DCL00-A. Declare immutable values using const or enum

Immutable (constant values) should be declared as const-qualified objects (unmodifiable lvalues), enumerations values, or as a last resort, using #define.

In general, it is preferable to declare immutable values as const-qualified objects rather than as macro definitions. Using a const declared value means that the compiler is able to check the type of the object, the object has scope, and (certain) debugging tools can show the name of the object. Const-qualified objects cannot be used where compile-time integer constants are required, namely to define the:

• size of a bit-field member of a structure
• size of an array
• value of an enumeration constant
• value of a case constant.

If any of these are required, then an integer constant (an rvalue) must be used. For integer constants, it is preferable to use an enum instead of a const-qualified object as this eliminates the possibility of taking the address of the integer constant and does not required that storage is allocated for the value.

Non-Compliant Code Example 1

In this example, PI is defined using a macro. In the code, the value is introduced by textual subsitution.

#define PI 3.14159
...
float degrees;
float radians;
...
radians = degrees*PI/180;

Compliant Solution 1

In this compliant solution, the constant is defined as a const variable.

float const pi = 3.14159;
...
float degrees;
float radians;
...
radians = degrees*pi/180;

Non-Compliant Code Example 2

Delcaring immutable integer values as const-qualified objects still allows the programmer to take the address of the object. Also, the constant cannot be used in locations where an integer constant is required, such as the size of an array.

int const max = 15; 
int a[max]; /* invalid declaration */
int const *p; 

p = &max; /* legal to take the address of a const-qualified object */

Most C compilers will also allocate memory for the const-qualified object.

Compliant Solution 2

This compliant solution uses an enum rather than a const-qualified object or a macro definition.

enum { max = 15 }; 
int a[max]; /* OK */
int const *p; 

p = &max; /* error: '&' on constant */

Risk Assessment

References

[[ISO/IEC 9899-1999]] Sections 6.3.2.1, "Lvalues, arrays, and function designators"; 6.7.2.2, "Enumeration specifiers"; 6.10.3, "Macro replacement"