Check if pair is empty or uninitialized

Question

 int main() {

   pair<int, int> res;
   // check here pair is empty ?
   // res = make_pair(1 , 2);


   return 0;
}

In the above snippet what is the ideal way to check if a pair has been initilaized or not?

EDIT: As pointed out in some answers below, using the word "uninitialized" is wrong here, to be more specific how do I check if the value has been explicitly set (other than the default constructor)

Answer 1

There's no such thing as unitialized std::pair.

Default constructor for std::pair will value-initialize both it's members, which for pair of ints means both int will have value 0.

Answer 2

In the above snippet what is the ideal way to check if a pair has been initilaized or not?

You check your favorite language reference and realize that

• std::pair requires default-constructible types and

• the std::pair default constructor default-initializes its data members.

And hence,

std::pair<int, int> res;

will result in res.first and res.second both be zero.

---

When you need to add an additional empty state to an object, it's always an option to explicitly wrap it into a std::optional (or boost::optional if you can't use C++17). This way, you can always check whether you have a meaningful std::pair or not:

#include <optional>

std::optional<std::pair<int, int>> res;

if (res) // the pair is initialized and usable
   doStuff(*res);
else // ... it's not, hence initialize it
   res = std::make_pair(42, 43);