std::optional<T>::value_or
From cppreference.com
template< class U > constexpr T value_or( U&& default_value ) const&; |
(1) | (since C++17) |
template< class U > constexpr T value_or( U&& default_value ) &&; |
(2) | (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)).
Contents |
[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
Run this code
#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)
[edit] See also
returns the contained value (public member function) |