The example does not actually use compound literals. (i.e. _arg's initializer would work in C89.) It does use two other C99 features, though -- variadic macros and a for-loop-scoped variable declaration. The biggest compatibility issue, though, are its use of GCC extensions:
- Statement expressions: e.g.: int x = ({ int y = 0; y; });
- If zero-args are allowed (e.g. bar()), then _args is 0-sized, which is also non-standard.
These GCC extensions are somewhat common. Clang has them, and (I think) EDG's frontend does too. MSVC does not have either of them, even in MSVC 2015.
That said, a compound literal might be a good way to remove the extension use, but as long as bar() returns void, the do-while(0) trick is also sufficient for replacing the statement expression.
I think this compound literal usage works:
#define bar(...) (bar( \
sizeof((const char*[]) { NULL, __VA_ARGS__ }) / sizeof(const char*) - 1, \
(const char*[]) { NULL, __VA_ARGS__ } + 1))
bar() results in trailing commas. My impression is that C99 allows them in array initializers. (I saw an example in the n1256 C draft.) I don't recall whether C89 also had them. __VA_ARGS__ is expanded twice, but I think that's OK, because sizeof() only evaluates its operand when it has VLA type, and it never will. This code will work in MSVC 2013 (if not earlier).
- Statement expressions: e.g.: int x = ({ int y = 0; y; });
- If zero-args are allowed (e.g. bar()), then _args is 0-sized, which is also non-standard.
These GCC extensions are somewhat common. Clang has them, and (I think) EDG's frontend does too. MSVC does not have either of them, even in MSVC 2015.
That said, a compound literal might be a good way to remove the extension use, but as long as bar() returns void, the do-while(0) trick is also sufficient for replacing the statement expression.
I think this compound literal usage works:
bar() results in trailing commas. My impression is that C99 allows them in array initializers. (I saw an example in the n1256 C draft.) I don't recall whether C89 also had them. __VA_ARGS__ is expanded twice, but I think that's OK, because sizeof() only evaluates its operand when it has VLA type, and it never will. This code will work in MSVC 2013 (if not earlier).