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Doc. no.:N4389
Date:2015-02-23
Project:Programming Language C++, Library Working Group
Revises:N4334
Reply-to:Zhihao Yuan <zy at miator dot net>

Wording for bool_constant, revision 1

Rationale seehttps://issues.isocpp.org/show_bug.cgi?id=51.

This paper consolidatesN4334 with LWG’s wording improvements to<ratio>.

This wording is relative to N4296.

Modify 20.10.2 [meta.type.synop]:

namespace std {  // 20.10.3, helper class:  template <class T, T v> struct integral_constant;
template <bool B>
using bool_constant = integral_constant<bool, B>;
 
 typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;
 typedef bool_constant<true> true_type;
typedef bool_constant<false> false_type;

…

Modify 20.10.3 [meta.help]:

namespace std {  template <class T, T v>  struct integral_constant {

…

  };
typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;
}

The class templateintegral_constant, alias templatebool_constant, and its associated typedefstrue_type andfalse_type are used as base classes to define the interface for various type traits.

Modify Table 49 in 20.10.4.3 [meta.unary.prop]:

template <class T>
struct is_signed;
Ifis_arithmetic<T>::value istrue, the same result asintegral_constant<bool, T(-1) < T(0)>::valuebool_constant<T(-1) < T(0)>::value; otherwise,false
template <class T>
struct is_unsigned;
Ifis_arithmetic<T>::value istrue, the same result asintegral_constant<bool, T(0) < T(-1)>::valuebool_constant<T(0) < T(-1)>::value; otherwise,false

Modify 20.11.5 [ratio.comparison]:

template <class R1, class R2> struct ratio_equal
: integral_constant<bool,see below> { };
 : bool_constant<R1::num == R2::num &&R1::den == R2::den> { };
IfR1::num == R2::num andR1::den == R2::den,ratio_equal<R1, R2>shall be derived fromintegral_constant<bool, true>; otherwise it shall be derived fromintegral_constant<bool, false>.
template <class R1, class R2> struct ratio_not_equal
: integral_constant<bool, !ratio_equal<R1, R2>::value> { };
 : bool_constant<!ratio_equal<R1, R2>::value> { };
template <class R1, class R2> struct ratio_less
: integral_constant<bool,see below> { };
 : bool_constant<see below> { };

IfR1::num * R2::den < R2::num * R1::denR1::num ×R2::den is less thanR2::num ×R1::den ,ratio_less<R1, R2> shall be derived fromintegral_constant<bool, true>bool_constant<true> ; otherwise it shall be derived fromintegral_constant<bool, false>bool_constant<false> . Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed.

template <class R1, class R2> struct ratio_less_equal
: integral_constant<bool, !ratio_less<R2, R1>::value> { };
template <class R1, class R2> struct ratio_greater
: integral_constant<bool, ratio_less<R2, R1>::value> { };
template <class R1, class R2> struct ratio_greater_equal
: integral_constant<bool, !ratio_less<R1, R2>::value> { };
 template <class R1, class R2> struct ratio_less_equal
: bool_constant<!ratio_less<R2, R1>::value> { };
template <class R1, class R2> struct ratio_greater
: bool_constant<ratio_less<R2, R1>::value> { };
template <class R1, class R2> struct ratio_greater_equal
: bool_constant<!ratio_less<R1, R2>::value> { };

Acknowledgments

Thanks Tony Van Eerd for bringing this to the reflector.

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