The C++Course provides a general introduction to programming in C++. It is based on A.B. Downey's book, How to Think Like a Computer Scientist. Click here for details.

Point Objects

As a simple example of a compound structure, consider the concept of a mathematical point. At one level, a point is two numbers (coordinates) that we treat collectively as a single object. In mathematical notation, points are often written in parentheses, with a comma separating the coordinates. For example, (0, 0) indicates the origin, and (x, y) indicates the point x units to the right and y units up from the origin.

A natural way to represent a point in C++ is with two doubles. The question, then, is how to group these two values into a compound object, or structure. The answer is a struct definition:

struct Point {
  double x, y;

struct definitions appear outside of any function definition, usually at the beginning of the program (after the include statements).

This definition indicates that there are two elements in this structure, named x and y. These elements are called instance variables, for reasons I will explain a little later.

It is a common error to leave off the semi-colon at the end of a structure definition. It might seem odd to put a semi-colon after a squiggly-brace, but you'll get used to it.

Once you have defined the new structure, you can create variables with that type:

  Point blank;
  blank.x = 3.0;
  blank.y = 4.0;

The first line is a conventional variable declaration: blank has type Point. The next two lines initialize the instance variables of the structure. The "dot notation" used here is similar to the syntax for invoking a function on an object, as in fruit.length(). Of course, one difference is that function names are always followed by an argument list, even if it is empty.

The result of these assignments is shown in the following state diagram:

As usual, the name of the variable blank appears outside the box and its value appears inside the box. In this case, that value is a compound object with two named instance variables.

Last Update: 2005-11-21