I have a two classes: ------------ --------------- X Y ------------ ------------

Question

I have a two classes:

------------                         ---------------
     X                                       Y
------------                         ---------------
                      relation
------------    ------------------   ---------------
      A()                                   C()
      B()                           
      D()
      E()
------------                         ---------------
I want to inherit just these two functions from X class, where they are defined with the friend access modifier. My class will be:

---------------
      Y                                     
---------------

---------------
    C()
    A()
    D()
---------------
Y class uses some functions of X class namely A and D. How can I model this relationship in a UML class diagram?

Explanation / Answer

If the methods of X are instance methods, you can use the association relationship, which is a solid line between the two classes, however this might indicate to the reader of the model that the relationship is bidirectional. If an instance of Y maintains a reference to X in it and X does not maintain any kind of reference to 'Y', you can add the open-ended arrow to the end pointing toward X to add directionality. If Y can't exist without X, you can also consider using aggregation to indicate this.

If the methods of X are static, consider the dependency relationship, represented by a dotted line and open ended arrow from Y to X. This relationship is used Y calls static methods of X or if the functions of Y take X as a parameter but a reference to X is not maintained within an instance of Y.

In your specific case, when you're trying to model the friend relationship in C++, the information that I can find (such as this question on Stack Overflow) seems to suggest that you should annotate the members of X with the <> stereotype. From this point, I'd use either the association or dependency relationship discussed above, based on how Y is accessing X, with association if Y maintains an instance of X and dependency if Y receives instances of X as parameters to functions as they are needed.

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