C++ By Example: UnderC Learning Edition

Constants: const and enum

It is very useful to define symbolic constants. The following example calculates the areas of circles, and so PI is defined as a constant:

NOTE

Constants are commonly written in uppercase so that it's easy to identify them in code.

;> double PI = 3.14;
;> double r = 2.3;
;> PI*r*r;
(double) 16.6106

Using const Declarations

Even though it is a constant, it is possible to assign values to PI, which seems to contradict the idea of a constant. (One of Murphy's Laws for software development is “Variables won't and constants aren't.”) One C++ solution is the const modifier. When applied to a type in a declaration, it flags the declared symbol as not being modifiable:

;> const double PI = 3.14;
;> PI = 3; CON 3: Cannot ...