A code doesn't work but the equivalent code works - objective-c

Question

#define CLog( s, ... ) NSLog( #"%#", [NSString stringWithFormat:(s), ##__VA_ARGS__] )
#define PO(x) CLog(##x ": %#", x)
Then I do:
NSString * hello =[NSString stringWithFormat:#"%#, %#",theCatalogData.id,#(theCatalogData.images.count)];
PO(hello);
Works
Of course, a shortened version of this is simply:
PO([NSString stringWithFormat:#"%#, %#",theCatalogData.id,#(theCatalogData.images.count)]);
That doesn't work. I wonder why.
No emergency. No problem. The longer equivalent works anyway. I just want to understand how the compiler parses the macro to see why things don't work.

Answer 1

Since the pre-processor does not really understand syntax in itself, macro parameters containing commas will cause problems.
In the second case, since the parameter contains two commas outside of quotes, the compiler thinks the macro is getting 3 parameters instead of one, and since the macro takes only one parameter, the compiler will complain.
A simplified test case similar to your second case;
#define TEST(a,b,c) a
TEST([d e:#"%#, %#", f, g])
will expand to;
[d e:#"%#, %#"
which shows that the a parameter only contains all characters up to the first un-quoted comma.
In your case, you define the TEST macro to take one parameter and since the pre-processor considers it 3, compilation will fail.