CODEWITHSUNDEEP
cmacrosc-preprocessorstring-concatenation
John
John

Reputation: 2326

Getting a macro to concat AND stringify

The approach to concatenate in C/C++ in a preprocessor macro is to use ##. The approach to stringify is to use #. I'm trying to concat AND stringify. This is generating a warning from g++ (3.3.2)

#define TOKENPASTE(x, y) x ## y
#define TOKENPASTE2(x, y) TOKENPASTE(x, y)      // concat
#define TOKENPASTE3(x, y) TOKENPASTE(#x, #y)    // concat-stringify (warnings)
const char* s = TOKENPASTE3(Hi, There)

It is not acceptable to get the warning

"test_utils/test_registration.h:34:38: warning: pasting ""Hi"" and ""There"" does not give a valid preprocessing token"

although (using the -E option) I see that it generates:

const char* s = "Hi""There";

Which looks right to me.

Any help will be appreciated.

Upvotes: 6

Views: 11684

Answers (1)

user3920237
user3920237

Reputation:

The preprocessor already concatenates adjacent string literals. So your macro is unnecessary. Example:

#define TOKENPASTE3(x, y) #x #y
const char* s = TOKENPASTE3(Hi, There);

becomes "Hi" "There". However, if you wanted to stick with your approach, you need to use an extra level of indirection to macro expand your new token:

#define STRINGIFY(x) #x
#define TOKENPASTE(x, y) STRINGIFY(x ## y)

becomes "HiThere".

Upvotes: 9

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