Lesson 147 +10 XP

Multiple Inheritance

A class normally has one base class. Multiple inheritance lets one derived class inherit from several base classes at the same time.

Inheriting from two bases

List the base classes after the colon, separated by commas:

class A {
public:
  void printA() { cout << "A"; }
};

class B {
public:
  void printB() { cout << "B"; }
};

class D : public A, public B {
};

The syntax is class D : public A, public B.
An object of D has the members of both A and B.

int main() {
  D obj;
  obj.printA();   // from A
  obj.printB();   // from B
  return 0;
}

The diamond problem

Multiple inheritance gets tricky when two base classes share a common ancestor:

class A { public: int value; };
class B : public A { };
class C : public A { };
class D : public B, public C { };

Here A appears twice inside D, once through the B branch and once through the C branch. If code refers to value, the compiler cannot tell which copy you mean. That ambiguity is the famous diamond problem.

Caveats

Multiple inheritance gives features from several sources at once.
Name collisions appear when both bases declare the same member.
The diamond shape duplicates the shared ancestor.
Virtual base classes exist to fix the diamond, but they add their own complexity.

TL;DR

Multiple inheritance: one class inherits from several bases: class D : public A, public B.
Use commas to separate the base classes.
The derived class gets every member of every listed base.
A diamond (two bases over a shared ancestor) duplicates that ancestor.
Duplicate ancestors cause ambiguity: the diamond problem.

Multiple Inheritance