How to declare data members that are objects of any type in a class

Question

In this piece I'm trying to declare in Class B a list that can hold objects of Class A of any type, such as A<int>, A<double>, A<float>. I intend to add A objects to the list during runtime:

#include <list>

template <class T> class A {};

class B {
    template<class T> std::list<A<T>*> objects;
};

It seems like making a list like this should work but compiling it gives an error:

Line 6: error: data member 'objects' cannot be a member template

compilation terminated due to -Wfatal-errors.

Can somebody explain why this doesn't work and how I can fix it?

Answer 1

That's just not how C++ works. If you want to group different objects together, they need to have at least some relation. Being instantiations of the same class template doesn't imply that they are related, they're completely distinct types. If you want a list of A<T>*s, better make a list of base-class pointers and forward operations through virtual functions:

class A_base{
public:
  virtual void foo() = 0;
  virtual ~A_base() { }
};

template<class T>
class A : public A_base{
public:
  void foo(){
    // ...
  }
};

class B{
  std::list<A_base*> objects;
};

Answer 2

Member variables aren't allowed to be templates. Only member functions can be templates. You'll have to templatize the enclosing class B instead:

template <class T>
class B {
  std::list<A<T>*> objects;
};

Answer 3

Unfortunately you cannot have template variables. Only option to declare a member data is to make the class template:

template<class T>
class B {
    std::list<A<T>*> objects;
};