Getting a macro to concat AND stringify

Question

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.

Why are you using a version of GCC that's over a decade old? — user3920237
– user3920237, Commented Feb 11, 2015 at 14:00
It is actually a safety-critical fork from a decade ago. It is from WindRiver 653. That's out of my hands. — John
– John, Commented Feb 11, 2015 at 14:01
Well, you already have two strings, after #x and #y; you don't need to concat them into a single token, just use #x #y. — Tim Čas
– Tim Čas, Commented Feb 11, 2015 at 14:03
That concat is unnecessary. "Hi" "There" will be interpreted as "HiThere" even without doing so. — user694733
– user694733, Commented Feb 11, 2015 at 14:03
The preprocessor simply doesn't allow concatenation of strings using the concatenation operator. From this reference: "Only tokens that form a valid token together may be pasted". String-literal concatenation is handled by another compiler phase. — Some programmer dude
– Some programmer dude, Commented Feb 11, 2015 at 14:04

user3920237 · Accepted Answer · 2015-02-11 14:04:46Z

9

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".

answered Feb 11, 2015 at 14:04

user3920237

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