1、說(shuō)明

This template is designed to provide compile-time constants as types.

It is used by several parts of the standard library as the base class for trait types, especially in their bool variant: see true_type and false_type.

1、實(shí)現(xiàn)分析

/// integral_constant
  template<typename _Tp, _Tp __v>
    struct integral_constant
    {
      static constexpr _Tp                  value = __v;
      typedef _Tp                           value_type;
      typedef integral_constant<_Tp, __v>   type;
      constexpr operator value_type() const noexcept { return value; }
#if __cplusplus > 201103L

#define __cpp_lib_integral_constant_callable 201304

      constexpr value_type operator()() const noexcept { return value; }
#endif
    };

  template<typename _Tp, _Tp __v>
    constexpr _Tp integral_constant<_Tp, __v>::value;

  /// The type used as a compile-time boolean with true value.
  typedef integral_constant<bool, true>     true_type;

  /// The type used as a compile-time boolean with false value.
  typedef integral_constant<bool, false>    false_type;