Question

The code below gives the error:

error: type ‘std::list<T,std::allocator<_Tp1> >’ is not derived from type ‘Foo<T>’
error: expected ‘;’ before ‘iter’

#include <list>

template <class T> class Foo 
{
  public:
      std::list<T>::iterator iter;

  private:
      std::list<T> elements;
};

Why and what should this be to be correct?

Answer 1

You need typename std::list<T>::iterator. This is because list depends on the template parameter, so the compiler cannot know what exactly is the name iterator within it going to be (well, technically it could know, but the C++ standard doesn’t work that way). The keyword typename tells the compiler that what follows is a name of a type.