std::optional::value_or(3)

NAME
std::optional::value_or - std::optional::value_or

Synopsis
   template< class U > (1) (since C++17)
   constexpr T value_or( U&& default_value ) const&;
   template< class U > (2) (since C++17)
   constexpr T value_or( U&& default_value ) &&;
   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
   value returns the contained value
         (public member function)