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...

/* // ... */
if (age >= 18) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* // ... */

Compliant Solution

The compliant solution replaces 18 with the symbolic constant ADULT_AGE to clarify the meaning of the code.

...

enum { ADULT_AGE=18 };
/* // ... */
if (age >= ADULT_AGE) {
   /* Take action */
}
else {
  /* Take a different action */
}
/* // ... */

Exceptions

While replacing numeric constants with a symbolic constant is often a good practice, it can be taken too far. Exceptions can be made for constants that are themselves the abstraction you want to represent, as in this compliant solution.

...

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);

...

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[http://www.doc.ic.ac.uk/lab/cplus/c++.rules/chap10.html]
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7, "Declarations"