Pair as a key in map for Memoization

Question

I want to you use unordered_map for Memoization on a function f(row, int) . Unfortunately, I get a weird compilation error (very long and cryptic).

#include <vector>
#include <unordered_map>
#include <utility>


using namespace std;

typedef vector<bool> row;

int main(void) {

    unordered_map< pair<int, row>, int > x;

}

Answer 1

The key type for std::unordered_map needs to have an implementation of std::hash , I'd guess your error is telling you that std::pair<int, row> does not have an std::hash implementation. I don't think the standard specifies a specialization of std::hash for std::pair so you'll need to provide your own.