Multiple Inheritance in C++

1. Use Multiple Inheritance to Apply Multiple Properties of the Two Given Classes to Another
2. Use the virtual Keyword to Fix Multiple Copies of Base Classes

This article will demonstrate multiple methods about how to use multiple inheritances in C++.

Use Multiple Inheritance to Apply Multiple Properties of the Two Given Classes to Another

Classes in C++ can have multiple inheritances, which provides the possibility to derive a class from more than one direct base class.

This implies that there might be particular unusual behavior when the classes are not implemented carefully. E.g., consider the following snippet code, where the class C is derived from class B and A. All of them have the default constructor that outputs the special string. Although, when we declare a C type object in the mainc function, three constructors print the output. Notice that constructors get called in the same order they were inherited. On the other hand, the destructors are called in reverse order.

#include <iostream>

using std::cin;
using std::cout;
using std::endl;
using std::string;

class A {
 public:
  A() { cout << "A's constructor called" << endl; }
};

class B {
 public:
  B() { cout << "B's constructor called" << endl; }
};

class C : public B, public A {
 public:
  C() { cout << "C's constructor called" << endl; }
};

int main() {
  C c;
  return 0;
}

Output:

Bs constructor called
As constructor called
Cs constructor called

Use the virtual Keyword to Fix Multiple Copies of Base Classes

Note that the following program outputs the two instances of the Planet class constructor, Mars and Earth constructors each, and finally the Rock class constructor. Also, the Planet class’s destructor will get called twice when the object goes out of scope. Mind though, this issue can be solved by adding the virtual keyword to the Mars and Earth classes. When a class has multiple base classes, there is a possibility that the derived class will inherit a member with the same name from two or more of its base classes.

#include <iostream>

using std::cin;
using std::cout;
using std::endl;
using std::string;

class Planet {
 public:
  Planet(int x) { cout << "Planet::Planet(int ) called" << endl; }
};

class Mars : public Planet {
 public:
  Mars(int x) : Planet(x) { cout << "Mars::Mars(int ) called" << endl; }
};

class Earth : public Planet {
 public:
  Earth(int x) : Planet(x) { cout << "Earth::Earth(int ) called" << endl; }
};

class Rock : public Mars, public Earth {
 public:
  Rock(int x) : Earth(x), Mars(x) { cout << "Rock::Rock(int ) called" << endl; }
};

int main() { Rock tmp(30); }

Output:

Planet::Planet(int ) called
Mars::Mars(int ) called
Planet::Planet(int ) called
Earth::Earth(int ) called
Rock::Rock(int ) called