Erase elements from multimap, based on key values?

Question

How would I erase all elements from an std::multimap who have a key less than or equal to 20?

I know how to erase, I don't know how to pass in the condition "key less than 20".

Answer 1

The next code should work:

std::multimap<int, int> M;
// initialize M here
auto it = M.upper_bound(20);
M.erase(M.begin(), it);

Just use upper_bound and then erase.