• dan <dan_member pathlink.com> Dec 03 2003

I'd think...

     #defina a(x) x
     a(the_file).ext

should result in 

     the_file.ext

But then I'm not sure what the compiler should do if I write,

     a(the_file)ext  //no dot  



Rob Williscroft wrote:
It should produce the output "the_fileext" (not the quotes), however this is 2
preproesor tokens (though only one C++ identifier). Add:

#define the_fileext Expansion 
#define B() a(the_file)

B()

should produce: "the_fileext" Not "Expansion". 

I.e. juxtaposition is fine for producing C++ tokens but not for preprocessor
tokens (i.e. tokens that can themselves be expanded).

The pre ## way was something like:

#define cat(a, b ) a/**/b
#define XY Expanded

cat(X, Y)

produced "Expanded".

This is what (IMO) ## was designed to fix.


Wow!  My head is spinning. Ok, you're saying that in the old days a
macro could modify the instance of another macro before the latter was
applied.  But I thought that was the way it is currently, isn't it?
Would your last example NOT do that now?  And how does ## prevent...
what exactly?
Maybe if you could give me a 4-way example: What worked and didn't
work / then and now.  Thx.



Rob Williscroft wrote:

The problem with this method is when the C standard came out it required
comments to be replaced with a single space character.

Presumably this was so you couldn't write print/* whatever */f( "\n" ); and have
the compiler see printf( "\n" );

So code that relied on this behaviour needed another way of achiving the same
thing hence the invention of ##.

Rob.