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Comparing C++ and C (Inheritance and Virtual Functions)

We have covered C++ and C performance in a previous article. In this article we will dig deeper into C++ performance by analyzing the overhead of inheritance and virtual functions. 

Virtual Functions and Inheritance

This section presents the C++ code for a typical virtual function invocation scenario. This is then compared to the equivalent C code.

C++ code

// 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();
}

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 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);
}   

C code implementing the above C++ functionality is shown below. The code also includes compiler generated constructs like vtables. Virtual function access using vtables is also covered. (The presentation here has been simplified here to aid understanding).

Analysis

This section analyses the C++ code and its C translation and identifies the performance impact.

Object Construction Inheritance does increase the object construction overhead, as constructors for all the parent classes in the class hierarchy are invoked. There is the additional overhead of setting up vtable in the constructor.
Object Destruction Inheritance does increase the object destruction overhead, as destructors for all the parent classes in the class hierarchy are invoked. There is the additional overhead of setting up vtable in the destructor. Virtual destructors also increase the overhead of object destruction.
Virtual Function Invocation Virtual function invocation is slightly more expensive than invoking a function through a function pointer. In many scenarios, intelligent compilers can use normal method invocation instead of a virtual function invocation.

In a well designed object oriented system, a virtual function call would typically replace a switch statement so virtual function invocation might actually be faster than conventional coding techniques. For example, a generic draw statement in a "C" based paint program would involve switching over the type of shape and then invoking the corresponding draw function. In C++, this logic will be replaced by a virtual function call.

In our experience we have found that poorly designed and excessive use of constructors and destructors reduces performance much more than virtual function calls.

Memory Overhead Using plain inheritance has no memory overhead. Inheritance with virtual functions however does introduce the following memory overhead:
  • A vtable array pointer is added to all classes that use virtual functions.
  • Global vtable arrays are declared for every call with virtual functions (this should be a very small overhead).

 

Locality of Reference In the current computing environment, processor speeds have increased considerably but memory access speeds haven't kept pace. In such a scenario, cache hit ratio of an application plays a very important role in determining application performance.

In general this turns out to be an advantage for programs written in C++. With C++ code and data locality of reference is much better than C, as all the class code manipulating object data is located together. Also all object data is located at one place. In C code and data are scattered all over the place. Thus a C++ program should offer a better locality of reference than a C program. In many cases this might more than compensate for the performance overhead of C++.

 

Multiple Inheritance and Virtual Base Classes In this article we have not covered multiple inheritance and virtual base classes. There is a significant increase in overhead due to these features. We would recommend that you stay away from using these features.

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