// Example class A contains regular and
// static member variables and methods.class A
{
private:int m_x;static int g_y;int m_z;// Should be invoked when the object endsvoid InformEnd();public:A(int x);~A();void UpdateX(int newX);static void UpdateY(int newY);
};// Initialization of the static variable
int A::g_y = 0;// The non-static member variables
// are initialized in the constructor
A::A(int x)
{m_x = x;m_z = 0;
}// Destructor invokes a private variable
A::~A()
{InformEnd();
}// UpdateX checks the value of X against  a static variable before updating the value
void A::UpdateX(int newX)
{if (g_y != 0 && m_x < newX){m_x = newX;}
}// Unconditional update of static variable m_y
void A::UpdateY(int newY)
{g_y = newY;
}main()
{// Create a object on the heapA *pA = new A(5);// Create an object on the stackA a(6);// Example of an access via a pointerpA->UpdateX(8);// Example of a direct accessa.UpdateX(9);// Example of static method callA::UpdateY(1000);// Deleting the object
    delete pA;
}
/*
This code maps from the C++ code to the equivalent C code.
Mapping of the following entities is covered:
- classes                 - methods
- this pointer            - member variables
- constructors            - static methods
- destructors             - static variables
*/#include <stdio.h>
#include <stdlib.h>
#define TRUE 1
#define FALSE 0
typedef int BOOLEAN;/*
Structure A represents the class A. Only the non-static member
variables are present in the structure
*/
struct A
{int m_x;int m_z;
};/* Notice that g_y is not a part of struct A. Its a separate global variable. */
int g_y = 0;/*
Prototype for the InformEnd method. The C++ version of this method
did not have any parameters but the C mapped function needs the this
pointer to obtain the address of the object. Note that all non-static
methods in the C++ code would map to a C function the additional this
pointer as the first parameter.
*/
void InformEnd(A *this_ptr);/*
The constructor maps to function with the this pointer and the size of the
structure as parameters. this_ptr passed to the constructor is NULL when
the operator new is used to create the object. this_ptr contains a valid
pointer if the memory for the object to be constructed is already
allocated. (e.g. local variable or part of another structure.)
*/
A *A_Constructor(A *this_ptr, int x)
{/*Check if memory has been allocated for struct A. */if (this_ptr == NULL){/*Allocate memory of size A. */this_ptr = (A *) malloc(sizeof(A));}/* Once the memory has been allocated for A, initialise members of A. */if (this_ptr){this_ptr->m_x = x;this_ptr->m_z = 0;}return this_ptr;
}/*
The following function is equivalent to a destructor. The this
pointer and a dynamic flag are passed as the two parameters to
this function. The dynamic flag is set to true if the object is
being deleted using the delete operator.
*/
void A_Destructor(A *this_ptr, BOOLEAN dynamic)
{InformEnd(this_ptr);/* If the memory was dynamically allocated for A, explicitly free it. */if (dynamic){free(this_ptr);}
}/*
A pointer this is passed as first argument. All member variables
in the code will be accessed through an indirecion from the this
pointer. Notice that static variables are accessed directly as
they do not belong to any instance.
*/
void A_UpdateX(A *this_ptr, int newX)
{if (g_y != 0 && this_ptr->m_x < newX){this_ptr->m_x = newX;}
}/*
Notice that this is not passed here. This is so because
A_UpdateY is a static function. This function can only access
other static functions and static or global variables. This
function cannot access any member variables or methods of class A
as a static function does not correspond to an instance.
*/
void A_UpdateY(int newY)
{g_y = newY;
}main()
{/*Dynamically allocate memory by passing NULL in this arguement.Also initialize members of struct pointed to by pA.*/A *pA = A_Constructor(NULL, 5);/* Define local variable a of type struct A. */A a;/*Initialize members of struct variable a. Note that theconstructor is called with the address of the object asa has been pre-allocated on the stack.*/A_Constructor(&a, 6);/*Method invocations in C++ are handled by calling thecorresponding C functions with the object pointer.*/A_UpdateX(pA, 8);A_UpdateX(&a, 9);/* UpdateY is a static method, so object pointer is not passed */A_UpdateY(1000);/*Delete memory pointed to by pA (explicit delete inoriginal code).*/A_Destructor(pA, TRUE);/*Since memory was allocated on the stack for local struct variable a, it will be deallocated when a goes out of scope.The destructor will also be invoked. Notice that dynamic flagis set to false so that the destructor does not try tofree memory.*/A_Destructor(&a, FALSE);
}

// 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 coordinateint m_y;  // Y coordinatepublic:// Pure virtual function for drawingvirtual void Draw() = 0; // A regular virtual functionvirtual void MoveTo(int newX, int newY);// Regular method, not overridable.void Erase();// Constructor for ShapeShape(int x, int y);// Virtual destructor for Shapevirtual ~Shape();
};// Circle class declaration
class Circle : public Shape
{
private:int m_radius;    // Radius of the circlepublic:// Override to draw a circlevirtual void Draw();   // Constructor for CircleCircle(int x, int y, int radius);// Destructor for Circlevirtual ~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 25Shape *pShape = new Circle(50, 100, 25);// Define a circle with a center at (5,5) and a radius of 2Circle aCircle(5,5, 2);// Various operations on a Circle via a Shape pointerpShape->Draw();pShape->MoveTo(100, 100);pShape->Erase();delete pShape;// Invoking the Draw method directly
  aCircle.Draw();
}/*
C code
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 virtualbase classes are involved. We will be ignoring them for thisdiscussion.*/int d;int i;/*A function pointer to the virtual function to be called isstored 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 whichwill keep a function pointer to the virtual function thatshould 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 entryshould never be invoked, so call error handler*/{ 0, 0, (VirtualFunctionPointer) pure_virtual_called_error_handler },/*This vtable entry invokes the base class'sMoveTo 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'sDraw method is invoked*/{ 0, 0, (VirtualFunctionPointer) Circle_Draw },/*This vtable entry invokes the base class'sMoveTo 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 isrequired so that the destructor does not invokea virtual function defined by a inheriting class.(The base class destructor is invoked after inheritingclass actions have been completed. Thus it is notsafe to invoke the ineriting class methods from thebase class destructor)*/this_ptr->pVTable = VTableArrayForShape;/*...*//*If the memory was dynamically allocatedfor 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);/* Noting that here pointer is to circle rather than shapeThis overwriting the pointer set to shape in shape's constructor */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 classdestructor actions have been completed. Also note thatthat the dynamic flag is set to false so that the shapedestructor does not free any memory.*/Shape_Destructor((Shape *) this_ptr, FALSE);/*If the memory was dynamically allocatedfor 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 compilerhas 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 compilerhas 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 isnot virtual and it is only defined in the base class. Thusthe 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 allocated2 offset entry to the virtual destructorThis 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 ofgoing through the vtable, since the type of aCircle is fullyknown. (This is very much compiler dependent. Dumb compilers willstill 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 tofalse so that the destructor does not try to free memory. Again, thecompiler does not need to go through the vtable to invoke the destructor.*/Circle_Destructor(&aCircle, FALSE);
}