What are the implecations of making a function pure virtual function?

Only class instance methods can be rendered as pure-virtual functions (or pure-virtual methods, to be precise). Non-member functions and static member methods cannot be declared pure-virtual.

The implication of declaring a pure-virtual function is that the class to which it is a member becomes an abstract data type (or abstract base class). This means you cannot instantiate an object of that class, you can only derive classes from it. Moreover, the derived classes must also implement the pure-virtual functions or they, too, become abstract data types. Only concrete classes that contain a complete implementation can be instantiated, although they can inherit implementations from their base classes (but not from the class that initially declared the method).

Pure-virtual functions ensure that you do not instantiate base class objects that are not intended to be instantiated (they are conceptual rather than actual objects) and that derived objects provide a specific implementation. Typically, all methods of an abstract base class will be declared pure-virtual to ensure correct behaviour in derived classes through a common interface. Abstract base classes will also have few member variables, preferably none at all.

For example, a shape is a conceptual object whereas a rectangle, triangle or circle are actual objects. As such they can all be derived from shape. The shape base class need only declare the interface that is common to all shapes, such as Draw(), Rotate(), Mirror() and so on. But since a shape doesn't have enough information to implement these methods, they must be declared pure-virtual. This ensures that shape cannot be instantiated (since it would be meaningless) and that every object derived from shape not only has a common interface, but that each type of shape provides its own specific implementation of those methods. This is not unlike overriding standard virtual functions, however the difference is that you must override the pure-virtual methods; it is not optional unless the derived class is intended to become abstract itself.