Preprocessor define concatenation

[michael.martin@asahq.com]

Hi,

I am wondering if something like this is possible?

#define COLON ":"
#define PERIOD "."

#define WORD "word"

#define WORD_COLON WORD+COLON
#define WORD_PERIOD WORD+PERIOD

....

I know the above is incorrect syntax. But is there a way to
concatenate the above strings in some way? I would rather not have to
define WORD_COLON as "word:" and WORD_PERIOD as "word.". I would like
to be able to change the value of WORD, with just modifying one
constant, rather than modifying all 3, as I am doing this on a much
larger scale.

Thanks for any help,
mike martin

[Joona I Palaste]

Re: Preprocessor define concatenation

michael.martin@ asahq.com scribbled the following:[color=blue]
> Hi,[/color]
[color=blue]
> I am wondering if something like this is possible?[/color]
[color=blue]
> #define COLON ":"
> #define PERIOD "."[/color]
[color=blue]
> #define WORD "word"[/color]
[color=blue]
> #define WORD_COLON WORD+COLON
> #define WORD_PERIOD WORD+PERIOD[/color]
[color=blue]
> ...[/color]
[color=blue]
> I know the above is incorrect syntax. But is there a way to
> concatenate the above strings in some way? I would rather not have to
> define WORD_COLON as "word:" and WORD_PERIOD as "word.". I would like
> to be able to change the value of WORD, with just modifying one
> constant, rather than modifying all 3, as I am doing this on a much
> larger scale.[/color]

The C compiler concatenates any adjacent string literals into a single
string literal. Therefore
#define WORD_COLON WORD COLON
#define WORD_PERIOD WORD PERIOD
should do the trick.

--
/-- Joona Palaste (palaste@cc.hel sinki.fi) ------------- Finland --------\
\-------------------------------------------------------- rules! --------/
"We're women. We've got double standards to live up to."
- Ally McBeal

[Yan]

Re: Preprocessor define concatenation

Joona I Palaste wrote:[color=blue]
> michael.martin@ asahq.com scribbled the following:
>[color=green]
>>Hi,[/color]
>
>[color=green]
>>I am wondering if something like this is possible?[/color]
>
>[color=green]
>>#define COLON ":"
>>#define PERIOD "."[/color]
>
>[color=green]
>>#define WORD "word"[/color]
>
>[color=green]
>>#define WORD_COLON WORD+COLON
>>#define WORD_PERIOD WORD+PERIOD[/color]
>
>[color=green]
>>...[/color]
>
>[color=green]
>>I know the above is incorrect syntax. But is there a way to
>>concatenate the above strings in some way? I would rather not have to
>>define WORD_COLON as "word:" and WORD_PERIOD as "word.". I would like
>>to be able to change the value of WORD, with just modifying one
>>constant, rather than modifying all 3, as I am doing this on a much
>>larger scale.[/color]
>
>
> The C compiler concatenates any adjacent string literals into a single
> string literal. Therefore
> #define WORD_COLON WORD COLON
> #define WORD_PERIOD WORD PERIOD
> should do the trick.
>[/color]
you need the ## operator

i.e.:

#define concat(a, b) a ## b

and if you call it with:

concat(12,45), the pre-processor will replace it with 1245

-yan

[Keith Thompson]

Re: Preprocessor define concatenation

Yan <rottled@gmail. com> writes:[color=blue]
> Joona I Palaste wrote:[color=green]
>> michael.martin@ asahq.com scribbled the following:
>>[color=darkred]
>>>Hi,[/color]
>>[color=darkred]
>>>I am wondering if something like this is possible?[/color]
>>[color=darkred]
>>>#define COLON ":"
>>>#define PERIOD "."[/color]
>>[color=darkred]
>>>#define WORD "word"[/color]
>>[color=darkred]
>>>#define WORD_COLON WORD+COLON
>>>#define WORD_PERIOD WORD+PERIOD[/color]
>>[color=darkred]
>>>...[/color]
>>[color=darkred]
>>>I know the above is incorrect syntax. But is there a way to
>>>concatenat e the above strings in some way? I would rather not have to
>>>define WORD_COLON as "word:" and WORD_PERIOD as "word.". I would like
>>>to be able to change the value of WORD, with just modifying one
>>>constant, rather than modifying all 3, as I am doing this on a much
>>>larger scale.[/color]
>> The C compiler concatenates any adjacent string literals into a
>> single
>> string literal. Therefore
>> #define WORD_COLON WORD COLON
>> #define WORD_PERIOD WORD PERIOD
>> should do the trick.
>>[/color]
> you need the ## operator
>
> i.e.:
>
> #define concat(a, b) a ## b
>
> and if you call it with:
>
> concat(12,45), the pre-processor will replace it with 1245[/color]

The OP was trying to concatenate string literals. The ## operator is
valid only if the result is a single token. concat(12,45) does yield
the single token 1245, but concat("word"," :") yields "word"":", which
is not a single token.

Joona was correct. Implicit string literal concatenation will do what
the OP wants; the ## operator (useful though it is for other things)
will not.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.