11.1. Objects and functions

C++ is generally considered an object-oriented programming language, which means that it provides features that support object-oriented programming.

It’s not easy to define object-oriented programming, but we have already seen some features of it:

  1. Programs are made up of a collection of structure definitions and function definitions, where most of the functions operate on specific kinds of structures (or objects).

  2. Each structure definition corresponds to some object or concept in the real world, and the functions that operate on that structure correspond to the ways real-world objects interact.

For example, the Time structure we defined in Section 9.1 obviously corresponds to the way people record the time of day, and the operations we defined correspond to the sorts of things people do with recorded times. Similarly, the Point and Rectangle structures correspond to the mathematical concept of a point and a rectangle.

So far, though, we have not taken advantage of the features C++ provides to support object-oriented programming. 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, in the Time program, there is no obvious connection between the structure definition and the function definitions that follow. With some examination, it is apparent that every function takes at least one Time structure as a parameter.

This observation is the motivation for member functions. Member function 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 struct definition, in order to make the relationship between the structure and the function explicit.

In the next few sections, we will take the functions from Section 9.1 and transform them into member functions. 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.

As I said, 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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