Template class pointer c++ declaration

Question

template <typename T> class Node {...};  int main {     Node* ptr;     ptr = new Node<int>; } 

Will fail to compile I have to to declare the pointer as

Node<int>* ptr; 

Why do I have to specify the type when declaring a pointer I haven't created the class yet, why does the compiler have to know what type it will be pointing to. And is it not possible to create a generic pointer and decide afterwards what type I want to assign it.

Answer 1

Templating resolves types at compile-time. When you assign the new Node<int> object to it, the pointer must know at compile-time what type exactly it is.

Node<int> and Node<std::vector> can be very different in the binaries (the binary layout of your object changes completely according the template parameter) so it doesn't make any sense to have an unresolved pointer type to a template.

You should define first a common parent class for your nodes:

class NodeBase { ... }  template<typename ValueT>   class Node : public NodeBase {  ... };  NodeBase* ptr; 

Answer 2

The simple answer is because C++ uses (fairly) strict static type checking. Node<int> is a completely unrelated type to Node<double>, and when the compiler sees ptr->doSomething(), it has to know whether to call Node<int>::doSomething() or Node<double>::doSomething().

If you do need some sort of dynamic generality, where the actual type ptr will point to will only be known at runtime, you need to define a base class, and derive from that. (It's a fairly common idiom for a class template to derive from a non-template base, precisely so that the generality in the pointers can be resolved at runtime.)