Implementing interfaces in c++? - c++

Question

So I saw this thread: What does it mean to "program to an interface"?. Which talks about declaring a Parent, but initializing it as a Child. Is it possible to do the same but with c++? For example: I have an interface Shape which can be implemented as Triangle or Square.
I tried to do the following but my program didn't compile:
Shape * myShape = new Square();
myShape->uniquetoSquare();
"typeinfo for Shape", referenced from:
typeinfo for Triangle in Triangle.o
class Shape {
public:
Shape(){};
int height;
int width;
string color;
void sayHello(){
cout << "Hello!";
}
int getArea(){
return 0;
}
}
class Triangle: public Shape {
public:
bool obtuse;
Triangle(){
obtuse = false;
};
void sayHello(){
cout << "Hello I'm a triangle!";
}
int getArea(){
return height*width / 2;
}
}
class Square: public Shape {
public:
bool rectangular
Square(){
rectangle = true;
};
void sayHello(){
cout << "Hello I'm a square!";
}
int getArea(){
return height*width;
}
void uniqueToSquare(){
cout << "This func is only in square!";
}
}

Answer 1

Shape does not have a function named uniqueToSquare. Remember that if you are using a Shape, then you can only use shape-like methods.
If you want to use it as a Square, then do something like:
Square* mySquare = dynamic_cast<Square*>(myShape);
if (mySquare != nullptr) mySquare->uniqueToSquare();

Answer 2

This is a simple polymorphism example. Yes it is possible in C++ and is actually the heart of the benefits of Object-Oriented design
With C++ it isn't as simple as you might hope. You need virtual functions to do what you want. Modifying your example to be "good" C++.
class Shape {
public:
Shape(){}
int height;
int width;
string color;
virtual void sayHello() const {
cout << "Hello!";
}
virtual int getArea() const {
return 0;
}
//virtual destructor, no need to declare this on the derived types
virtual ~Shape() {}
}
class Triangle : public Shape {
public:
bool obtuse;
Triangle() {
obtuse = false;
}
void sayHello() const {
cout << "Hello I'm a triangle!";
}
int getArea() const {
return height*width / 2;
}
}
class Rectangle : public Shape {
public:
bool square;
Rectangle() {
square = false;
}
void sayHello() const {
cout << "Hello I'm a rectangle!";
}
int getArea() const {
return height*width;
}
void uniqueToRectangle() const {
cout << "This func is only in rectangle!";
}
}
The error your getting seems to be because of missing runtime type information RTTI. You may want to enable this but it really is unnecessary for what you want to achieve. Use dynamic cast as others have suggested (which also uses RTTI).
Shape* myShape = new Rectangle();
((Rectangle*)myShape)->uniquetoRectangle();
This is OK but remember to delete myShape; before it goes out of scope. See RAII for the use of destructors for that.

Answer 3

Your variable myShape is a pointer to a Shape. Your class Shape does not have a method called uniqueToSquare(). Therefore, you have an error.
The solution is to initialise myShape as a Square simply by Square myShape = Square();
Remember that it is always better to preserve the real type of an object, this is better for performance, and basically gives more information to the compiler. You only resort to dynamic polymorphism when you really have to (ie, storing polymorphic objects in an array).