Returns the contained value if *this contains a value, otherwise returns default_value.

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

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

Parameters

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

#include <cstdlib>
#include <iostream>
#include <optional>

std::optional<const char*> maybe_getenv(const char* n)
{
    if (const char* x = std::getenv(n))
        return x;
    else
        return {};
}

int main()
{
    std::cout << maybe_getenv("SHELL").value_or("(none)") << '\n';
    std::cout << maybe_getenv("MYPWD").value_or("(none)") << '\n';
}

Defect reports

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

See also