std::optional::value_or - cppreference.com (original) (raw)
| 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.
[edit] Parameters
| default_value | - | the value to be returned if *this does not contain a value |
|---|
[edit] Return value
has_value() ? **this : static_cast<T>(std::forward<U>(default_value));
has_value() ? std::move(**this) : static_cast<T>(std::forward<U>(default_value))
[edit] Example
#include #include #include 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:
[edit] 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 |
[edit] See also
| | returns the contained value (public member function) [edit] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |