Question on ANSI ## pre-processor operator.

[Stephen Vinoski]

In article <28302040.69ED at deneva.sdd.trw.com> knurlin at spf.trw.com (Scott Karlin) writes:
>I am looking for a clarification on the ANSI-C token merging
>operator (##).  According to H&S:  "After *all* [emphasis mine]
>macro replacements have been done, the two tokens surrounding
>any ## operator are combined into a single token."  I interpret
>this to mean that:
>
>#define INDEX 0
>#define FN_NAME(x) name ## x
>void FN_NAME(INDEX) () { printf("Hello\n"); }

(I have cross-posted this to comp.std.c since a similar question
recently came up there.  Follow-ups have been directed to
comp.lang.c.)

I had a problem with token pasting similar to yours.  A compiler
writer said that the problem was caused by the fact that I was
attempting to paste arbitrary text with macro parameters; he said that
token pasting is only meant to work on macro parameters.

The ANSI C standard appears to agree with him; I believe it is section
3.8.3.3 (I don't have the standard handy) that says that ## causes
macro parameters to be pasted together.  If I have misinterpreted the
standard, I'm sure I will hear about it.  :-)

I have also noticed that K&R II only shows macro parameters being
pasted together.

What if you change your example to:

#define INDEX 0
#define REAL_FN_NAME(x,y) x ## y
#define FN_NAME(x)        REAL_FN_NAME(name,x)

Then your example usage:

>void FN_NAME(INDEX) () { printf("Hello\n"); }

should produce the desired result.


-steve

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