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Nov 16, 2012

Interface class

Sometimes it is simple and helpful to define a class that does nothing but defines a standardised interface that can be used by other classes to make sure the implementation follows a standard set of input and output paramaters. Also this approach results in faster code runs and more maintainable code.

The interface class can only be approached by using a pointer to the virtual table that exposes the methods in the interface. Interface does not come by itself, it usually comes with an inherited class that implements the exposed method in the interface. Such a class that implements the interface exposed methods is often called a co-class.


The basic idea of an interface using C++

class CExampleArray
{
public:
    int getLength() { return m_iLength; }

private:
    int m_iLength;
    int m_ArrVec[100];
};

If we create an instance of the above class, it will waste memory space because of the pre-definition of the m_ArrVec to be at the size of 100.What if we need an array of more than 100 integers? This class is inadequate for a definition of a general array.

We can resolve the problem by altering the fixed size to be a pointer, and storing the size of the array in another data member as in the following class:

class CExampleArray
{
public:
    int getLength() { return m_iLength; }

private:
    int m_iLength;
    int* m_ArrVec;
    short    m_iArrSize;
};
Another Simple example

#include<iostream>

using namespace std;

//Shape is an Interface Class. No data and everything pure virtual
class Shape {
public:
  virtual void Area(int length, int breadth) = 0;
  virtual void Perimeter(int length, int breadth) = 0;
  //Note, no data
};

//Derived class - Inherits Shape as Public
class Rectangle : public Shape {
public:
  void Area(int length, int breadth);
  void Perimeter(int length, int breadth);
private:
  int someData;
};

//Derived class - Inherits Shape as Public
class Triangle : public Shape {
public:
  void Area(int length, int breadth);
  void Perimeter(int length, int breadth);
private:
  int someData;
};

int main()
{
  Rectangle r;
  Triangle t;

  cout<<"\n\n";
  r.Area(3,4);
  r.Perimeter(3,4);

  t.Area(3,4);
  t.Perimeter(3,4);

  cout<<"\n\n";
  return 0;
}

void Rectangle::Area(int length, int breadth)
{
  cout<<"\nThe Area of Rectangle for length = "<<length<<" and\
  breadth = "<<breadth<<" is "<<(length * breadth)<<endl;
}

void Rectangle::Perimeter(int length, int breadth)
{
  cout<<"\nThe Perimeter of Rectangle for length = "<<length<<" and\
  breadth = "<<breadth<<" is "<<2 * (length + breadth)<<endl;
}

void Triangle::Area(int length, int breadth)
{
  cout<<"\nThe Area of Triangle for length = "<<length<<" and\
  breadth = "<<breadth<<" is "<<(length * breadth)/2<<endl;
}

void Triangle::Perimeter(int length, int breadth)
{
  cout<<"\nThe Perimeter of Triangle for length = "<<length<<" and\
  breadth = "<<breadth<<" is "<<(length * breadth)/3<<endl;
}

In the above example, Shape class is an 'abstract class'. All the interfaces are 'pure interfaces'. It contains no data. The derived classes now have to define the interace functions.

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