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std::optional<T>::value_or

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

#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)

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