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• size of a bit-field member of a structure
• size of an array (except in the case of variable length arrays)
• 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

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(object-like macro)

A preprocessing directive of the form:

# define identifier replacement-list new-line

defines an _object-like_ macro that causes each subsequent instance of the macro name to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\].

In this non-compliant code In this example, PI is defined using a as an object-like macro. In Following the codedefinition, the value is introduced each subsequent occurrence of the string "PI" is replaced by the string "3.14159" by textual substitution.

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

An unsuffixed floating constant, as in this example, has type double. If suffixed by the letter f or F, it has type float. If suffixed by the letter l or L, it has type long double.

Compliant Solution

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In this compliant solution, the constant pi is defined declared as a const variable-qualified object.

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

This is the best solution for non-integer constants.

Non-Compliant Code Example

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(immutable integer values)

In this non-compliant code example, max is declared as a const-qualified object. While declaring non-integer constants as const-qualified object is the best that can be done in C, for integer constants we can do better. Declaring Delcaring immutable integer values as const-qualified objects still allows the programmer to take the address of the object. Also, the constant const-qualified integers cannot be used in locations where an integer constant is required, such as the size of an arrayvalue of a case constant.

int const max = 15;
int a[max]; /* invalid declaration outside of a function */
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 objectobjects.

Compliant Solution

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(enum)

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

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