integral_constant

定义 ¹

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

template <class T, T v>
struct integral_constant;

实现

template <class T, T v>
struct integral_constant {
  static constexpr T value = v;
  typedef T value_type;
  typedef integral_constant<T,v> type;
  constexpr operator T() { return v; }
};

应用: 编译器求值

// factorial as an integral_constant
#include <iostream>
#include <type_traits>

template <unsigned n>
struct factorial : std::integral_constant<int,n * factorial<n-1>::value> {};

template <>
struct factorial<0> : std::integral_constant<int,1> {};

int main() {
  std::cout << factorial<5>::value;  // constexpr (no calculations on runtime)
  return 0;
}

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1. integral_constant . ↩