std::optional<T>::value_or

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std::optional
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) )

Parameters

default_value - the value to use in case *this is empty
Type requirements
-
T must meet the requirements of CopyConstructible
-
T must meet the requirements of MoveConstructible
-
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 <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)

See also

 returns the contained value
(public member function)
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