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Showing content from https://cplusplus.com/reference/type_traits/integral_constant/ below:

class template

<type_traits>

template <class T, T v>struct integral_constant;

Integral constant

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.

Its definition in the Standard Library has the same behavior as:

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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; }
};

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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() const noexcept { return v; }
  constexpr T operator()() const noexcept { return v; }
};

T

Type of the integral constant.
Aliased as member type integral_constant::value_type.

v

Value of the integral constant.
Accessible as member integral_constant::value, or through type casting.

operator value_type

Returns value (public member function)

true_type

True type (class)

false_type

False type (class)

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// 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;
}

true_type

True type (class)

false_type

False type (class)

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