CODEWITHSUNDEEP
cc-preprocessorpreprocessor-directive
Avi
Avi

Reputation: 325

Using '#' and '##' preprocessing operators together

From what I have read I understand that # operator is used with a parametric macro to convert it's parameter to a string and ## is used to join two parameters or a parameter with some other identifier (Correct me if my understanding is wrong).

But how can I use both # and ## operator together? I tried it by doing:

#define str(n)  #n ## #n

I thought then 

printf("%s",str(Hello))

will be expanded as

printf("%s", "Hello""Hello")

And as adjacent strings are joined automatically to make one string in C so, this will lead to printf("%s", "HelloHello") and output will beHelloHello. But the story was different, it throws an error:

pasting "hello" and "hello" does not give a valid preprocessing token

Please explain me how these parametric macros with # and ## operator are expanded.

Upvotes: 0

Views: 57

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170239

## "joining two parameters" is a vast oversimplification. This operator joins tokens. And the result must be a single valid token. Two string literal tokens cannot be token pasted into a single token.

Furthermore, string literal concatenation is handled at a later translation phase. So an obvious fix to your macro is to not use ## at all.

#define str(n) #n #n

But if you really want to use both, then you need to token paste before stringifying. And do that via an intermediate macro expansion.

#define str(n) str_(n ## n)
#define str_(nn) #nn

Upvotes: 3

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