C++

Reference

<type_traits>

integral_constant

class template
<type_traits>

std::integral_constant

template <class T, T v>struct integral_constant;
Integral constant
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.

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; } };

Template parameters

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.

Member types

member typedefinition
value_typeThe type of the constant (template parameter T)
typeThe integral_constant type itself

Member functions

operator value_type
Returns value (public member function)

Instantiations

true_type
True type (class)
false_type
False type (class)

Example

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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; }

Output:
120


See also

true_type
True type (class)
false_type
False type (class)