Erasing in map in a loop. if you run this code: Consider 'mp' as map<int,int> for ( auto & it : mp) { a.erase(it.first); } This would give you a runtime error as follows: terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc This is because when you are erasing it.first from a map and then in the loop you are doing it++ and it do not exist after erasing. So, this loop will work instead: auto it = a.begin(); while (it != a.end()) { it = a.erase(it); }