When using the token pasting operator (##), the resulting token always has type ID. This is a problem in expressions. Test case:
#define PASTE(x, y) x ## y
#if PASTE(1, 2) == 12
works
#else
fails
#endif
Both gcc and mcpp generate 'works'. pcpp generates 'fails'. The cause seems to be that the type of token generated is always CPP_ID, therefore the pasted token 12 is replaced with 0 because it's interpreted as an identifier.
I guess that a solution would be to convert the tokens to string before concatenating them, and lex the resulting string again. Now, if the result is not 1 token, an error should be emitted, e.g. PASTE(+,-) is not correct; however I don't think that pcpp can catch that, because it doesn't seem to recognize tokens like ++ or *= which are valid, therefore it could fail on valid code if it did.
ned14
commented
5 years ago
When using the token pasting operator (
##), the resulting token always has type ID. This is a problem in expressions. Test case:Both gcc and mcpp generate 'works'. pcpp generates 'fails'. The cause seems to be that the type of token generated is always CPP_ID, therefore the pasted token
12is replaced with0because it's interpreted as an identifier.I guess that a solution would be to convert the tokens to string before concatenating them, and lex the resulting string again. Now, if the result is not 1 token, an error should be emitted, e.g.
PASTE(+,-)is not correct; however I don't think that pcpp can catch that, because it doesn't seem to recognize tokens like++or*=which are valid, therefore it could fail on valid code if it did.