I had a really new but very interesting compiling issue these days. Let's see the following construct:

#include <iostream>
#include <vector>

//------------------------------------------------------------------
template<class T>
class A {
public:
  A();
};

//------------------------------------------------------------------
template<class T>
A<T>::A()
{
  std::vector<T*>::iterator iter;
}

//------------------------------------------------------------------
int main() {
  std::cout << "Test template issues" << std::endl;
  A<unsigned> a;
  return( 0 );
}

That's what I'd expect to work and it even would with some compilers but not with gcc. The problem here is the question if std::vector<T*>::iterator is a type, a variable (member) or a function (method). We get the following error:

In constructor ‘A<T>::A()’:

error: expected `;' before ‘iter’

In constructor ‘A<T>::A() [with T = unsigned int]’:

error: dependent-name ‘std::vector<T*,std::allocator<T*>
>::iterator’ is parsed as a non-type, but instantiation yields
a type

note: say ‘typename std::vector<T*,std::allocator<T*>
>::iterator’ if a type is meant

The default behaviour is that the parser assumes it to be a non-type (he expects a member or a method). But during instantiation it proves to be wrong - std::vector<T*>::iterator is a type.

The situation is easy to resolve if we read the error message We have to declare the iterator like:

typename std::vector<T*>::iterator iter;

After knowing what problem I have I also found the right chapter in Stroustrup's bible at

  C.13.5 Typename and Template (The C++ Programming Language - Bjarne Stroustrup)