Question

I want to make an empty base class called “Node”, and then have other classes derived from this such as “DecisionNode” and “Leaf.” It makes sense to do this so I can take advantage of polymorphism to pass these different kinds of nodes to methods without knowing at compile time what will be passed to the method, but each of the derived classes do not share any state or methods.

I thought the best way to implement this, without creating an additional pure virtual method in the base class, which would add clutter, would be to make the constructor pure virtual. In the header file for the class, “Node.h” I therefore wrote:

class Node {
 private:
  virtual Node();
};

and in “Node.cpp” I wrote:

#include "Node.h"
virtual Node::Node() = 0;

This implementation prevents Node from ever being instantiated by another class, since the only constructor is private and uses the pure virtual specifier to indicate that the class is abstract. However, this gives the compiler errors:

Node.h:6:21: error: return type specification for constructor invalid
Node.h:6:21: error: constructors cannot be declared virtual [-fpermissive]

My question is: is there a neat way to make an empty abstract base class?

Answer 1

C++ doesn’t support virtual constructor.

§ 12.1 Constructors

12.1.4 A constructor shall not be virtual (10.3) or static (9.4).

Below code won’t compile:

virtual Node::Node() = 0;

My question is: is there a neat way to make an empty abstract base
class?

Yes, make destructor a pure virtual function, also provides destructor function definition

class Node 
{
public:
    virtual ~Node()=0
    {
    }
};