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C++ Interface

In this tutorial we will discuss interfaces in C++ and why one would be of benefit.  First off, an interface serves as a means to have a commonality of usage for varies objects.  What I mean by that is that say you have a class representing a basketball and a class that represents a soccer ball, it is understood that both are used for playing but how they are played with is different.  That difference of how they are used for playing is where interfaces come in.

An Interface provides a common way of using a class of type basketball and a class of type soccer ball without having will being able to refer to each different object(basketball or soccer ball) with the same interface.

Okay okay, this is confusing so let us take a couple of examples.

Without an interface, to have an object of basketball and soccer ball perform the same behavior, playing, you could do this:


class BasketBall{
public:
     void play(){
          cout << "playing with basketball" << endl;
     }
};
class SoccerBall{
public:
     void play(){
            cout << "playing with basketball" <<endl;
     }
}
int main(){
     BasketBall* bbball = new BasketBall();
     bball->play();
     delete bball;

     SoccerBall *sball = new SoccerBall();
     sball->play();
     delete sball;
     return 0;
}

From the preceding, we declared two class and declare two objects: bball and sball.  Take note that the two objects have a common usage.  That is all well and good, but the cost is that if you have a function that is used to execute the play method of these two functions, then you have to declare two functions of the type of the two respective objects.  


class BasketBall{
public:
     void play(){
          cout << "playing with basketball" << endl;
     }
};
class SoccerBall{
public:
     void play(){
            cout << "playing with basketball" <<endl;
     }
}
void executeBasketBallPlay(BasketBall* ball){
     ball->play();
}
void executeSoccerPlay(SoccerBall* ball){
     ball->play();
}
int main(){
     BasketBall* bbball = new BasketBall();
    executeBasketBallPlay(bball);
     delete bball;

     SoccerBall *sball = new SoccerBall();
     executeSoccerPlay(sball);
     delete sball;
     return 0;
}

Here we see that it is inefficient to have two separate functions that will execute essentially the same type of behavior from different class types.  So, the prudent thing to do would be to design a more efficient way of interacting with our balls (hey, that is kind of funny).  So, by now we have noted that our two class a common interface: the play method.  So we need to design a way to have other functions or classes in our code to respect this common interface/commonality between these two balls.

So here we go.

//here we define an interface, it is nothing more than a class with only pure
//virtual functions, denoted by the virtual keyword and the "=0" suffix
class PlayInterface {
public:

     virtual void play() = 0;
};


class BasketBall : public PlayInterface {
public:
     void play(){
          cout << "playing with basketball" << endl;
     }
};
class SoccerBall : public PlayInterface {
public:
     void play(){
            cout << "playing with basketball" <<endl;
     }
}

void executePlay(PlayInterface* ball){
     ball->play();
}

int main(){
     BasketBall* bbball = new BasketBall();
     executePlay (bball);
     delete bball;

     SoccerBall *sball = new SoccerBall();
     executePlay(sball);
     delete sball;
     return 0;
}

Now we have significantly increased the effectiveness of our code because we have only on method that will execute a simple command: playing with balls.  What this does is that executePlay takes a valid object of type of any type so long as it derives or implements PlayInterface, if it does then it executes it.

This is beneficial in many ways but namely because it allows use to not care about type matching and allows us to focus on common behaviors of objects.



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