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

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

# Declarations

# Primary template

template< class U >
constexpr T value_or( U&& default_value ) const&;

(since C++23)

template< class U >
constexpr T value_or( U&& default_value ) &&;

(since C++23)

# Parameters

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

# Return value

1. For the const& overload, has_value() ? **this : static_cast<T>(std::forward<U>(default_value)).
2. For the && overload, has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(default_value)).

# Notes

These member functions exist only on the primary template. std::expected<void, E> does not provide value_or().

1. The const& overload requires that T is copy-constructible and that U is convertible to T; otherwise the program is ill-formed.
2. The && overload requires that T is move-constructible and that U is convertible to T; otherwise the program is ill-formed.

# Example

#include <expected>
#include <iostream>
#include <string>

std::expected<int, std::string> parse_port(bool ok)
{
    if (ok)
        return 8080;
    return std::unexpected("invalid configuration");
}

int main()
{
    std::cout << parse_port(true).value_or(80) << '\n';
    std::cout << parse_port(false).value_or(80) << '\n';
}

# See also

• value