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Member Functions or Methods

As mentioned earlier, C++ is generally considered an object-oriented programming (OOP) language . One of the features of OOP is that it provides a way to define objects that behave very much like real world objects. For example, the class Point we defined before, obviously corresponds to the way people think about coordinates on the screen or in the movie, and the operations we defined ( GetDistance , Reflect and GetReflectedCopy ) correspond to the sorts of things people do with these coordinates.

So far, though, we have not taken advantage of the features C++ provides to support object-oriented programming (except using internal WCM objects). Strictly speaking, these features are not necessary. For the most part they provide an alternate syntax for doing things we have already done, but in many cases the alternate syntax is more concise and more accurately conveys the structure of the program.

For example, at first glance there is no obvious connection between class Point and functions GetDistance , Reflect and GetReflectedCopy . With some examination, it is apparent that these functions take at least one point object as an argument.

This observation is the motivation for member functions or so called methods . Member functions differ from the other functions we have written in two ways:

  1. When we call the function, we invoke it on an object, rather than just call it. People sometimes describe this process as “performing an operation on an object,” or “sending a message to an object.”

  2. The function is declared inside the class definition, in order to make the relationship between the structure and the function explicit.

Note: It was mentioned earlier that the names “function” and “method” are used differently in different languages and, in informal discussions, are essentially the same. “Method” and “Member Function” as used in this section have very specific C++ definitions.

In the next few sections, we will take some functions and transform them into member functions or C++ methods. One thing you should realize is that this transformation is purely mechanical; in other words, you can do it just by following a sequence of steps.

Anything that can be done with a member function can also be done with a nonmember function (sometimes called a free-standing function). But sometimes there is an advantage to one over the other. If you are comfortable converting from one form to another, you will be able to choose the best form for whatever you are doing.

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