[expected.void.general]

# 22 General utilities library [[utilities]](./#utilities)

## 22.8 Expected objects [[expected]](expected#void.general)

### 22.8.7 Partial specialization of expected for void types [[expected.void]](expected.void#general)

#### 22.8.7.1 General [expected.void.general]

template<class T, class E> requires is_void_v<T>class expected<T, E> {public:using [value_type](#lib:expected%3cvoid%3e,value_type "22.8.7.1 General [expected.void.general]") = T; using [error_type](#lib:expected%3cvoid%3e,error_type "22.8.7.1 General [expected.void.general]") = E; using [unexpected_type](#lib:expected%3cvoid%3e,unexpected_type "22.8.7.1 General [expected.void.general]") = unexpected<E>; template<class U>using [rebind](#lib:expected%3cvoid%3e,rebind "22.8.7.1 General [expected.void.general]") = expected<U, error_type>; // [[expected.void.cons]](expected.void.cons "22.8.7.2 Constructors"), constructorsconstexpr expected() noexcept; constexpr expected(const expected&); constexpr expected(expected&&) noexcept(*see below*); template<class U, class G>constexpr explicit(*see below*) expected(const expected<U, G>&); template<class U, class G>constexpr explicit(*see below*) expected(expected<U, G>&&); template<class G>constexpr explicit(*see below*) expected(const unexpected<G>&); template<class G>constexpr explicit(*see below*) expected(unexpected<G>&&); constexpr explicit expected(in_place_t) noexcept; template<class... Args>constexpr explicit expected(unexpect_t, Args&&...); template<class U, class... Args>constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...); // [[expected.void.dtor]](expected.void.dtor "22.8.7.3 Destructor"), destructorconstexpr ~expected(); // [[expected.void.assign]](expected.void.assign "22.8.7.4 Assignment"), assignmentconstexpr expected& operator=(const expected&); constexpr expected& operator=(expected&&) noexcept(*see below*); template<class G>constexpr expected& operator=(const unexpected<G>&); template<class G>constexpr expected& operator=(unexpected<G>&&); constexpr void emplace() noexcept; // [[expected.void.swap]](expected.void.swap "22.8.7.5 Swap"), swapconstexpr void swap(expected&) noexcept(*see below*); friend constexpr void swap(expected& x, expected& y) noexcept(noexcept(x.swap(y))); // [[expected.void.obs]](expected.void.obs "22.8.7.6 Observers"), observersconstexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr void operator*() const noexcept; constexpr void value() const &; // freestanding-deletedconstexpr void value() &&; // freestanding-deletedconstexpr const E& error() const & noexcept; constexpr E& error() & noexcept; constexpr const E&& error() const && noexcept; constexpr E&& error() && noexcept; template<class G = E> constexpr E error_or(G&&) const &; template<class G = E> constexpr E error_or(G&&) &&; // [[expected.void.monadic]](expected.void.monadic "22.8.7.7 Monadic operations"), monadic operationstemplate<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &; template<class F> constexpr auto and_then(F&& f) const &&; template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &; template<class F> constexpr auto or_else(F&& f) const &&; template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &; template<class F> constexpr auto transform(F&& f) const &&; template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &; template<class F> constexpr auto transform_error(F&& f) const &&; // [[expected.void.eq]](expected.void.eq "22.8.7.8 Equality operators"), equality operatorstemplate<class T2, class E2> requires is_void_v<T2>friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y); template<class E2>friend constexpr bool operator==(const expected&, const unexpected<E2>&);

private:bool *has_val*; // *exposition only*union { E *unex*; // *exposition only*};};

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L9513)

Any object of type expected<T, E> either
represents a value of type T, or
contains a value of type E nested within ([[intro.object]](intro.object "6.8.2 Object model")) it[.](#1.sentence-1)

Member *has_val* indicates whether the expected<T, E> object
represents a value of type T[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L9521)

A program that instantiates
the definition of the template expected<T, E> with
a type for the E parameter that
is not a valid template argument for unexpected is ill-formed[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L9527)

E shall meet the requirements of[*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [35](utility.arg.requirements#tab:cpp17.destructible "Table 35: Cpp17Destructible requirements"))[.](#3.sentence-1)