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─────C/C++ Language─────────────────────────────────────────────────────────
 
     Virtual base classes offer a way to save space and avoid
     ambiguities in class hierarchies that use multiple inheritance.
     Consider the following example:
 
         class Employee
         {
         private:
             char name[30];
         };
 
         class SalesPerson : public Employee
         {
         };
 
         class Manager : public Employee
         {
         };
 
         class SalesManager : public SalesPerson, public Manager
         {
         };
 
     In this hierarchy, the Employee class acts as an indirect base
     class to SalesManager twice. The following diagram describes a
     SalesManager object:
 
         Employee                   Employee
            │                          │
         SalesPerson                Manager
                    \             /
                      SalesManager
 
     Each SalesManager object contains two copies of the data members
     defined in Employee. This duplication wastes space and requires
     you to specify which copy of Employee's members you want whenever
     you access them.
 
     The use of the virtual keyword eliminates these problems. When a
     base class is specified as a virtual base, it can act as an
     indirect base more than once without duplication of its data
     members. A single copy of its data members is shared by all the
     base classes that use it as a virtual base. For example:
 
         class Employee
         {
         private:
             char name[30];
         };
 
         class SalesPerson : virtual public Employee
         {
         };
 
         class Manager : virtual public Employee
         {
         };
 
         class SalesManager : public SalesPerson, public Manager
         {
         };
 
     A SalesManager object can now be described by the following
     diagram:
 
                       Employee
                     /          \
         SalesPerson              Manager
                    \           /
                     SalesManager
 
     The SalesPerson and Manager base classes share the same copy of
     Employee's data members. As a result, each SalesManager object
     contains just one copy of Employee's data members, and references
     to Employee's members are unambiguous. Notice that the virtual
     keyword appears in the base lists of the SalesPerson and Manager
     classes, not SalesManager.
                                    -♦-