On Mon, 21 Feb 2011 12:12:15 -0500, Lorenzo Caminiti wrote:
On Mon, Feb 21, 2011 at 9:19 AM, Edward Diener <eldiener@tropicsoft.com> wrote:
However, DATA_SIZE() => 0 is ill-conceived. An empty argument is still an argument to the preprocessor. A better correlation is:
DATA_SIZE(,,) => 3 DATA_SIZE(,) => 2 DATA_SIZE() => 1
Thanks ! I will just have to further document that the data size returned can never be 0, even when the variadic macro is invoked with an empty argument.
IMO, that makes sense -- so documentation is a good option.
However, I still don't understand why MSVC accepts this DATA_SIZE() invocation:
#define DATA_SIZE(...) DATA_SIZE(1) DATA_SIZE() // No error -- why??
#define SIZE(x) SIZE(1) SIZE() // Error -- as it should!
Neither case should be an error. According to the C99 standard (and C+ +0x), the sequence of tokens (and whitespace separations) that makes up a macro argument may be empty or contain no tokens. If the sequence of tokens (and whitespace separations) contains no tokens, the formal parameters in the replacement list are replaced by a "placemarker" which is a sort of virtual token. So, in the following scenarios: #define MACRO(x) [x] MACRO(123) // [123] MACRO( 123 ) // [ 123 ] MACRO( 123 ) // [ 123 ] // adjacent whitespace is combined // in an earlier phase of translation MACRO( ) // [<placemarker>] -> [] MACRO() // [<placemarker>] -> [] For variadics, for the purposes of substitution, token-pasting, and stringizing, all of the variadic arguments act as one argument: #define MACRO(...) [__VA_ARGS__] MACRO() // [<placemarker>] -> [] MACRO( ) // [<placemarker>] -> [] MACRO(a) // [a] MACRO(a,b) // [a,b] MACRO(a, b) // [a, b] MACRO( a,b, c ) // [ a,b, c ] * Note that I don't know of a single preprocessor that actually handles whitespace correctly in all cases. Regards, Paul Mensonides