C/C++ macro parameter containing dot (member access operator)

Question

How can one use in C/C++ macro parameter containing dot (member access operator)?

Example:

#define M(obj,y) obj.y##x
struct S { struct {int x;} c; int x; };
S s;
s.c.x = 1;
s.x = 2;
M(s,)   // works, 2 (resolves to s.x)
M(s,c.) // error: pasting formed '.x', an invalid preprocessing token

How can one make M(s,c.) to resolve to s.c.x ?

Thank you for your help!

Are you using C or C++? If you are using C++ I would suggest ditching the macro and making a template function. — NathanOliver, Oct 11 '19 at 17:34
Macros tend to make code harder to read, understand and maintain. Especially macros like the one you want to make. Please try to avoid them. — Some programmer dude, Oct 11 '19 at 17:34
You probably need another layer of indirection like what is done with concat macros. I'm not a macro pro though so it might not help. — NathanOliver, Oct 11 '19 at 17:39
It's okay to have whitespace on either side (or both) of the `.`. — Pete Becker, Oct 11 '19 at 18:07

score 5 · Accepted Answer · answered Oct 11 '19 at 17:42

The token pasting operator ## requires its two operands to be valid preprocessing tokens, and yields a single preprocessing token. It is often used to concatenate two identifiers into a single identifier.

What you are trying to do here is not token pasting. Instead, you are seeking to create expressions like s.x or s.c.x where the x part is always a single token. Therefore, the ## operator should not be used. Instead, you can just do this:

#define M(obj, y) obj.y x

When you try to use the ## operator, the preprocessor tries to combine the last token in the argument y with the token x. In c., the . is a token, so the result is .x, which is not a valid token. Rather, .x is only valid as a sequence of two tokens.

This is also applicable if you are using the arrow operator -> — Joniale, Jun 04 '20 at 08:45

C/C++ macro parameter containing dot (member access operator)

1 Answers1