Question

I’m writing my own implementation of the C++ STL map container. Now I’m trying to implement the iterator. It should allow you to do something like iter->first and iter->second that returns the key/value respectively and where iter is an object not a pointer. I’m wondering how I should overload this? It’s a bit confusing because I’m not sure what the return type should be; it has to be an object with members first/second I suppose. Is it typical to return a reference to a wrapper/interface object or something like that?

Answer 1

The value_type of a standard map is std::pair<const KeyType, MappedType>.

To achieve normal pointer semantics, operator* returns a reference, whereas operator-> returns a pointer.

//minimal example
#include <utility>
#include <cstdio>

struct It
{
  std::pair<const int, int> pair;
  std::pair<const int, int>* operator->() { return &pair; }
  std::pair<const int, int>& operator*() { return pair; }
};

int main()
{
  It it = {std::make_pair(10, 20) };
  (*it).second = 30;
  std::printf("%d %d\n", it->first, it->second);
}