If a method returns an interface, what does it mean?

Question

I see many methods that specify an interface as return value. Is my thought true that it means: my method can return every class type that inherits from that interface? if n

Answer 1

C++ supports a programming technique called polymorphism. That is a derived class can look like a base class to other code that knows nothing about the derived classes. Take a look at his example:

class Shape
{
public:
    virtual float Area () const = 0;
};

class Rectangle: public Shape
{
public:
    Rectangle (float width, float height)
        : m_width(width)
        , m_height(height)
    {}

    virtual float Area () const
    {
        return m_width * m_height;
    }

private:
    float m_width;
    float m_height;
};

class Circle: public Shape
{
public:
    Circle (float radius)
        : m_radius(radius)
    {}

    virtual float Area () const
    {
        return 3.141592653f*(m_radius*m_radius);
    }

private:
    float m_radius;
};

Now you can see from this code we've created a base class Shape (our interface) and two derived classes that specialise this class, one a rectangle, another a circle. Now let's create a function that prints out the area of a shape:

void PrintArea (const Shape& shape)
{
    printf("Area of shape = %f",shape.Area());
}

This function doesn't care if its a circle of rectangle. Or it cares about is that it's passed a shape and that you can get the area of it, regardless of type.

So this code uses this function:

 Rectangle r (5.0f,4.0f);
 Circle c (25.0f);

 PrintArea(r);      // Print the area of the rectangle
 PrintArea(c);      // Print the area of the circle

Hope this helps.