10. Constants

Rule 36
Constants are to be defined using const or enum; never using #define.
Rule 37
Avoid the use of numeric values in code; use symbolic values instead.

The preprocessor performs a textual substitution for macros in the source code which is then compiled. This has a number of negative consequences. For example, if a constant has been defined using #define, the name of the constant is not recognized in many debuggers. If the constant is represented by an expression, this expression may be evaluated differently for different instantiations, depending on the scope of the name. In addition, macros are, at times, incorrectly written.

Numerical values in code ("Magic Numbers") should be viewed with suspicion. They can be the cause of difficult problems if and when it becomes necessary to change a value. A large amount of code can be dependent on such a value never changing, the value can be used at a number of places in the code (it may be difficult to locate all of them), and values as such are rather anonymous (it may be that every `2' in the code should not be changed to a `3').

From the point of view of portability, absolute values may be the cause of more subtle problems. The type of a numeric value is dependent on the implementation. Normally, the type of a numeric value is defined as the smallest type which can contain the value.

Exception to Rule 36
No exceptions.
Exception to Rule 37
Certain numerical values have a well established and clear meaning in a program. For example, '1' and '0' are often used to represent `true' and `false' respectively. These may be used directly in code without being considered to be "Magic".

Example 42: Different ways of declaring constants.

   // Constants using macros
   #define BUFSIZE 7            // No type checking
   // Constants using const
   const int bufSize = 7;       // Type checking takes place
   // Constants using enums
   enum SIZE { BufSize = 7 };   // Type checking takes place

Example 43: Declaration of const defined in another file.

   extern const char constantCharacter;
   extern const String fileName;