interface and implementation of a class,

An interface describes the behavior or capabilities of a C++ class without committing to a particular implementation of that class.

The C++ interfaces are implemented using abstract classes and these abstract classes should not be confused with data abstraction which is a concept of keeping implementation details separate from associated data.

A class is made abstract by declaring at least one of its functions as pure virtual function. A pure virtual function is specified by placing "= 0" in its declaration as follows:

class Box {
   public:
      // pure virtual function
      virtual double getVolume() = 0;
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};

implementation of a Interface class

#include <iostream>
 
using namespace std;
 
// Base class
class Shape  {

public:
   // pure virtual function providing interface framework.
   virtual int getArea() = 0;
 
   void setWidth(int w) {
      width = w;
   }
 
   void setHeight(int h) {
      height = h;
   }
 
protected:
   int width;
   int height;
};
 
// Derived classes
class Rectangle: public Shape {

public:
   int getArea() { 
      return (width * height); 
   }
};

class Triangle: public Shape {

public:
   int getArea() { 
      return (width * height)/2; 
   }
};
 
int main(void) {
   Rectangle Rect;
   Triangle  Tri;
 
   Rect.setWidth(5);
   Rect.setHeight(7);
   // Print the area of the object.
   cout << "Total Rectangle area: " << Rect.getArea() << endl;

   Tri.setWidth(5);
   Tri.setHeight(7);
 
   // Print the area of the object.
   cout << "Total Triangle area: " << Tri.getArea() << endl; 

   return 0;
}

When the above code is compiled and executed, it produces the following result:

Total Rectangle area: 35
Total Triangle area: 17