std::unexpected

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std::expected
Defined in header <expected>
template< class E >
class unexpected;
(since C++23)

The class template std::unexpected represents an unexpected value stored in std::expected. In particular, std::expected has constructors with std::unexpected as a single argument, which creates an expected

A program is ill-formed if it instantiates an unexpected with a non-object type, an array type, a specialization of std::unexpected, or a cv-qualified type.

Template parameters

E - the type of the unexpected value. The type must not be an array type, a non-object type, a specialization of std::unexpected, or a cv-qualified type.

Member functions

 constructs the unexpected object
(public member function)
(destructor)
(implicitly declared)
 destroys the unexpected object, along with the stored value
(public member function)
operator=
(implicitly declared)
 assigns the stored value
(public member function)
accesses the stored value
(public member function)
swaps the stored value
(public member function)

Non-member functions

(C++23)
 compares the stored value
(function template)
(C++23)
 specializes the std::swap algorithm
(function template)

std::unexpected::unexpected

constexpr unexpected( const unexpected& ) = default ;
(1)
constexpr unexpected( unexpected&& ) = default ;
(2)
template < class Err = E >
constexpr explicit unexpected( Err&& e ) ;
(3)
template < class... Args >
constexpr explicit unexpected( std::in_place_t, Args&&... args ) ;
(4)
template < class U, class... Args >

constexpr explicit unexpected( std::in_place_t,

std::initializer_list <U> il, Args&&... args ) ;
(5)

Constructs a std::unexpected object.

1,2) Copy/move constructor. Copies or moves the stored value, respectively.
3) Constructs the stored value, as if by direct-initializing a value of type E from std::forward <Err> (e) .
4) Constructs the stored value, as if by direct-initializing a value of type E from the arguments std::forward <Args> (args) .
5) Constructs the stored value, as if by direct-initializing a value of type E from the arguments il, std::forward <Args> (args) .

Parameters

e - value with which to initialize the contained value
args... - arguments with which to initialize the contained value
il - initializer list with which to initialize the contained value

Exceptions

Throws any exception thrown by the constructor of E.

std::unexpected::error

constexpr const E& error( ) const & noexcept ;

constexpr E& error( ) & noexcept ;
constexpr const E&& error( ) const && noexcept ;

constexpr E&& error( ) && noexcept ;

Returns a reference to the stored value.

std::unexpected::swap

constexpr void swap( unexpected& other ) noexcept ( std::is_nothrow_swappable_v <E> ) ;

Swaps the stored values, as if by using std::swap ; swap(error( ), other.error ( ) ) ;

The program is ill-formed if std::is_swappable_v<E>

operator==(std::unexpected)

template < class E2 >
friend constexpr bool operator== ( unexpected& x, std:: unexpected <E2> & y ) ;

Compares the stored values, as if by return x.error ( ) == y.error ( )

If the expression x.error() == e.error() is not well-formed, or if its result is not convertible to bool

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup

swap(std::unexpected)

friend constexpr void
swap( unexpected& x, unexpected& y ) noexcept ( noexcept (x.swap (y) ) ) ;

Equivalent to x.swap(y).

This overload participates in overload resolution only if std::is_swappable_v<E>

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup

Deduction guides

template < class E >
unexpected(E) - > unexpected<E> ;
(since C++23)

The deduction guide is provided for unexpected to allow deduction from the constructor argument.

Notes

Prior to C++17, the name std::unexpected denoted the function called by the C++ runtime when a dynamic exception specification was violated.

Example

Run this code
#include <expected>
#include <iostream>
 
enum class error
{
    compile_time_error,
    runtime_error
};
 
[[nodiscard]] auto unexpected_runtime_error() -> std::expected<int, error>
{
    return std::unexpected(error::runtime_error);
}
 
int main()
{
    std::expected<double, int> ex = std::unexpected(3);
 
    if (!ex)
        std::cout << "ex contains an error value\n";
 
    if (ex == std::unexpected(3))
        std::cout << "The error value is equal to 3\n";
 
    const auto e = unexpected_runtime_error();
 
    e.and_then([](const auto& e) -> std::expected<int, error>
    {
        std::cout << "and_then: " << int(e); // not printed
        return {};
    })
    .or_else([](const auto& e) -> std::expected<int, error>
    {
        std::cout << "or_else: " << int(e); // prints this line
        return {};
    });
}

Output: 

ex contains an error value
The error value is equal to 3
or_else: 1

See also

 constructs the expected object
(public member function)
(C++23)
 compares expected objects
(function template)
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