stdintegralconstant

std::integral_constant is a class template in the C++ standard library that represents a compile-time constant value as a type. It is defined as template<class T, T v> struct integral_constant { static constexpr T value = v; using value_type = T; using type = integral_constant; constexpr operator T() const noexcept { return value; } }; This construct makes a value available both as a type and as a value.

The primary purpose of std::integral_constant is to support type traits and template metaprogramming. It provides a fixed, compile-time value that can be queried by other templates, enabling decisions to be made at compile time. The value member is a static constant, and the class defines a conversion operator to its underlying type, allowing convenient use in contexts that expect a value.

std::integral_constant is the base for a family of type wrappers such as std::true_type and std::false_type, which are aliases for integral_constant<bool, true> and integral_constant<bool, false>, respectively. These wrappers allow boolean results of type traits to be represented as types, which can then be used in template specialization and SFINAE-based techniques.

In practice, std::integral_constant is used to convey constant information without runtime cost, to parameterize templates with non-type values, and to enable static assertions and conditional compilation. It resides in the <type_traits> header and, together with its common aliases, plays a central role in modern C++ metaprogramming. For example, std::integral_constant<bool, true>::value is true, and the type can be implicitly converted to bool via its conversion operator.