or implied warranty, and with no claim -as to its suitability for any purpose.
+ +Revised July 19, 2004
+ +© Copyright Daryle Walker 2001.
+ + Distributed under the Boost Software License, Version 1.0. + (See accompanying file LICENSE_1_0.txt or copy at + + http://www.boost.org/LICENSE_1_0.txt) + +
+ © Copyright Gennaro Prota 2004.
+ + +