Multiple Inheritance

A subclass can inherit from multiple base classes which requires a comma-separated base-specifier list:

class Base1 {};
class Base2 {};
class Child : public Base1, public Base2 {};

class Child : public Base1, public Base1 {};  // Error.

Order

The order of construction/destruction will be:

1. Construction of each base class in the base-specifier list (left-to-right).
2. Construct derived object.
3. Destroy derived object.
4. Destroy each base class in base-specifier list in reverse (right-to-left).

Ambiguity

If two inherited base classes contain members with the same name, there will be a compiler error. Use the scope resolution (::) operator to target the class before accessing the member:

struct Base1 { int data; };
struct Base2 { int data; };
struct Child : public Base1, public Base2 {};

Child c;
// c.data;  // Error: ambiguous.
c.Base1::data;
c.Base2::data;

Virtual Inheritance

Consider when a class inherits two parent classes that in-turn inherit a common grandparent class:

struct Grandparent { int data; };
struct Parent1 : public Grandparent {};
struct Parent2 : public Grandparent {};
struct Child : public Parent1, public Parent2 {};

Child c;
c.data;  // Error: ambiguous.

The c object has two separate grandparent instances thus it is ambiguous if data is resolved through Parent1 or Parent2. This is the "dreaded diamond" problem:

     Grandparent
        /  \
       /    \
      /      \
Parent1      Parent2
      \      /
       \    /
        \  /
       Child

Virtual inheritance resolves the diamond problem so there is only a single instance of the grandparent object. Virtual inheritance should be applied on the classes deriving the grandparent (where the inheritance chain begins to split):

struct Grandparent { int data; };
struct Parent1 : virtual public Grandparent {};
struct Parent2 : virtual public Grandparent {};
struct Child : public Parent1, public Parent2 {};