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I am using the following sequence macro expansion. (I am not aware of any official term for it)

#define SEQ(F) F(1) F(2) F(3) #define F1(I) HELLO_ ## I SEQ(F1) // HELLO_1 HELLO_2 HELLO_3

It works as expected, but when I am trying to nest it into another macro expansion it doesn't work.

#define F2(I) SEQ(F1) WORLD_ ## I!!! SEQ(F2) // SEQ(F1) WORLD_1!!! SEQ(F1) WORLD_2!!! SEQ(F1) WORLD_3!!!

Instead, I expect it to expand to

HELLO_1 HELLO_2 HELLO_3 WORLD_1!!! HELLO_1 HELLO_2 HELLO_3 WORLD_2!!! HELLO_1 HELLO_2 HELLO_3 WORLD_3!!!

Why it doesn't work and how to fix it?

Update:

It seems the same mechanism that forbids the use of recursive macro breaks things for me: the preprocessor thinks that SEQ is already expanded in SEQ(F2) so it doesn't attempt to expand SEQ(F1).

My workaround is to copy-paste SEQ to SEQ1

#define SEQ(F) F(1) F(2) F(3) #define SEQ1(F) F(1) F(2) F(3)

and then use

#define F2(I) SEQ1(F1) WORLD_ ## I!!! SEQ(F2)
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  • It doesn't work because macro expansion is not recursive. You need to devise a solution that does not depend on recursing (of the SEQ macro in this case). Commented Jun 6, 2021 at 13:52
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