How can I use a std::map iterator inside a templated class?

Question

I am having trouble using a private std::map variable from within a class template. Can anyone explain why the following (simplified example) does not work, and what I should be doing instead?

#include <iostream>
#include <map>

template <class T>
class Container
{
    private:
        typedef std::map<long, T> Map;
        typedef Map::iterator Iterator; // <-- this is line 10
        Map items;
    public:
        void insert(const long id, const T& item) { items[id] = item; }
        void print()
        {
            for (Iterator iter = items.begin(); iter != items.end(); iter++)
            { std::cout << iter->second << std::endl; }
        }
};

int main()
{
    Container<int> container;

    container.insert(300, 1);
    container.insert(200, 2);
    container.insert(100, 3);

    container.print();

    return 0;
}

The resulting error is somewhat cryptic:

t.cpp:10: error: type 'std::map, std::allocator > >' is not derived from type 'Container'

t.cpp:10: error: ISO C++ forbids declaration of 'iterator' with no type

t.cpp:10: error: expected ';' before "Iterator"

Answer 1

You need to qualify the dependant typename:

    typedef typename Map::iterator Iterator;

Rationale: the compiler, at the time of parsing the template, cannot work out whether is a type or a value expression. This is because the names depend on the template argument(s), which are only known at instantiation time.

You have to help the compiler out.

---

Sidenotes:

• MSVC seems to be lax with this rule, as it's template instantiation engine behaves in non-standard ways, and the name lookups aren't done until instantiation time anyway

• sometime, dependent member templates need to be qualified as well:

  template <typename aTuple> struct ContrivedSample
  {
      aTuple data;
  
      void foo()
      {
         auto v = data.template get<0>(); // template required
      }
  };