[const.wrap.class]

# 21 Metaprogramming library [[meta]](./#meta)

## 21.3 Metaprogramming and type traits [[type.traits]](type.traits#const.wrap.class)

### 21.3.5 Class template constant_wrapper [const.wrap.class]

template<class T>struct *cw-fixed-value* { // *exposition only*using *type* = T; // *exposition only*constexpr *cw-fixed-value*(*type* v) noexcept : *data*(v) {} T *data*; // *exposition only*};

template<class T, size_t Extent>struct *cw-fixed-value*<T[Extent]> { // *exposition only*using *type* = T[Extent]; // *exposition only*constexpr *cw-fixed-value*(T (&arr)[Extent]) noexcept;
 T *data*[Extent]; // *exposition only*};

template<class T, size_t Extent>*cw-fixed-value*(T (&)[Extent]) -> *cw-fixed-value*<T[Extent]>; // *exposition only*struct *cw-operators* { // *exposition only*// unary operatorstemplate<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator+(T) noexcept -> constant_wrapper<(+T::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator-(T) noexcept -> constant_wrapper<(-T::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator~(T) noexcept -> constant_wrapper<(~T::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator!(T) noexcept -> constant_wrapper<(!T::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator&(T) noexcept -> constant_wrapper<(&T::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>friend constexpr auto operator*(T) noexcept -> constant_wrapper<(*T::value)>{ return {}; }// binary operatorstemplate<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator+(L, R) noexcept -> constant_wrapper<(L::value + R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator-(L, R) noexcept -> constant_wrapper<(L::value - R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator*(L, R) noexcept -> constant_wrapper<(L::value * R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator/(L, R) noexcept -> constant_wrapper<(L::value / R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator%(L, R) noexcept -> constant_wrapper<(L::value % R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator<<(L, R) noexcept -> constant_wrapper<(L::value << R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator>>(L, R) noexcept -> constant_wrapper<(L::value >> R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator&(L, R) noexcept -> constant_wrapper<(L::value & R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator|(L, R) noexcept -> constant_wrapper<(L::value | R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator^(L, R) noexcept -> constant_wrapper<(L::value ^ R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") 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*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") 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 {}; }// comparisonstemplate<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator<=>(L, R) noexcept -> constant_wrapper<(L::value <=> R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator<(L, R) noexcept -> constant_wrapper<(L::value < R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator<=(L, R) noexcept -> constant_wrapper<(L::value <= R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator==(L, R) noexcept -> constant_wrapper<(L::value == R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator!=(L, R) noexcept -> constant_wrapper<(L::value != R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator>(L, R) noexcept -> constant_wrapper<(L::value > R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator>=(L, R) noexcept -> constant_wrapper<(L::value >= R::value)>{ return {}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator,(L, R) noexcept = delete; template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") L, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>friend constexpr auto operator->*(L, R) noexcept -> constant_wrapper<L::value->*(R::value)>{ return {}; }// call and indextemplate<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]")... Args>constexpr auto operator()(this T, Args...) noexceptrequires requires(Args...) { constant_wrapper<T::value(Args::value...)>(); }{ return constant_wrapper<T::value(Args::value...)>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]")... Args>constexpr auto operator[](this T, Args...) noexcept-> constant_wrapper<(T::value[Args::value...])>{ return {}; }// pseudo-mutatorstemplate<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>constexpr auto operator++(this T) noexceptrequires requires(T::value_type x) { ++x; }{ return constant_wrapper<[] { auto c = T::value; return ++c; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>constexpr auto operator++(this T, int) noexceptrequires requires(T::value_type x) { x++; }{ return constant_wrapper<[] { auto c = T::value; return c++; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>constexpr auto operator--(this T) noexceptrequires requires(T::value_type x) { --x; }{ return constant_wrapper<[] { auto c = T::value; return --c; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T>constexpr auto operator--(this T, int) noexceptrequires requires(T::value_type x) { x--; }{ return constant_wrapper<[] { auto c = T::value; return c--; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator+=(this T, R) noexceptrequires requires(T::value_type x) { x += R::value; }{ return constant_wrapper<[] { auto v = T::value; return v += R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator-=(this T, R) noexceptrequires requires(T::value_type x) { x -= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v -= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator*=(this T, R) noexceptrequires requires(T::value_type x) { x *= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v *= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator/=(this T, R) noexceptrequires requires(T::value_type x) { x /= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v /= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator%=(this T, R) noexceptrequires requires(T::value_type x) { x %= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v %= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator&=(this T, R) noexceptrequires requires(T::value_type x) { x &= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v &= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator|=(this T, R) noexceptrequires requires(T::value_type x) { x |= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v |= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator^=(this T, R) noexceptrequires requires(T::value_type x) { x ^= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v ^= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator<<=(this T, R) noexceptrequires requires(T::value_type x) { x <<= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v <<= R::value; }()>{}; }template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") T, [*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator>>=(this T, R) noexceptrequires requires(T::value_type x) { x >>= R::value; }{ return constant_wrapper<[] { auto v = T::value; return v >>= R::value; }()>{}; }};

template<*cw-fixed-value* X, class>struct constant_wrapper : cw-operators {static constexpr const auto & value = X.*data*; using type = constant_wrapper; using value_type = typename decltype(X)::*type*; template<[*constexpr-param*](meta.type.synop#concept:constexpr-param "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") R>constexpr auto operator=(R) const noexceptrequires requires(value_type x) { x = R::value; }{ return constant_wrapper<[] { auto v = value; return v = R::value; }()>{}; }constexpr operator decltype(auto)() const noexcept { return value; }};

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L851)

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]](expr.const "7.7 Constant expressions")),
regardless of the context in which they appear[.](#1.sentence-1)

In particular, this enables use of constant_wrapper values
that are passed as arguments to constexpr functions to be used in constant expressions[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L859)

[*Note [1](#note-1)*:

The unnamed second template parameter to constant_wrapper is present
to aid argument-dependent lookup ([[basic.lookup.argdep]](basic.lookup.argdep "6.5.4 Argument-dependent name lookup"))
in finding overloads for which constant_wrapper's wrapped value is a suitable argument,
but for which the constant_wrapper itself is not[.](#2.sentence-1)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L867)

[*Example [1](#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*]

[🔗](#itemdecl:1)

`constexpr cw-fixed-value(T (&arr)[Extent]) noexcept;
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L904)

*Effects*: Initialize elements of *data* with corresponding elements of arr[.](#4.sentence-1)