22 General utilities library [utilities]

22.2 Utility components [utility]

22.2.10 Class template constant_wrapper [const.wrap.class]

namespace std { struct cw-operators { // exposition only // unary operators template<constexpr-param T> friend constexpr auto operator+(T) noexcept -> constant_wrapper<(+T::value)> { return {}; } template<constexpr-param T> friend constexpr auto operator-(T) noexcept -> constant_wrapper<(-T::value)> { return {}; } template<constexpr-param T> friend constexpr auto operator~(T) noexcept -> constant_wrapper<(~T::value)> { return {}; } template<constexpr-param T> friend constexpr auto operator!(T) noexcept -> constant_wrapper<(!T::value)> { return {}; } template<constexpr-param T> friend constexpr auto operator&(T) noexcept -> constant_wrapper<(&T::value)> { return {}; } template<constexpr-param T> friend constexpr auto operator*(T) noexcept -> constant_wrapper<(*T::value)> { return {}; } // binary operators template<constexpr-param L, constexpr-param R> friend constexpr auto operator+(L, R) noexcept -> constant_wrapper<(L::value + R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator-(L, R) noexcept -> constant_wrapper<(L::value - R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator*(L, R) noexcept -> constant_wrapper<(L::value * R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator/(L, R) noexcept -> constant_wrapper<(L::value / R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator%(L, R) noexcept -> constant_wrapper<(L::value % R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator<<(L, R) noexcept -> constant_wrapper<(L::value << R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator>>(L, R) noexcept -> constant_wrapper<(L::value >> R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator&(L, R) noexcept -> constant_wrapper<(L::value & R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator|(L, R) noexcept -> constant_wrapper<(L::value | R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator^(L, R) noexcept -> constant_wrapper<(L::value ^ R::value)> { return {}; } template<constexpr-param L, constexpr-param R> requires (!is_constructible_v<bool, decltype(L::value)> || !is_constructible_v<bool, decltype(R::value)>) friend constexpr auto operator&&(L, R) noexcept -> constant_wrapper<(L::value && R::value)> { return {}; } template<constexpr-param L, constexpr-param R> requires (!is_constructible_v<bool, decltype(L::value)> || !is_constructible_v<bool, decltype(R::value)>) friend constexpr auto operator||(L, R) noexcept -> constant_wrapper<(L::value || R::value)> { return {}; } // comparisons template<constexpr-param L, constexpr-param R> friend constexpr auto operator<=>(L, R) noexcept -> constant_wrapper<(L::value <=> R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator<(L, R) noexcept -> constant_wrapper<(L::value < R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator<=(L, R) noexcept -> constant_wrapper<(L::value <= R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator==(L, R) noexcept -> constant_wrapper<(L::value == R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator!=(L, R) noexcept -> constant_wrapper<(L::value != R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator>(L, R) noexcept -> constant_wrapper<(L::value > R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator>=(L, R) noexcept -> constant_wrapper<(L::value >= R::value)> { return {}; } template<constexpr-param L, constexpr-param R> friend constexpr auto operator,(L, R) noexcept = delete; template<constexpr-param L, constexpr-param R> friend constexpr auto operator->*(L, R) noexcept -> constant_wrapper<L::value->*(R::value)> { return {}; } // pseudo-mutators template<constexpr-param T> constexpr auto operator++(this T) noexcept -> constant_wrapper<(++T::value)> { return {}; } template<constexpr-param T> constexpr auto operator++(this T, int) noexcept -> constant_wrapper<(T::value++)> { return {}; } template<constexpr-param T> constexpr auto operator--(this T) noexcept -> constant_wrapper<(--T::value)> { return {}; } template<constexpr-param T> constexpr auto operator--(this T, int) noexcept -> constant_wrapper<(T::value--)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator+=(this T, R) noexcept -> constant_wrapper<(T::value += R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator-=(this T, R) noexcept -> constant_wrapper<(T::value -= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator*=(this T, R) noexcept -> constant_wrapper<(T::value *= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator/=(this T, R) noexcept -> constant_wrapper<(T::value /= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator%=(this T, R) noexcept -> constant_wrapper<(T::value %= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator&=(this T, R) noexcept -> constant_wrapper<(T::value &= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator|=(this T, R) noexcept -> constant_wrapper<(T::value |= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator^=(this T, R) noexcept -> constant_wrapper<(T::value ^= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator<<=(this T, R) noexcept -> constant_wrapper<(T::value <<= R::value)> { return {}; } template<constexpr-param T, constexpr-param R> constexpr auto operator>>=(this T, R) noexcept -> constant_wrapper<(T::value >>= R::value)> { return {}; } }; template<auto X, class T = decltype(X)> struct constant_wrapper : cw-operators { static constexpr decltype(auto) value = (X); using type = constant_wrapper; using value_type = decltype(X); template<constexpr-param R> constexpr auto operator=(R) const noexcept -> constant_wrapper<(value = R::value)> { return {}; } constexpr operator decltype(value)() const noexcept { return value; } template<class... Args> static constexpr decltype(auto) operator()(Args&&... args) noexcept(see below); template<class... Args> static constexpr decltype(auto) operator[](Args&&... args) noexcept(see below); }; }
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The class template constant_wrapper aids in metaprogramming by ensuring that the evaluation of expressions comprised entirely of constant_wrapper are core constant expressions ([expr.const.core]), regardless of the context in which they appear.
In particular, this enables use of constant_wrapper values that are passed as arguments to constexpr functions to be used in constant expressions.
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If a specialization of constant_wrapper is instantiated with a type T such that is_same_v<T, value_type> is false, the program is ill-formed.
[Note 1: 
The second template parameter to constant_wrapper is present to aid argument-dependent lookup ([basic.lookup.argdep]) in finding overloads for which constant_wrapper's wrapped value is a suitable argument, but for which the constant_wrapper itself is not.
— end note]
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[Example 1:  constexpr auto initial_phase(auto quantity_1, auto quantity_2) { return quantity_1 + quantity_2; } constexpr auto middle_phase(auto tbd) { return tbd; } void final_phase(auto gathered, auto available) { if constexpr (gathered == available) std::cout << "Profit!\n"; } void impeccable_underground_planning() { auto gathered_quantity = middle_phase(initial_phase(std::cw<42>, std::cw<13>)); static_assert(gathered_quantity == 55); auto all_available = std::cw<55>; final_phase(gathered_quantity, all_available); } void deeply_flawed_underground_planning() { constexpr auto gathered_quantity = middle_phase(initial_phase(42, 13)); constexpr auto all_available = 55; final_phase(gathered_quantity, all_available); // error: gathered == available // is not a constant expression } — end example]
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template<class... Args> static constexpr decltype(auto) operator()(Args&&... args) noexcept(see below);
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Let call-expr be constant_wrapper<INVOKE(value, remove_cvref_t<Args>​::​value...)>{} if all types in remove_cvref_t<Args>... satisfy constexpr-param and constant_wrapper<INVOKE(value, remove_cvref_t<Args>​::​value...)> is a valid type, otherwise let call-expr be INVOKE(value, std​::​forward<Args>(args)...).
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Constraints: call-expr is a valid expression.
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Effects: Equivalent to: return call-expr;
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Remarks: The exception specification is equivalent to noexcept(call-expr).
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template<class... Args> static constexpr decltype(auto) operator[](Args&&... args) noexcept(see below);
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Let subscr-expr be constant_wrapper<value[remove_cvref_t<Args>​::​value...]>{} if all types in remove_cvref_t<Args>... satisfy constexpr-param and constant_wrapper<value[remove_cvref_t<Args>​::​value...]> is a valid type, otherwise let subscr-expr be value[std​::​forward<Args​>(args)...].
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Constraints: subscr-expr is a valid expression.
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Effects: Equivalent to: return subscr-expr;
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Remarks: The exception specification is equivalent to noexcept(subscr-expr).