C++ Operator Overloading & Friend Functions: Examples & UML Diagrams

1. C++ Lecture 7  Function/operator overloading  Friend functions – C++ example  UML – UML diagrams Use-case diagram Class diagram sequence diagram  Unit Assessment

2. Function/operator overloading  Function overloading – constructor functions – actual function called determined by the number and type of the arguments  Operators – Unary e.g. -, !, ~, ++, -- – Binary e.g. +,-,*,/,<<,>> etc.  Operators are treated as functions by C++  Therefore the operators can be overloaded

3. Operator overloading  It is therefore possible to write operator functions for the operators.  The functions will be called operator@, where @ is the actual operator  There will possibly be many operator functions all with the same name. They must differ in the number and type of their arguments.  This is operator overloading.

4. Operators as functions Consider an object obj of class CAny CAny obj; The normal way of invoking a member function of the class CAny is :- obj.func() Now consider a binary operator e.g. objx @ objy where objx and objy are instances of some class and @ is a binary operator such as +,-,*,/, << or >>

5. Operators as functions objx @ objy is interpreted by the C++ compiler in on of two ways  objx.operator@(objy) – i.e. the compiler looks for a a member function called operator@ in the class of which objx is an instance. – if such a member function cannot be found in the class then the following is tried operator@(objx, objy) – i.e. the compiler looks for an ordinary function called operator@ that takes two arguments with the appropriate data types. 

6. Operator overloading  We have a choice over which method to implement : – either as a member function of a class – or as an ordinary function  objx @ objy is treated as objx.operator(objy) – If the operator is a member function then this implies that the object to the left of the operator MUST be an instance of the class containing the operator function. – What if the object on the left is not an instance of the class containing the operator function?

7. Friend Functions  We might therefore implement the operator as an ordinary function:- – operator@(objx, objy) – Since this is NOT a member function how can it access the private data of the objects the objx, objy class could have appropriate accessor and mutator functions. This would be less efficient than if the operator function could access the private data directly. C++ added Friends!

8. Friend Functions  Friends are functions or whole classes which are granted access to the private data of a class.This breaks the encapsulation  A friend is declared within the class which is allowing friendship.  A function cannot make itself a friend of a class.  Friendship should be used thoughtfully and sparingly  A common use of friend functions is to allow the use of << and >> with cout and cin for output and input of user defined types.

9. Unified Modelling Language - UML  UML is a modelling tool to capture and represent in the form of diagrams and text the various stages of the software development process  There are 8 different diagram types. We shall only consider 3 – Use-case : capture user interaction with the system – Class : show relationship between objects and classes – Sequence : show the interaction of objects.  Rational Rose – UML development tool