Maps, lookup, and accidental insertion

The idea

std::map stores unique keys in comparator order and supports logarithmic lookup. std::unordered_map uses hashing and does not promise sorted iteration. Pick the container according to the operations and ordering your interface requires, rather than assuming that every dictionary iteration has the same order.

The indexing operator on a map is convenient for accumulating counts because a missing mapped integer is value-initialized to zero. That same insertion is a bug when a read-only existence check must leave the map unchanged. Use find and compare the iterator with end for a noninserting lookup. Never dereference the end iterator. An explicit lookup policy also keeps a missing key distinct from a present key whose value is zero.

#include <iostream>
#include <map>
#include <string>
int main() {
  std::map<std::string, int> counts;
  ++counts["pear"]; ++counts["pear"];
  auto it = counts.find("apple");
  std::cout << counts.size() << ' ' << (it == counts.end());
}

1 1

find does not insert apple, so the map still contains one key.