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Now let's say that we want to create a structure to represent a rectangle. The question is, what information do I have to provide in order to specify a rectangle? To keep things simple let's assume that the rectangle will be oriented vertically or horizontally, never at an angle.

There are a few possibilities: I could specify the center of the rectangle (two coordinates) and its size (width and height), or I could specify one of the corners and the size, or I could specify two opposing corners.

The most common choice in existing programs is to specify the upper left corner of the rectangle and the size. To do that in C++, we will define a structure that contains a Point and two doubles.

struct Rectangle {
  Point corner;
  double width, height;

Notice that one structure can contain another. In fact, this sort of thing is quite common. Of course, this means that in order to create a Rectangle, we have to create a Point first:

  Point corner = { 0.0, 0.0 };
  Rectangle box = { corner, 100.0, 200.0 };

This code creates a new Rectangle structure and initializes the instance variables. The figure shows the effect of this assignment.

We can access the width and height in the usual way:

  box.width += 50.0;
  cout << box.height << endl;

In order to access the instance variables of corner, we can use a temporary variable:

  Point temp = box.corner;
  double x = temp.x;

Alternatively, we can compose the two statements:

  double x = box.corner.x;

It makes the most sense to read this statement from right to left: "Extract x from the corner of the box, and assign it to the local variable x."

While we are on the subject of composition, I should point out that you can, in fact, create the Point and the Rectangle at the same time:

  Rectangle box = { { 0.0, 0.0 }, 100.0, 200.0 };

The innermost squiggly braces are the coordinates of the corner point; together they make up the first of the three values that go into the new Rectangle. This statement is an example of nested structure.

Last Update: 2005-12-05