How can I delete a list which is in a generic container?

Question

I'm doing a list of lists with pointers and dinamic memory, the main list it's composed by nodes, and each node can be another list or whatever kind of type (int, double...). When I try to delete everything appears the error:

C2541: 'delete' : cannot delete objects that are not pointers.

List's destructor:

While the list isn't empty, I erase the first element of it.

template <class T> Lista<T>::~Lista()
{
  while ( !es_vacia() )
  {
    borrar(1);
  }
}

Node's destructor:

The function borrar(1) call the node's destructor. It put the pointers to null (it's a doubly linked list) and then if the class T is a pointer delete it (because there's a list contained) but it doesn't work.

template <class T> Nodo<T>::~Nodo()
{
  sig = 0;
  ant = 0;
  if (std::is_pointer<T>::value)
  {
    delete valor;
  } 
}

Answer 1

I'm with the commenters that a better way to handle this would be that if users want your list to take ownership of pointers they should declare it Lista<std::unique_ptr<T>>. That way if they want to continue to own their pointers in some other way that would still be an option.

But to answer your question...if you want to be able to have a generic type that deletes pointers when the template parameter is a pointer you have to move your delete into a class or function where it is always a pointer. Here is one way of doing that:

template <class T, bool isPointer = std::is_pointer<T>::value>
struct ItemCleanup
{
    // default case, does nothing
    static void cleanup(T value) { }
};

template <class T>
struct ItemCleanup<T, true>
{
    // specialized case if is_pointer<T>::value is true
    static void cleanup(T value) { delete value; }
};

template <class T>
Nodo<T>::~Nodo()
{
    ItemCleanup<T>::cleanup(valor);
}