pitfalls of macro substitution

[johnmerlino]

"If you examine the expansion of max, you will notice some pitfalls. The expressions are evaluated twice; this is bad if they involve side effects like increment operators or input and output. For instance, the below example will increment the larger twice."

#define max(A, B) ((A) > (B) ? (A) : (B))

max(i++, j++)	/* WRONG */

I don't see what the problem is with the code above. i is incremented and j is incremented and then it performs a ternary operation to see which is greater. Am I missing something?

[Catfish666]

Macros are textual substitution, they're basically a copy-paste mechanism.
By the way, did you know that every time you #include something, that entire file just gets copy-pasted into the current one?

So:

#define max(A, B) ((A) > (B) ? (A) : (B))

max(i++, j++) // after preprocessor expands this macro, it becomes

((i++) > (j++) ? (i++) : (j++))

Here's another example:

#define MULTILINE_MACRO(x) {\
    function_a(x); \
    function_b(x); \
    function_c(x); \
}

if (x != 0)
    MULTILINE_MACRO(x); // boom
else
    something_else();

// the above if() looks like

if (x != 0)
{
    function_a(x);
    function_b(x);
    function_c(x);
}; // <--- boom
else
    something_else();

Which is why multi-line macros are sometimes written like:

#define MULTILINE_MACRO(x) do {\
    function_a(x); \
    function_b(x); \
    function_c(x); \
} while (false)