C pre-processor macro expansion

Question

I'm trying to do (what I would have thought) was a simple macro expansion

#define CLEAR_DIGIT(a,b)    iconMap[a] &= ~(b)
#define R1 4, 16
CLEAR_DIGIT(R1);

Now I would expect that to expand to CLEAR_DIGIT(4,16) which expands to iconMap[4] &= ~16 However, it doesn't... If I make CLEAR_DIGIT a function:

void ClearDigit(unsigned char a, unsigned char b)
{
    iconMap[a] &= ~b;
}
#define R1 4, 16
ClearDigit(R1);

then it works fine, so R1 being expanded out to the two arguments isn't an issue... Is there any way of forcing it to expand R1 before doing the macro function expansion?

Related: stackoverflow.com/questions/3776750/…

MByD
– MByD

2011-12-21 10:09:28 +00:00
Commented Dec 21, 2011 at 10:09 — MByD
– MByD, Commented Dec 21, 2011 at 10:09

Community · Accepted Answer · 2017-05-23 11:45:06Z

11

You could use a helper macro. See also double-stringize problem

#define CLEAR_DIGIT_HELPER(a,b) iconMap[a] &= ~(b)
#define CLEAR_DIGIT(x) CLEAR_DIGIT_HELPER(x)
#define R1 4, 16
CLEAR_DIGIT(R1);

edited May 23, 2017 at 11:45

CommunityBot

11 silver badge

answered Dec 21, 2011 at 10:17

Benoit

79.7k24 gold badges226 silver badges241 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

jwd Over a year ago

It's a fine answer, but it does not explain why this works. The linked double-stringize problem is notably different - there are special rules when # and ## are used. Those rules do not apply here.

hburd Over a year ago

This solution did not work for me in visual studio; R1 is expanded, yet still treated as a single argument. In this case, CLEAR_DIGIT(R1) would expand to iconMap[4, 16] &= ~(), outputting a warning for not enough arguments. This seems to be related to the problem in this post. The solution there fixes this problem too.

Collectives™ on Stack Overflow

C pre-processor macro expansion

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related