std greater on a an std pair of a double and a class

Question

When you have a std::pair of int and a class, does std::greater work?

I'm attempting to construct a priority queue of pairs that is ordered by the first element:

std::priority_queue<std::pair<double, classA>, std::vector<std::pair<double, classA>>, std::greater<std::pair<double, classA>>> priorityQueue

But I get an error message that says

"operator" has no match.

It also refers to the class type of the second element of the std::pair.

Is std::greater applied to the first and second std::pair elements?

Answer 1

std::greater is simply a wrapper for a template type operator call.

We can look at the reference site for std::pair and see that it says

Compares lhs and rhs lexicographically by operator, that is, compares the first elements and only compares the second elements if they are equivalent.

As a result, it employs the operator of both types, implying that your class type must provide it.

Since it doesn't you get the compiler error.