std::expected<T,E>::value_or

Primary template
template< class U = std::remove_cv_t<T> > constexpr T value_or( U&& default_value ) const&;	(1)	(since C++23)
template< class U = std::remove_cv_t<T> > constexpr T value_or( U&& default_value ) &&;	(2)	(since C++23)

Returns the expected value if it exists, otherwise returns default_value.

The void partial specialization does not have these member functions.

1) If std::is_copy_constructible_v<T> or std::is_convertible_v<U, T> is false, the program is ill-formed.

2) If std::is_move_constructible_v<T> or std::is_convertible_v<U, T> is false, the program is ill-formed.

Parameters

default_value - the value to use in case *this does not contain an expected value

Return value

1) has_value() ? **this : static_cast<T>(std::forward<U>(default_value))

2) has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(default_value))

Example

Can be checked out on Compiler Explorer.

#include <cstdlib>
#include <expected>
#include <print>
#include <system_error>
#include <utility>

std::expected<const char*, std::errc> try_getenv(const char* name)
{
    if (const char* env{std::getenv(name)})
        return env;
    return std::unexpected{std::errc::function_not_supported};
}

int main()
{
    for (auto env : {"SHELL", "OS"})
        std::println("{}: {}", env, try_getenv(env).value_or("(unknown)"));
}

Possible output:

SHELL: /usr/bin/bash
OS: (unknown)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3886	C++23	`U` does not have a default template argument	specified

std::expected<T,E>::value_or

Parameters

Return value

Example

Defect reports

See also