Gfrktyl

I'm reading the Standard N1570 about macro replacement and misunderstand some wording from 6.10.3.4.

1 After all parameters in the replacement list have been substituted
and # and ## processing has taken place, all placemarker preprocessing
tokens are removed. The resulting preprocessing token sequence is then
rescanned, along with all subsequent preprocessing tokens of the
source file, for more macro names to replace

So after all # and ## are resolved we rescan the replacement list. But the section 2 specifies:

2 If the name of the macro being replaced is found during this scan of
the replacement list (not including the rest of the source file’s
preprocessing tokens), it is not replaced. Furthermore, if any nested
replacements encounter the name of the macro being replaced, it is not
replaced.

It looks contradictory to me. So what kind of replacement possible in that rescan? I tried the following example:

#define FOOBAR(a, b) printf(#a #b)



#define INVOKE(a, b) a##b(a, b)



int main() {

    INVOKE(FOO, BAR); //expands to printf("FOO" "BAR")

}

So INVOKE(FOO, BAR) expands to FOOBAR(FOO, BAR) after substitution of ##. Then the replacement list FOOBAR(FOO, BAR) is rescanned. But the section 2. specifies that the name of the macro being replaced (FOOBAR) is found (yes, defined above) it is not replaced (but actually replaced as can be seen in th demo).

Can you please clarify that wording? What did I miss?

LIVE DEMO

The (original) macro being replaced is not FOOBAR, it's INVOKE. When you're expanding INVOKE and you find FOOBAR, you expand FOOBAR normally. However, if INVOKE had been found when expanding INVOKE, it would no longer be expanded.

Let's take the following code:

#define FOOBAR(a, b) printf(#a #b)



#define INVOKE(a, b) e1 a##b(a, b)



int main() {

    INVOKE(INV, OKE);

}

I added the e1 to the expansion of INVOKE to be able to visualise how many expansions happen. The result of preprocessing main is:

e1 INVOKE(INV, OKE);

This proves that INVOKE was expanded once and then, upon rescanning, not expanded again.

[Live example]

Consider the following simple example:

#include<stdio.h>



const int FOO = 42;



#define FOO (42 + FOO) 



int main()

{

   printf("%d", FOO);

}

Here the output will be 84.

The printf will be expanded to:

printf("%d", 42 + 42);

This means that when the macro FOO is expanded, the expansion will stop when the second FOO is found. It will not be further expanded. Otherwise, you will end up with endless recursion resulting in: 42 + (42 + (42 + (42 + ....)

Live demo here.