std::optional<T>::value_or
From cppreference.com
| template< class U > constexpr T value_or( U&& default_value ) const&; |
(since C++17) | |
| template< class U > constexpr T value_or( U&& default_value ) &&; |
(since C++17) | |
Returns the contained value if *this has a value, otherwise returns default_value.
1) Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value))
2) Equivalent to bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(default_value))
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
| ||
Return value
The current value if *this has a value, or default_value otherwise.
Exceptions
Any exception thrown by the selected constructor of the return value T.
Example
Run this code
#include <optional> #include <iostream> #include <cstdlib> 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("MYPWD").value_or("(none)") << '\n'; }
Possible output:
(none)
See also
| returns the contained value (public member function) |