Question

The folowing code generates a syntax error at the line where the iterator is declared:

template <typename T> class A {   public:      struct B     {        int x, y, z;     };      void a()     {         std::map<int, B>::const_iterator itr; // error: ; expected before itr     }      std::vector<T> v;     std::map<int, B> m; }; 

This only happens when A is a templated class. What’s wrong with this code? If I move B out of A, the code compiles fine.

Answer 1

You need a typename:

 typename std::map<int, B>::const_iterator itr; 

The iterator is a dependant type (depends on B) and when you have this situation the compiler requires you to clarify it with a typename.

There is a reasonable discussion of the issue here.