CODEWITHSUNDEEP
cmacrosembedded
Abhijit K Rao
Abhijit K Rao

Reputation: 1097

Understanding recursive Macro Expansions

I came across this question in an Embedded interview question set. 

#define cat(x,y) x##y

concatenates x to y. But cat(cat(1,2),3) does not expand but gives preprocessor warning. Why?

Does C not encourage Recursive Macro expansions ? My assumption is the expression should display 1##2##3. Am i wrong ?

Upvotes: 4

Views: 1370

Answers (1)

haccks
haccks

Reputation: 106082

The problem is that cat(cat(1,2),3) isn't expanded in a normal way which you expect that cat(1,2) would give 12 and cat(12, 3) would give 123.

Macro parameters that are preceded or followed by ## in a replacement list aren't expanded at the time of substitution. As a result, cat(cat(1,2),3) expands to cat(1,2)3, which can't be further expanded since there is no macro named cat(1,2)3.
So the simple rule is that, macros whose replacement lists depends on ## usually can't be called in a nested fashion.

Upvotes: 3

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