Question

I’m looking at the following code in my book:

class Shape
{
    public:
        Shape(){}
        ~Shape(){}
        virtual long getArea() = 0; // Pure virtual function
        virtual long getPerim() = 0;
        virtual void draw() = 0;
};

Now it says that these virtual functions make the class abstract (which I understand from Java), so the class cannot be instantiated.

However, it says: “a class is in abstract data type by including one or more virtual functions in the class declaration.”

Would this mean if I declared a class with ONE pure virtual function:

class Shape
{
    public:
        Shape(){}
        ~Shape(){}
        virtual long getArea() = 0; // Only pure virtual function
        virtual long getPerim(){}
        virtual void draw(){}
};

does the whole class become abstract? Because if a class has 100+ methods, it’ll be tedious to write =0 for every method if I decide to make it abstract later.

Answer 1

Yes, a single pure virtual method is enough to make the class abstract.

Moreover, you can always make the destructor pure virtual if no other methods are suited to be pure.

Also, your destructor should probably be virtual, since you’re obviously going to inherit from this class. This is mandatory if you plan on deleting objects of a derived type through a pointer to a base type.