C++ inheritance, is it possible to have only 1 copy of a shared grandparent class

Question

I have a project with some very small classes that act as minimal interfaces for larger classes. Sometimes I have objects that need several of these interfaces. And sometimes these interfaces share the same derived base class. Is there anyway that I can structure my code so that I only have one copy of that derived base class that all of the interfaces share?

Here's a code example:

class A {
public:
  virtual void f(int a)=0;
  virtual int g() const {
    return m_inner;
  }
protected:
  int m_inner;
};

class B : public A {
public:
  virtual void f(int a)=0;
  virtual void h(int b) {
    m_stuff = b;
    m_inner = 0;
  }
protected:
  int m_stuff;
};

class C : public A {
public:
  virtual void f(int a)=0;
  virtual int i() const {
    return m_otherstuff;
  }
protected:
  int m_otherstuff;
};

class D : public B, public C {
public:
  void f(int a) { //do stuff }
  //fill in the rest of a big actual class.
};

D has two copies of A and referencing A:: is ambiguous. What's the best way to structure my classes so that I get only one copy of A at the end of the day? I'm willing to restructure A, B and C in whatever fashion. Is this even possible?

Have you considering making A be a virtual base? – Vaughn Cato Sep 25 '12 at 04:47 — Vaughn Cato, Sep 25 '12 at 04:47
It isn't clear why C would derive from A in your example. – Vaughn Cato Sep 25 '12 at 04:55 — Vaughn Cato, Sep 25 '12 at 04:55
I know, it was kinda fakey fake. – OmnipotentEntity Sep 25 '12 at 16:08 — OmnipotentEntity, Sep 25 '12 at 16:08

score 5 · Accepted Answer · answered Sep 25 '12 at 04:50

5

Virtual inheritance is what you need. Your case is a typical diamond problem. Have a brief search on google or any c++ book should give u plenty of related info

answered Sep 25 '12 at 04:50

Adrian Shum

38,812
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Today, I learned about the diamond problem and virtual inheritance. Thank you. – OmnipotentEntity Sep 25 '12 at 04:53

score 3 · Answer 2 · answered Sep 25 '12 at 04:51

3

I'm not much into C++ but I know this is called Diamond Problem.

and, this is your solution.

answered Sep 25 '12 at 04:51

Azodious

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score 1 · Answer 3 · edited May 23 '17 at 11:56

1

Virtual inheritance will solve your immediate problem (already answered here). But, I strongly suggest you to re-think the hierarchy. I have never seen virtual inheritance used except in academic papers and in some cases of extreme refactoring. Perhaps, you may get away by using aggregation instead of inheritance.

edited May 23 '17 at 11:56

Community

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answered Sep 25 '12 at 04:53

blinnov.com

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C++ inheritance, is it possible to have only 1 copy of a shared grandparent class

3 Answers3