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When should I parenthesize macro arguments in C?
I've seen macros use parentheses to enclose its arguments. Most of these make sense, as in
#define sum(a, b) ((a) + (b))
The outer prevents the following:
#define sum_bad(a, b) (a) + (b)
s = sum_bad(x, y) * z; // (x) + (y) * z
The inner prevents the following:
#define mul_bad(a, b) (a * b)
m = mul_bad(x, y + z); // (x * y + z)
However, I've seen some parentheses that don't seem justified.
#define foo(a, b) bar((a), (b)) // Why not just `bar(a, b)`
#define asd(a, b) do \
{
int x = (b); // Why not just `= b;`?
zxc(x);
} while (0)
Is it really necessary to always enclose macro arguments in parentheses, or is it superfluous in some cases?
2 answers
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With macros, you are better off using parens at any time you use one of the parameters in an expression context.
The problem with operator precedence is not limited to + and *. Did you know that comma (,) is an operator? Did you know that comma has a lower precedence than assignment? Did you know that assignment is considered an operator in C?
Did you know that comma is also used to declare multiple variables of the same type?
What does the expression x = a, b do? What about int x = a, b; ?
In general, if you are using macro parameters as "values", you should go ahead and parenthesize them. It generally doesn't hurt, and it might help.
Only if you are using macro parameters as text or code or strings should you (maybe) not parenthesize them. That is, if you are using a macro parameter to construct a name, like:
#define unique_name(base) base ## __COUNTER__
then don't use parens, because you want the "raw" value of base (and because it wouldn't make sense with parens). Likewise if you are stringizing the parameter:
#define DO_PRAGMA(x) _Pragma (#x)
then don't use parens, because you don't want parens in the string you are creating. (Or, if you do want them, then go ahead!) Also, parens interfere with string literal concatenation. So it's generally better to leave stringized expressions bare. And finally if the parameter is itself code, not an expression:
#define DO_ONCE(code, ...) do code, ## __VA_ARGS__ while(0)
then don't use parens because they are likely to be invalid syntax. Maybe use braces {...} instead. (But really that's a horrible macro that I just made up to make a point. Don't do things like that in real life!)
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The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| alx | (no comment) | Nov 13, 2023 at 16:29 |
Simply put: parenthesis are used whenever we suspect that there may be operator precedence issues.
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Either because the user passed an expression containing several operands and operators to the macro.
To deal with this we need to surround the use of each macro parameter with parenthesis.
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Or because the macro is used in an expression together with other operators.
To deal with this, the function-like macro needs to be wrapped in an outer parenthesis.
And yes there exists plenty of situations where we don't really need to worry about what the user passed, since it wouldn't make a difference for the macro. Or if they pass something weird and therefore get a compiler error, then that's not necessarily a bad thing.
But it is good to keep ones coding style consistent and analyzable by tools. If we always follow both of the above mentioned best practices with parenthesis, then we can also verify, by means of a static analyzer tool, that no bugs caused by missing parenthesis exist.
It's kind of the same thing as writing {} after each if. While writing the code we can conclude that the braces after if aren't needed in some cases, yet we know that if we go lax with our coding style and start skipping them, that lax coding style will eventually lead to bugs.
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