Compiler errors using inheritance in C++

Question

I'm a newbie in C++. I've written an example code that illustrates the problem and causes compiler errors.

class Quantifier;

class Node {
public:
    virtual void Match(/* args */) = 0;
};

class QuantifiableNode : public Node {
public:
    virtual void SetQuantifier(Quantifier *quantifier) = 0;
};

class NodeBase : public Node {
public:
    void Match() override {
        // Base behavior
    }
};

class QuantifiableNodeImpl : public NodeBase, // Needed to inherit base behavior
                             public QuantifiableNode // Needed to implement QuantifiableNode interface
{
public:
    void SetQuantifier(Quantifier *quantifier) override {}

    void Method() {
        this->Match();
    }
};

int main() {
    QuantifiableNodeImpl node;
    node.Match();
    return 0;
}

I get these errors:

main.cpp(27): error C2385: ambiguous access of 'Match'
main.cpp(27): note: could be the 'Match' in base 'NodeBase'
main.cpp(27): note: or could be the 'Match' in base 'Node'
main.cpp(32): error C2259: 'QuantifiableNodeImpl': cannot instantiate abstract class
main.cpp(20): note: see declaration of 'QuantifiableNodeImpl'
main.cpp(32): note: due to following members:
main.cpp(32): note: 'void Node::Match(void)': is abstract
main.cpp(5): note: see declaration of 'Node::Match'
main.cpp(33): error C2385: ambiguous access of 'Match'
main.cpp(33): note: could be the 'Match' in base 'NodeBase'
main.cpp(33): note: or could be the 'Match' in base 'Node'

As far as I understand, the compiler can't compile this code because class QuantifiableNodeImpl inherits class NodeBase and interface QuantifiableNode which both have a method Match (NodeBase implements it from Node, QuantifiableNode inherits abstract method from Node). I need to have both of the interfaces Node and QuantifiableNode and use them in another code. Also, I need to have NodeBase class in order to separate base functionality (in my real code I have many derivatives of NodeBase).

Also, an object of class QuantifiableNodeImpl can't be created too, the compiler says that it has an unimplemented abstract Match method.

So, what should I do? Hope for your help!

You probably need to look up virtual inheritance. Related: [https://stackoverflow.com/questions/21558/in-c-what-is-a-virtual-base-class](https://stackoverflow.com/questions/21558/in-c-what-is-a-virtual-base-class) — drescherjm, Jun 05 '21 at 15:56

score 2 · Accepted Answer · answered Jun 05 '21 at 16:01

It seems you're not entirely familiar with diamond inheritance. QuantifiableNodeImpl has two Node sub-objects, one via NodeBase and one via QuantifiableNode. The second Node subobject lacks a QuantifiableNode::Match.

It seems that this might not be intentional: is QuantifiableNode really supposed to be a Node in its own right? How is QuantifiableNode::Match supposed to work?

It might be that Quantifiable is more accurately modelled as a mixin

Compiler errors using inheritance in C++

1 Answers1