Question

So I have the following code:

template<class T>
class A
{
public:

  class B
  {
  public:
    virtual void Destroy(T obj);
  };

  B &_b;

  A(B b) : _b(b)
  {
  }

  void Go(T obj)
  {
    _b.Destroy(obj);
  }
};

class X : public A<int>::B
{
public:
  void Destroy(int x)
  {
    //do something
  }
};

int main()
{
  X x;
  A<int> a(x);
  a.Go(5);
  return 0;
}

But I get a compile error:

undefined reference to 'A<int>::B::Destroy(int)'

I’ve seen issues before when doing templates in separate .hpp and .cpp files… but this is all in one file.

Thanks.

Answer 1

Add a definition for virtual void Destroy(T obj);:

• Either in place:

  class B
  {
  public:
      virtual void Destroy(T obj) { /* here! */ }
      // ==== ALTERNATIVELY: ====
      virtual void Destroy(T obj) = 0;  // pure-virtual
  };
  

• Or afterwards:

  template <typename T>
  void A<T>::B::Destroy(T obj) { /* here. */ }
  

In any event, the constructor for A should take a reference:

A(B & b) : _b(b) { }
//^^^^^