Re: What's the purpose of these parentheses in the #define's?
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- Subject: Re: What's the purpose of these parentheses in the #define's?
- From: Philipp Janda <siffiejoe@...>
- Date: 2015年1月29日 14:53:13 +0100
Am 29.01.2015 um 11:33 schröbte Philippe Lhoste:
On 26/01/2015 10:00, Philipp Janda wrote:
Am 26.01.2015 um 08:51 schröbte William Ahern:
The macro arguments might themselves be comma expressions. So the
parentheses aren't useless.
You can't have comma expressions without parentheses in argument lists.
#define X(a, b) ...
X(1, 2); /* ok */
X(1, 2, 3); /* constraint violation: wrong number of arguments */
X((1, 2), 3); /* ok, using comma expression as first argument */
What about vararg functions? (My C is rusty, so it can be an invalid
remark!)
The argument lists of vararg functions (or functions without prototype)
are the same. You won't get a compiler diagnostic, but you still have
the wrong number of arguments. But we are talking about function-like
macros anyway. Basically, if it parses as one argument in the argument
list of a function-like macro, it will parse as one argument in the
argument list of a function call in the macro replacement without
additional parentheses. However, there is one corner case (and it in
fact has to do with a combination of comma expressions and vararg
functions):
You *can* have comma expressions without parentheses in the argument
list of functions (but not function-like macros) if the comma expression
is in the middle part of a ternary expression. So
f(a<b ? a,b : b);
will call `f` with one argument if `f` is a function, but two arguments
if `f` is a function-like macro. As a consequence the following
#define X(a, b) printf("%d %d\n", a, b)
X(1<2 ? 1,2 : 3);
will lead to a `printf` call with one missing argument. On the other hand
#define X(a, b) printf("%d %d\n", (a), (b))
X(1<2 ? 1,2 : 3);
will be flagged as a syntax error by the compiler. So in some rare cases
the extra parentheses can help you catch incorrect usage of a macro.
Of course, this discussion is mostly theoretical, I would say as above
the parentheses are here mostly for consistency (with other macros) and
common practice.
As the OP noted, the Lua source isn't consistent in this regard.
Philipp