最近面試被問到多重繼承相關的問題，簡單做個紀錄～

Multiple Inheritance (多重繼承)

An object or class can inherit characteristics and features from more than one parent object or parent class.

Virtual base classes

Virtual base classes are used in virtual inheritance, is a way of preventing multiple "instance" of a given class appearing in an inheritance hierarchy when using multiple inheritance.

Explain

Consider the follow scenario:

class A { public: void func() {} };
class B : public A {};
class C : public A {};
class D : public B, public C {};

The above class hierarchy results in the "dreaded diamond" which looks like below:

    A
  /   \
 B     C
  \   /
    D
(D has now 2 A, B's A and C's A)

Class A is inherited twice to class D. One is through class B and second is through class C.

When you want to access any data or function of class A by class D, ambiguity arises as to which data or function should be called? One inherited through and the other inherited through C. This confuses compiler, as below:

Solution

Virtual inheritance is there to solve this problem. When a base class is specified as a virtual base, it can acts as an indirect base more than once without duplication of its data members. A single copy of its data members is shared by all the base classes that use virtual base.

class B : virtual public A {};
class C : virtual public A {};

This means that there is only "one instance" of A included in the hierarchy.

Example

#include <iostream>

using namespace std;

class A
{
    public:
        virtual void func()
        {
            cout << "function form A" << endl;
        }
};

class B : virtual public A {};

class C : virtual public A {};

class D : public B, public C
{
    public:
        void func()
        {
            // use the func() of class A
            B::A::func();
        }
};

int main()
{
    D d;
    d.func();

    return 0;
}

Output

function form A