std::optional<T>::value_or
template< class U = std::remove_cv_t<T> > constexpr T value_or( U&& default_value ) const&; | (1) | (since C++17) |
template< class U = std::remove_cv_t<T> > constexpr T value_or( U&& default_value ) &&; | (2) | (since C++17) |
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
| default_value | - | the value to be returned if *this does not contain a 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
#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';
}
Possible output:
/usr/bin/zsh (none)
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++17 |
U does not have a default template argument | specified |
See also
| returns the contained value (public member function) |
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https://en.cppreference.com/cpp/utility/optional/value_or
