Construct fixed string by concatenating macro with string

Question

I've went through a number of similar questions and almost got it to work, but not quite.

What I have is the standard Macro-Expansion-Stringification-method.

#define QUOTEME(M) #M #define DOQUOTE(M) QUOTEME(M) #define XCONCAT(X, Y) X##Y #define CONCAT(X, Y) XCONCAT(X, Y)

Then I have a macro PREFIX like this:

#define SW_PREFIX XY2Ar-

What I was trying to do was:

#define SW_FILE DOQUOTE(CONCAT(SW_PREFIX, update))

What this is supposed to output is: "XY2Ar-update" What it outputs on my GCC-type compiler is:

error: pasting "-" and "update" does not give a valid preprocessing token

Now, I assume that he's replacing the macro correctly, but apparently doesn't want to append update to XY2Ar- because of the -.

I also tried:

#define SW_FILE DOQUOTE(SW_PREFIX.update)

which, once again, almost does what I want, however it outputs XY2Ar-.update, which is not what I want either.

I'm out of ideas.

Kerrek SB · Accepted Answer · 2015-12-14 10:43:05Z

3

Here is an alternative:

#define QUOTEME(M) #M #define COMPOSE(A, B) QUOTEME(A) #B #define SW_PREFIX XY2Ar- #define SW_FILE COMPOSE(SW_PREFIX, update)

edited Dec 14, 2015 at 10:43

answered Dec 14, 2015 at 10:27

Kerrek SB

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5 Comments

Refugnic Eternium Over a year ago

I did think that I tried something like that before. Error message: error: pasting ""XY2Ar-"" and ""update"" does not give a valid preprocessing token. Thanks for the input though.

Kerrek SB Over a year ago

@RefugnicEternium: You're right, sorry. That was messed up. Check now, I think this works. I think it has to be asymmetric in the two arguments, because one is a bareword and the other needs replacement. Demo.

Refugnic Eternium Over a year ago

This seems to work, though I think, quite personally, that my solution is a little less...shall we say 'verbose', wouldn't you agree? ;) I discovered it shortly after I left the comment on your previous solution.

Kerrek SB Over a year ago

@RefugnicEternium: Yes, absolutely. Feel free to make yours the accepted answer. (That said, your answer also changed the question by allowing SW_PREFIX to be a string rather than a bareword, so it's somewhat different situation.)

Refugnic Eternium Over a year ago

Yes, I understand that the situation is a different one, which is why yours remains the accepted one. Otherwise, Jens' answer has a very similar effect to that end.

Jens · Accepted Answer · 2015-12-14 10:52:01Z

Your thinking seems too complicated :-) Do not create a single token to stringify. Simply use string concatenation for separate parts.

$ cat x.c #define QUOTEME(M) #M #define DOQUOTE(M) QUOTEME(M) #define SW_PREFIX XY2Ar- #define SW_FILE DOQUOTE(SW_PREFIX) QUOTEME(.update) char *x = SW_FILE; $ gcc -E x.c # 1 "x.c" # 1 "<built-in>" # 1 "<command-line>" # 1 "x.c" char *x = "XY2Ar-" ".update";

Yeah, I get that a lot. I also came to the same conclusion a little while ago. Thanks for pointing it out again though. :)

Refugnic Eternium · Accepted Answer · 2015-12-14 10:45:12Z

Sometimes I keep finding myself forgetting the KISS-principle...and it's times like these where it shows.

#define SW_PREFIX "XY2Ar-" #define SW_FILE SW_PREFIX"update"

These simple lines, no further expansion or anything return the desired output. As for the reason why it does that:

Since there aren't any # in there, it expands SW_PREFIX to "XY2Ar-" and appends "update" to it, which results in "XY2Ar-""update", which, for the compiler, is the same as XY2Ar-update.

Thanks to the guy who erased his answers for reminding me.

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