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The 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 { typedefimplementation-definedstatic_log2_argument_type; typedefimplementation-definedstatic_log2_result_type; 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 `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 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 < boost::static_log2_argument_type value > bool is_it_what() { typedef boost::static_log2<value> lb_type; int temp = lb_type::value; //... return (temp % 2) != 0; } //... int main() { bool temp = is_it_what<2000>(); //... # if 0 temp = is_it_what<0>(); // would give an error # endif //... temp = is_it_what<24>(); //... }

The program static_log2_test.cpp is a simplistic demonstration of the results from instantiating various examples of the binary logarithm class template.

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 (*i.e.*
at compile-time) available.

*New in version 1.32.0:*

The argument type and the result type of`boost::static_log2`

are now typedef'd. Formerly, they were hardcoded as`unsigned long`

and`int`

respectively. Please, use the provided typedefs in new code (and update old code as soon as possible).

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 July 19, 2004

© Copyright Daryle Walker 2001.

© Copyright Gennaro Prota 2004.