ct_gcd_lcm - Compile-Time GCD and LCM

Introduction

detail/ct_gcd_lcm.hpp provides two compile-time algorithms: greatest common divisor and least common multiple.

Synopsis

namespace details {
namespace pool {

template <unsigned A, unsigned B>
struct ct_gcd
{
  static const unsigned value = ...;
};
template <unsigned A, unsigned B>
struct ct_lcm
{
  static const unsigned value = ...;
};

} // namespace pool
} // namespace details

Semantics

*Symbol Table*
Symbol	Meaning
A, B	compile-time unsigned integer constants^[5.19/1]

*Semantics*
Expression	Result Type	Value	Precondition
ct_gcd<A, B>::value	compile-time unsigned integer constant	The greatest common divisor of A and B	A != 0 && B != 0
ct_lcm<A, B>::value	compile-time unsigned integer constant	The least common multiple of A and B	A != 0 && B != 0

Notes

Since these are compile-time algorithms, violation of the preconditions will result in a compile-time error.

Dependencies

<boost/static_assert.hpp> (see Boost.Static_Assert), to ensure preconditions are met.
<boost/type_traits/ice.hpp> (see Coding Guidelines for Integral Constant Expressions), to help with portability.

Selected Quotations from the Standard

5.19/1: Expressions: Constant Expressions: ". . . An integral constant expression can involve only literals (2.13), enumerators, const variables or static data members of integral or enumeration types initialized with constant expressions (8.5), non-type template parameters of integral or enumeration types, and sizeof expressions. Floating literals (2.13.3) can appear only if they are cast to integral or enumeration types. Only type conversions to integral or enumeration types can be used. In particular, except in sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call, or comma operators shall not be used."

Future Directions

This header may be replaced by a Boost compile-time algorithms library.

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