Virtual Base Classes
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A virtual class is a base class that is passed to more than one derived class, as might happen with multiple inheritance.
You cannot specify a base class more than once in a derived class:
class B { ... };
class D : B; B { ... }; //ILLEGAL
However, you can indirectly pass a base class to the derived class more than once:
class X : public B { ... };
class Y : public B { ... };
class Z : public X, public Y { ... }; //OK
In this case, each object of class Z has two sub-objects of class B.
If this causes problems, add the keyword virtual to the base class specifier. For example,
class X : virtual public B { ... };
class Y : virtual public B { ... };
class Z : public X, public Y { ... };
B is now a virtual base class, and class Z has only one sub-object of class B.
Constructors for Virtual Base Classes
Constructors for virtual base classes are invoked before any non-virtual base classes.
If the hierarchy contains multiple virtual base classes, the virtual base class constructors invoke in the order they were declared.
Any non-virtual bases are then constructed before the derived class constructor is called.
If a virtual class is derived from a non-virtual base, that non-virtual base will be first, so that the virtual base class can be properly constructed. For example, this code
class X : public Y , virtual public Z X one;
produces this order:
Z(); // virtual base class initialization Y(); // non-virtual base class X(); // derived class