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Objects that are const-qualified have scope and can be type-checked by the compiler. Because these are named objects (unlike macro definitions), some debugging tools can show the name of the object. The object also consumes memory.

A const-qualified objects object allows you to specify the exact type of the constant. For example:

unsigned int const buffer_size = 256;

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{{const}}\-qualified objects are likely to incur some runtime overhead  \[[Saks 01b|AA. C References#Saks 02]\]. Most C compilers, for example, allocate memory for {{const}}\-qualified objects. {{const}}\-qualified objects declared inside a function body may have automatic storage duration. If so, the compiler will allocate storage for the object, and it will be on the stack. As a result, this storage will need to be allocated and initialized each time the containing function is invoked.

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A preprocessing directive of the form:

# define identifier replacement-list

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C programmers frequently define symbolic constants as object-like macros. For example, the code:

#define buffer_size 256

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In this compliant solution, the integer literal is replaced with an enumeration constant (see DCL00-A. Const-qualify immutable objects).

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Compliant Solution (sizeof)

Frequently, it is possible to obtain the desired readability by using a symbolic expression composed of existing symbols rather than by defining a new symbol. For example, a sizeof expression can work just as well as an enumeration constant (see EXP09-A. Use sizeof to determine the size of a type or variable).

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Using the {{sizeof}} expression in this example reduces the total number of names declared in the program, which is generally a good idea \[[Saks 02|AA. C References#Saks 02]\].  The {{sizeof}} operator is almost always evaluated at compile time (except in the case of variable -length arrays).

When working with sizeof(), keep in mind ARR01-A. Do not apply the sizeof operator to a pointer when taking the size of an array.

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Replacing numeric constants with symbolic constants in this example does nothing to improve the readability of the code, and may actually make the code more difficult to read:.

enum { TWO = 2 };     /* a scalar */
enum { FOUR = 4 };    /* a scalar */
enum { SQUARE = 2 };  /* an exponent */
x = (-b + sqrt(pow(b, SQUARE) - FOUR*a*c))/ (TWO * a);

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