使用virtual inheritance outside diamond inheritance问题允许越级

Question

Consider the following program:

#include <iostream>
#include <string>

class B
{
public:
    int n;

    B() : n(0) {}
    B(int m) : n(m) {}
};

class D1 : virtual public B
{
public:
    int a;
    
    D1() : a(0) {}
    D1(int m) : a(m) {}
};

class D2 : public D1
{
public:
    int d;
    
    D2() : d(0) {}
    D2(int m) : d(m) {}
    D2(int j, int k) : d(j), D1(k) {}
    D2(int i, int j, int k) : d(i), D1(j), B(k) {}  //Without virtual inheritance,
                                                    //must add new constructor to D1.
};

int main()
{
    std::string s;
    D2 d2(0, 1, 2);
    
    std::cout << "d2.n = " << d2.n << "\n";
    std::cout << "d2.a = " << d2.a << "\n";
    std::cout << "d2.d = " << d2.d << "\n";
    
    std::cout << "Press ENTER to exit.\n";
    getline(std::cin, s);
}

Having D1 inherit virtually from B allows me to call the constructor of B from the constructor of D2, leapfrogging over the class D1.

Can this use of virtual inheritance apart from the diamond-inheritance problem cause undefined behavior or other harm?

I have never seen virtual inheritance used except to solve the diamond inheritance problem.

Answer 1

Can this use of virtual inheritance apart from the diamond-inheritance problem cause undefined behavior or other harm?

Virtual inheritance, for any purposes, incurs these "harms":

• Access to the virtual base class is via pointer, which is potentially slower
• Every derived class, at all depths, must be responsible for constructing the virtual base
• Downcasting from the virtual base must be done via dynamic_cast . The faster static_cast is not possible.

Your example doesn't introduce any Undefined Behavior or harm that don't exist for any other usage of virtual inheritance.