5.4. Overloading

In the previous section you might have noticed that fred and area perform similar functions—finding the area of a circle—but take different parameters. For area, we have to provide the radius; for fred, we provide two points.

If two functions do the same thing, it is natural to give them the same name. In other words, it would make more sense if fred were called area.

Having more than one function with the same name, which is called overloading, is legal in C++ as long as each version takes different parameters. So we can go ahead and rename fred:

double area (double xc, double yc, double xp, double yp) {
  return area (distance (xc, yc, xp, yp));

This looks like a recursive function, but it is not. Actually, this version of area is calling the other version. When you call an overloaded function, C++ knows which version you want by looking at the arguments that you provide. If you write:

double x = area (3.0);

C++ goes looking for a function named area that takes a double as an argument, and so it uses the first version. If you write

double x = area (1.0, 2.0, 4.0, 6.0);

C++ uses the second version of area.

Many of the built-in C++ commands are overloaded, meaning that there are different versions that accept different numbers or types of parameters.


Although overloading is a useful feature, it should be used with caution. You might get yourself nicely confused if you are trying to debug one version of a function while accidentally calling a different one.

Actually, that reminds me of one of the cardinal rules of debugging: make sure that the version of the program you are looking at is the version of the program that is running! Some time you may find yourself making one change after another in your program, and seeing the same thing every time you run it. This is a warning sign that for one reason or another you are not running the version of the program you think you are. To check, stick in an output statement (it doesn’t matter what it says) and make sure the behavior of the program changes accordingly.

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