C 实现 C++继承多态 实例

引述自：http://www.eventhelix.com/realtimemantra/basics/ComparingCPPAndCPerformance2.htm#.UaG1Kdd3-fg

 1     // A typical example of inheritance and virtual function use.
    // We would be mapping this code to equivalent C.
     
    // Prototype graphics library function to draw a circle
    void glib_draw_circle (int x, int y, int radius);
     
    // Shape base class declaration
    class Shape
    {
    protected:
      int m_x;    // X coordinate
      int m_y;  // Y coordinate
     
    public:
      // Pure virtual function for drawing
      virtual void Draw() = 0;
     
      // A regular virtual function
      virtual void MoveTo(int newX, int newY);
     
      // Regular method, not overridable.
      void Erase();
     
      // Constructor for Shape
      Shape(int x, int y);
     
      // Virtual destructor for Shape
      virtual ~Shape();
    };
     
    // Circle class declaration
    class Circle : public Shape
    {
    private:
       int m_radius;    // Radius of the circle
     
    public:
       // Override to draw a circle
       virtual void Draw();  
     
       // Constructor for Circle
       Circle(int x, int y, int radius);
     
       // Destructor for Circle
       virtual ~Circle();
    };
     
    // Shape constructor implementation
    Shape::Shape(int x, int y)
    {
       m_x = x;
       m_y = y;
    }
     
    // Shape destructor implementation
    Shape::~Shape()
    {
    //...
    }
     
    // Circle constructor implementation
    Circle::Circle(int x, int y, int radius) : Shape (x, y)
    {
       m_radius = radius;
    }
     
    // Circle destructor implementation
    Circle::~Circle()
    {
    //...
    }
     
    // Circle override of the pure virtual Draw method.
    void Circle::Draw()
    {
       glib_draw_circle(m_x, m_y, m_radius);
    }
     
    main()
    {
      // Define a circle with a center at (50,100) and a radius of 25
      Shape *pShape = new Circle(50, 100, 25);
     
      // Define a circle with a center at (5,5) and a radius of 2
      Circle aCircle(5,5, 2);
     
      // Various operations on a Circle via a Shape pointer
      pShape->Draw();
      pShape->MoveTo(100, 100);
      pShape->Erase();
      delete pShape;
     
      // Invoking the Draw method directly
      aCircle.Draw();
    }

    /*
    The following code maps the C++ code for the Shape and Circle classes
    to C code.
    */
     
    #include <stdio.h>
    #include <stdlib.h>
    #define TRUE 1
    #define FALSE 0
    typedef int BOOLEAN;
     
    /*
    Error handler used to stuff dummy VTable
    entries. This is covered later.
    */
    void pure_virtual_called_error_handler();
     
    /* Prototype graphics library function to draw a circle */
    void glib_draw_circle (int x, int y, int radius);
     
    typedef void (*VirtualFunctionPointer)(...);
     
    /*
    VTable structure used by the compiler to keep
    track of the virtual functions associated with a class.
    There is one instance of a VTable for every class
    containing virtual functions. All instances of
    a given class point to the same VTable.
    */
    struct VTable
    {
       /*
       d and i fields are used when multiple inheritance and virtual
       base classes are involved. We will be ignoring them for this
       discussion.
       */
       int d;
       int i;
     
       /*
       A function pointer to the virtual function to be called is
       stored here.
       */
       VirtualFunctionPointer pFunc;
    };
     
    /*
    The Shape class maps into the Shape structure in C. All
    the member variables present in the class are included
    as structure elements. Since Shape contains a virtual
    function, a pointer to the VTable has also been added.
    */
     
    struct Shape
    {
      int m_x;
      int m_y;
     
      /*
      The C++ compiler inserts an extra pointer to a vtable which
      will keep a function pointer to the virtual function that
      should be called.
      */
      VTable *pVTable;
    };
     
    /*
    Function prototypes that correspond to the C++ methods
    for the Shape class,
    */
    Shape *Shape_Constructor(Shape *this_ptr, int x, int y);
    void Shape_Destructor(Shape *this_ptr, bool dynamic);
    void Shape_MoveTo(Shape *this_ptr, int newX, int newY);
    void Shape_Erase(Shape *this_ptr);
     
    /*
    The Shape vtable array contains entries for Draw and MoveTo
    virtual functions. Notice that there is no entry for Erase,
    as it is not virtual. Also, the first two fields for every
    vtable entry are zero, these fields might have non zero
    values with multiple inheritance, virtual base classes
    A third entry has also been defined for the virtual destructor
    */
     
    VTable VTableArrayForShape[] =
    {
        /*
        Vtable entry virtual function Draw.
        Since Draw is pure virtual, this entry
        should never be invoked, so call error handler
        */
        { 0, 0, (VirtualFunctionPointer) pure_virtual_called_error_handler },
     
        /*
        This vtable entry invokes the base class's
        MoveTo method.
        */
        { 0, 0, (VirtualFunctionPointer) Shape_MoveTo },
     
        /* Entry for the virtual destructor */
        { 0, 0, (VirtualFunctionPointer) Shape_Destructor }
    };
     
    /*
    The struct Circle maps to the Circle class in the C++ code.
    The layout of the structure is:
    - Member variables inherited from the the base class Shape.
    - Vtable pointer for the class.
    - Member variables added by the inheriting class Circle.
    */
     
    struct Circle
    {
       /* Fields inherited from Shape */
       int m_x;
       int m_y;
       VTable *pVTable;
     
       /* Fields added by Circle */
       int m_radius;
    };
     
    /*
    Function prototypes for methods in the Circle class.
    */
     
    Circle *Circle_Constructor(Circle *this_ptr, int x, int y, int radius);
    void Circle_Draw(Circle *this_ptr);
    void Circle_Destructor(Circle *this_ptr, BOOLEAN dynamic);
     
    /* Vtable array for Circle */
     
    VTable VTableArrayForCircle[] =
    {
        /*
        Vtable entry virtual function Draw.
        Circle_Draw method will be invoked when Shape's
        Draw method is invoked
        */
        { 0, 0, (VirtualFunctionPointer) Circle_Draw },
     
        /*
        This vtable entry invokes the base class's
        MoveTo method.
        */
        { 0, 0, (VirtualFunctionPointer) Shape_MoveTo },
     
        /* Entry for the virtual destructor */
        { 0, 0, (VirtualFunctionPointer) Circle_Destructor }
    };
     
    Shape *Shape_Constructor(Shape *this_ptr, int x, int y)
    {
      /* Check if memory has been allocated for struct Shape. */
      if (this_ptr == NULL)
      {
        /* Allocate memory of size Shape. */
        this_ptr = (Shape *) malloc(sizeof(Shape));
      }
     
      /*
      Once the memory has been allocated for Shape,
      initialise members of Shape.
      */
      if (this_ptr)
      {  
        /* Initialize the VTable pointer to point to shape */
        this_ptr->pVTable = VTableArrayForShape;
        this_ptr->m_x = x;
        this_ptr->m_y = y;
      }
     
      return this_ptr;
    }
     
    void Shape_Destructor(Shape *this_ptr, BOOLEAN dynamic)
    {
      /*
      Restore the VTable to that for Shape. This is
      required so that the destructor does not invoke
      a virtual function defined by a inheriting class.
      (The base class destructor is invoked after inheriting
      class actions have been completed. Thus it is not
      safe to invoke the ineriting class methods from the
      base class destructor)
      */
      this_ptr->pVTable = VTableArrayForShape;
     
      /*...*/
     
      /*
      If the memory was dynamically allocated
      for Shape, explicitly free it.
      */
      if (dynamic)
      {
        free(this_ptr);
      }
    }
     
    Circle *Circle_Constructor(Circle *this_ptr, int x, int y, int radius)
    {
      /* Check if memory has been allocated for struct Circle. */
      if (this_ptr == NULL)
      {
        /* Allocate memory of size Circle. */
        this_ptr = (Circle *) malloc(sizeof(Circle));
      }
     
      /*
      Once the memory has been allocated for Circle,
      initialise members of Circle.
      */
      if (this_ptr)
      {
          /* Invoking the base class constructor */
          Shape_Constructor((Shape *)this_ptr, x, y);
          this_ptr->pVTable = VTableArrayForCircle;
     
          this_ptr->m_radius = radius;
      }
      return this_ptr;
    }
     
    void Circle_Destructor(Circle *this_ptr, BOOLEAN dynamic)
    {
      /* Restore the VTable to that for Circle */
      this_ptr->pVTable = VTableArrayForCircle;
     
      /*...*/
     
      /*
      Invoke the base class destructor after ineriting class
      destructor actions have been completed. Also note that
      that the dynamic flag is set to false so that the shape
      destructor does not free any memory.
      */
      Shape_Destructor((Shape *) this_ptr, FALSE);
     
      /*
      If the memory was dynamically allocated
      for Circle, explicitly free it.
      */
      if (dynamic)
      {
        free(this_ptr);
      }
    }
     
    void Circle_Draw(Circle *this_ptr)
    {
       glib_draw_circle(this_ptr->m_x, this_ptr->m_y, this_ptr->m_radius);
    }
     
    main()
    {  
      /*
      Dynamically allocate memory by passing NULL in this arguement.
      Also initialse members of struct pointed to by pShape.
      */
      Shape *pShape = (Shape *) Circle_Constructor(NULL, 50, 100, 25);
     
      /* Define a local variable aCircle of type struct Circle. */
      Circle aCircle;
     
      /* Initialise members of struct variable aCircle. */
      Circle_Constructor(&aCircle, 5, 5, 2);
     
      /*
      Virtual function Draw is called for the shape pointer. The compiler
      has allocated 0 offset array entry to the Draw virtual function.
      This code corresponds to "pShape->Draw();"
      */
      (pShape->pVTable[0].pFunc)(pShape);
     
      /*
      Virtual function MoveTo is called for the shape pointer. The compiler
      has allocared 1 offset array entry to the MoveTo virtual function.
      This code corresponds to "pShape->MoveTo(100, 100);"
      */
      (pShape->pVTable[1].pFunc)(pShape, 100, 100);
     
      /*
      The following code represents the Erase method. This method is
      not virtual and it is only defined in the base class. Thus
      the Shape_Erase C function is called.
      */
      Shape_Erase(pShape);
     
      /* Delete memory pointed to by pShape (explicit delete in original code).
      Since the destructor is declared virtual, the compiler has allocated
      2 offset entry to the virtual destructor
      This code corresponds to "delete pShape;".
      */
      (pShape->pVTable[2].pFunc)(pShape, TRUE);
     
      /*
      The following code corresponds to aCircle.Draw().
      Here the compiler can invoke the method directly instead of
      going through the vtable, since the type of aCircle is fully
      known. (This is very much compiler dependent. Dumb compilers will
      still invoke the method through the vtable).
      */
      Circle_Draw(&aCircle);
     
      /*
      Since memory was allocated from the stack for local struct
      variable aCircle, it will be deallocated when aCircle goes out of scope.
      The destructor will also be invoked. Notice that dynamic flag is set to
      false so that the destructor does not try to free memory. Again, the
      compiler does not need to go through the vtable to invoke the destructor.
      */
      Circle_Destructor(&aCircle, FALSE);
    }