Programming for Beginners

Animation and Keyboard

Using Explicit Conversions, Again

There is a hidden type conversion in some programs mentioned in this chapter. As usual, we would like to make obvious where this conversion happens and what the consequences are.

The center of the circle to be drawn by the circlefill function is given by a pair of coordinates. Although it does make sense to place a center of the circle even on non-integer coordinates, the Allegro library lacks the ability to draw such circles. Allegro is only able to draw a circle that is centered in the middle of a pixel, and therefore the circlefill function accepts only integer values for coordinates of its center, as well as for the radius.

The Allegro library was selected as graphics library for programs in this book largely due to Allegro's enormous simplicity. In the future, you might use a different library to draw graphics, or even a completely different language. Most basic principles that you learn throughout this book remain the same regardless of the library or the language you use. We will, however, continue using Allegro because it makes the essentials of graphics programming quite easy.

To get back to the type conversion issue: in the program above, the value of variable x, being of type double, gets automatically converted to the type int because the circlefill function accepts only integers for coordinates. To make this conversion explicit, we could use the cast operator like this:

        circlefill(surface, int(x), 200, 50, makecol(0,0,255));

Also, note that the same applies to all the drawing functions from the Allegro library, including the functions line, rect and rectfill. They only take integer values for coordinates. The putpixel function has to use integer values in this library and in any other, since it makes no sense otherwise.

#pragma warning(push)
#include <allegro.h>
#pragma warning(pop)