Using a macro expansion as part of an identifier

Question

Consider the following code:

struct S {};

#define CREATE_INSTANCE S instance_##__LINE__

int main()
{
    CREATE_INSTANCE;
    CREATE_INSTANCE;
    return 0;
}

What I want it to do is create two instances of S named instance_7 and instance_8. What it actually does is creates instance___LINE__ twice.

How to achieve what I want?

score 2 · Answer 1 · answered Aug 31 '17 at 20:21

2

Using some indirection:

#define Concat_(a, b) a ## b
#define Concat(a, b) Concat_(a, b)
#define CREATE_INSTANCE S Concat(instance_, __LINE__)

answered Aug 31 '17 at 20:21

Jarod42

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Interesting! Are you saying that macros are expanded in the context of other macro's arguments, but not in the context in which I used it here? – Violet Giraffe Aug 31 '17 at 20:23
@VioletGiraffe Remember that the cpp is _one pass_ actually. – user0042 Aug 31 '17 at 20:24
And I still don't get why double indirection is required... – Violet Giraffe Aug 31 '17 at 20:24
@user0042: but aren't macros supposed to be expanded in the expansion of other macros? – Violet Giraffe Aug 31 '17 at 20:25
@Violet You accepted the dupe. It's pretty well explained there. – user0042 Aug 31 '17 at 20:28
@user0042: Indeed, I was unaware `#` and `##` inhibit macro expansion. – Violet Giraffe Aug 31 '17 at 20:29
@Violet The preprocessor is a _hairy beast_. I avoid using it whenever I can ;-). I gave your question an upvote, because it's a good signpost. BTW: A "violet giraffe" haz violet ground fur and black/brown patches or other way round? – user0042 Aug 31 '17 at 20:31
@user0042: so do I! But some things seem literally impossible without it. And the actual problem I'm solving is not as silly as my MCVE in the question :) – Violet Giraffe Aug 31 '17 at 20:34
@VioletGiraffe _"And the actual problem I'm solving is not as silly as my MCVE in the question"_ Sure, and I'm aware of that _over simplification_ problem ;-) – user0042 Aug 31 '17 at 20:36
@user0042: what is the essence of the _oversimplification problem_? :) – Violet Giraffe Aug 31 '17 at 20:38
@Violet _"what is the essence ..."_ That it moves it to be an XY-problem ;-) – user0042 Aug 31 '17 at 20:41
@user0042: Nah, I asked precisely what I needed to know and I've fixed my thing. I'm curious if it could be done without macros, but I seriously doubt that. – Violet Giraffe Aug 31 '17 at 20:49
1

@Violet No, such things can't be actually solved without macros. – user0042 Aug 31 '17 at 20:52

Using a macro expansion as part of an identifier

1 Answers1