Macros and #define
beginner14 min readLesson 61 of 148
Text substitution with rules: object macros, function-like macros, and the parenthesis trap.
Two kinds of macros
#define MAX_SCORE 100 // object macro: a named constant
#define SQUARE(x) ((x) * (x)) // function-like macro
The preprocessor replaces text BEFORE compilation. SQUARE(a+b) becomes
((a+b) * (a+b)) — the compiler never sees the name SQUARE.
The parenthesis trap
#define BAD_SQUARE(x) (x * x)
BAD_SQUARE(2 + 3) // becomes (2 + 3 * 2 + 3) = 11 — not 25!
Every parameter AND the whole expansion get parentheses. This is not style; it is correctness.
The double-evaluation trap
#define MAX(a, b) ((a) > (b) ? (a) : (b))
int i = 5;
MAX(i++, 3); // i++ may be evaluated TWICE — side effects multiply
Macros paste text; arguments with side effects are dangerous. A do-while wrapper gives statement-like macros sane semantics:
#define LOG(msg) do { fprintf(stderr, "%s\n", msg); } while (0)
#undef and scope
Macros live from their #define to the end of the file (or #undef). They do
not respect C scopes — a macro named SIZE will clobber any SIZE in every
header included after it. Name them deliberately (or not at all: a const
or static const is often the better tool).