fig22_21.cpp

《c＋＋大学教程》源码

// Fig. 22.21: fig22_21.cpp
// Using virtual base classes.
#include <iostream>

using std::cout;
using std::endl;

// class Base definition
class Base {
public:

   // implicit default constructor
   
   virtual void print() const = 0; // pure virtual
};  // end Base class

// class DerivedOne definition
class DerivedOne : virtual public Base {
public:

   // implicit default constructor calls
   // Base default constructor 

   // override print function
   void print() const { cout << "DerivedOne\n"; }

};  // end DerivedOne class

// class DerivedTwo definition
class DerivedTwo : virtual public Base {
public:

   // implicit default constructor calls
   // Base default constructor 

   // override print function
   void print() const { cout << "DerivedTwo\n"; }

}; // end DerivedTwo class

// class Multiple definition
class Multiple : public DerivedOne, public DerivedTwo {
public:

   // implicit default constructor calls
   // DerivedOne and DerivedTwo default constructors

   // qualify which version of function print
   void print() const { DerivedTwo::print(); }

}; // end Multiple class

int main()
{
   Multiple both;   // instantiate Multiple object
   DerivedOne one;  // instantiate DerivedOne object
   DerivedTwo two;  // instantiate DerivedTwo object

   // declare array of base-class pointers and initialize
   // each element to a derived-class type
   Base *array[ 3 ];

   array[ 0 ] = &both;
   array[ 1 ] = &one;
   array[ 2 ] = &two;

   // polymorphically invoke function print
   for ( int i = 0; i < 3; i++ )
      array[ i ]->print();

   return 0;

}  // end main

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