In C++, what is a virtual base class?

Question

I want to know what a \"virtual base class\" is and what it means.

Let me show an example:

class Foo
{
public:
    void DoSomething() { /* .         


        

Answer 1

Diamond inheritance runnable usage example

This example shows how to use a virtual base class in the typical scenario: to solve diamond inheritance problems.

Consider the following working example:

main.cpp

#include 

class A {
    public:
        A(){}
        A(int i) : i(i) {}
        int i;
        virtual int f() = 0;
        virtual int g() = 0;
        virtual int h() = 0;
};

class B : public virtual A {
    public:
        B(int j) : j(j) {}
        int j;
        virtual int f() { return this->i + this->j; }
};

class C : public virtual A {
    public:
        C(int k) : k(k) {}
        int k;
        virtual int g() { return this->i + this->k; }
};

class D : public B, public C {
    public:
        D(int i, int j, int k) : A(i), B(j), C(k) {}
        virtual int h() { return this->i + this->j + this->k; }
};

int main() {
    D d = D(1, 2, 4);
    assert(d.f() == 3);
    assert(d.g() == 5);
    assert(d.h() == 7);
}

Compile and run:

g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out

If we remove the virtual into:

class B : public virtual A

we would get a wall of errors about GCC being unable to resolve D members and methods that were inherited twice via A:

main.cpp:27:7: warning: virtual base ‘A’ inaccessible in ‘D’ due to ambiguity [-Wextra]
   27 | class D : public B, public C {
      |       ^
main.cpp: In member function ‘virtual int D::h()’:
main.cpp:30:40: error: request for member ‘i’ is ambiguous
   30 |         virtual int h() { return this->i + this->j + this->k; }
      |                                        ^
main.cpp:7:13: note: candidates are: ‘int A::i’
    7 |         int i;
      |             ^
main.cpp:7:13: note:                 ‘int A::i’
main.cpp: In function ‘int main()’:
main.cpp:34:20: error: invalid cast to abstract class type ‘D’
   34 |     D d = D(1, 2, 4);
      |                    ^
main.cpp:27:7: note:   because the following virtual functions are pure within ‘D’:
   27 | class D : public B, public C {
      |       ^
main.cpp:8:21: note:    ‘virtual int A::f()’
    8 |         virtual int f() = 0;
      |                     ^
main.cpp:9:21: note:    ‘virtual int A::g()’
    9 |         virtual int g() = 0;
      |                     ^
main.cpp:34:7: error: cannot declare variable ‘d’ to be of abstract type ‘D’
   34 |     D d = D(1, 2, 4);
      |       ^
In file included from /usr/include/c++/9/cassert:44,
                 from main.cpp:1:
main.cpp:35:14: error: request for member ‘f’ is ambiguous
   35 |     assert(d.f() == 3);
      |              ^
main.cpp:8:21: note: candidates are: ‘virtual int A::f()’
    8 |         virtual int f() = 0;
      |                     ^
main.cpp:17:21: note:                 ‘virtual int B::f()’
   17 |         virtual int f() { return this->i + this->j; }
      |                     ^
In file included from /usr/include/c++/9/cassert:44,
                 from main.cpp:1:
main.cpp:36:14: error: request for member ‘g’ is ambiguous
   36 |     assert(d.g() == 5);
      |              ^
main.cpp:9:21: note: candidates are: ‘virtual int A::g()’
    9 |         virtual int g() = 0;
      |                     ^
main.cpp:24:21: note:                 ‘virtual int C::g()’
   24 |         virtual int g() { return this->i + this->k; }
      |                     ^
main.cpp:9:21: note:                 ‘virtual int A::g()’
    9 |         virtual int g() = 0;
      |                     ^
./main.out

Tested on GCC 9.3.0, Ubuntu 20.04.