Friend Functions and Virtual Inheritance

Friend functions and classes work with virtual inheritance in a similar way as they do with non-virtual inheritance. However, there are a few nuances to be aware of.

Understanding Virtual Inheritance

Virtual inheritance is used to solve the diamond problem in multiple inheritance, ensuring that a base class is only inherited once, no matter how many times it appears in the inheritance hierarchy.

Example

Consider a scenario where Base is virtually inherited by both Derived1 and Derived2, and MostDerived inherits from both:

1#include <iostream>
2
3class Base {
4public:
5  Base(int val) : value{val} {}
6  friend void showValue(Base &b);
7
8protected:
9  int value;
10};
11
12void showValue(Base &b) {
13  std::cout << "Base value: " << b.value; 
14}
15
16class Derived1 : virtual public Base {
17public:
18  Derived1(int val) : Base{val} {}
19};
20
21class Derived2 : virtual public Base {
22public:
23  Derived2(int val) : Base{val} {}
24};
25
26class MostDerived
27  : public Derived1, public Derived2 {
28public:
29  MostDerived(int val)
30    : Base{val}, Derived1{val}, Derived2{val} {}
31};
32
33int main() {
34  MostDerived obj(42);
35  showValue(obj); 
36}

1Base value: 42

In this example, showValue() is a friend of Base and can access its protected member value. Since MostDerived virtually inherits from Base, showValue() can access value through the most derived object.

Key Points

• Single Instance: With virtual inheritance, there is only one instance of the virtually inherited base class, even if it appears multiple times in the inheritance hierarchy.
• Access Control: Friend functions declared in the base class have access to the base class’s protected and private members, regardless of how the base class is inherited.
• Initialization: Virtual base classes are initialized by the most derived class in the hierarchy.

Practical Considerations

• Design Complexity: Virtual inheritance adds complexity to the class design. Ensure that friend functions are used judiciously to maintain readability and manageability.
• Explicit Initialization: When using virtual inheritance, the most derived class must explicitly initialize the virtual base class.

Conclusion

Friend functions and classes can work seamlessly with virtual inheritance, providing access to private and protected members of virtually inherited base classes.

Be mindful of the complexity virtual inheritance introduces and use friend declarations judiciously to maintain clean and understandable code.