namespace std {
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 value_type() { return value; }
};
typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;
}
The class template integral_constant and its associated typedefs true_type and false_type are used as base classes to define the interface for various type traits.