std::experimental::optional::value_or - cppreference.com (original) (raw)
| template< class U > constexpr T value_or( U&& default_value ) const&; | | (library fundamentals TS) | | ------------------------------------------------------------------------ | | ------------------------- | | template< class U > constexpr T value_or( U&& default_value ) &&; | | (library fundamentals TS) |
Returns the contained value if *this has a value, otherwise returns default_value.
Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value)).
Equivalent to bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(default_value)).
[edit] Parameters
| default_value | - | the value to use in case *this is empty |
|---|---|---|
| Type requirements | ||
| -T must meet the requirements of CopyConstructible in order to use overload (1). | ||
| -T must meet the requirements of MoveConstructible in order to use overload (2). | ||
| -U&& must be convertible to T. |
[edit] Return value
The current value if *this has a value, or default_value otherwise.
[edit] Exceptions
Any exception thrown by the selected constructor of the return value T.
[edit] Example
#include #include <experimental/optional> #include std::experimental::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("MYPWD").value_or("(none)") << '\n'; }
Possible output:
[edit] See also
| | returns the contained value (public member function) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |