20 General utilities library [utilities]

20.15 Metaprogramming and type traits [meta]

20.15.3 Helper classes [meta.help]

namespace std {
  template<class T, T v> struct integral_constant {
    static constexpr T value = v;

    using value_type = T;
    using type = integral_constant<T, v>;

    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; }
  };
}
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The class template integral_­constant, alias template bool_­constant, and its associated typedef-names true_­type and false_­type are used as base classes to define the interface for various type traits.