Stringification – how does it work?

The relevant steps of macro expansion are (per C 2011 [n1570] 6.10.3.1 and C++ 1998 16.3.1):

1. Process tokens that are preceded by # or ##.
2. Apply macro replacement to each argument.
3. Replace each parameter with the corresponding result of the above macro replacement.
4. Rescan for more macros.

Thus, with xstr(foo), we have:

1. The replacement text, str(s), contains no # or ##, so nothing happens.
2. The argument foo is replaced with 4, so it is as if xstr(4) had been used.
3. In the replacement text str(s), the parameter s is replaced with 4, producing str(4).
4. str(4) is rescanned. (The resulting steps produce ”4”.)

Note that the problem with str(foo) is that step 2, which would replace foo with 4, comes after step 1, which changes the argument to a string. In step 1, foo is still foo; it has not been replaced with 4, so the result is ”foo”.

This is why a helper macro is used. It allows us to get a step 2 performed, then use another macro to perform step 1.