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std::unexpected - cppreference.com

template< class E >
class unexpected;

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 object that contains an unexpected value.

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.

(destructor)

(implicitly declared)

operator=

(implicitly declared)

constexpr unexpected( const unexpected& ) = default;

constexpr unexpected( unexpected&& ) = default;

template< class Err = E >
constexpr explicit unexpected( Err&& e );

Constructs a std::unexpected object.

1,2) Copy/move constructor. Copies or moves the stored value, respectively.

Throws any exception thrown by the constructor of E.

Returns a reference to the stored value.

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> is false.

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, the program is ill-formed.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::unexpected<E> is an associated class of the arguments.

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> is true.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::unexpected<E> is an associated class of the arguments.

template< class E >
unexpected(E) -> unexpected<E>;

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

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

#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

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