C Preprocessor: concatenate macro call with token-CodePudding

I'm trying to concatenate a macro call with a token to create a new one, for instance:

#define TEST(X) X ## _TEST
#define CONCAT(X) TEST(X) ## _CONCAT

CONCAT(OK);

Then I check the output with gcc -E; I would want to get OK_TEST_CONCAT; but instead I get an error:

error: pasting ")" and "_CONCAT" does not give a valid preprocessing token

If I remove ## I get no error but the output is OK_TEST _CONCAT;

This is a minimal example, so the easiest here would be to combine everything in a single macro, but know that it's impossible for me to get rid of the first call to TEST. Is there a way to remove that space?

Thanks

CodePudding user response：

All preprocessor tokens must be expanded before a function-like macro using ## or # is called. Because ## or # is applied before macro expansion. In your case TEST(X) only expands X into TEST(OK) and then the preprocessor attempts to paste TEST(OK) with _CONCAT which won't work. For each attempt to concatenate tokens, you must first expand all macros before ##, which is done by extra helper macros that force a rescanning/replacement.

The contrived solution given #define TEST(X) X ## _TEST would be this:

#define CONCAT(X) EXPAND_HELPER(TEST(X)) // expands TEST(X) to TEST(OK)

-->

#define EXPAND_HELPER(X) CONCAT_HELPER(X, _CONCAT) // expands TEST(OK) to OK_TEST

-->

#define CONCAT_HELPER(X,Y) X ## Y

That is:

// NOTE: contrived solution, avoid

#define TEST(X) X ## _TEST
#define CONCAT_HELPER(X,Y) X ## Y
#define EXPAND_HELPER(X) CONCAT_HELPER(X, _CONCAT)
#define CONCAT(X) EXPAND_HELPER(TEST(X))

...

int CONCAT(OK) = 1; // becomes int OK_TEST_CONCAT = 1;

A much simpler solution would be:

#define CONCAT(X) X ## _TEST ## _CONCAT

CodePudding user response：

Well, this is an another way to do this, the macro will take two values (X and Y) and combine them together with the "TEST" string in between.

#define CONCAT(X, Y) X ## _TEST ## _ ## Y
CONCAT(OK,CONCAT)

Hope it helps you!