CODEWITHSUNDEEP
c++cmacros
hhwangcocora
hhwangcocora

Reputation: 65

How to generate function name using c macro

Macro "VER" is defined as "((u_long)1)" in some other header file which I can't change.

In my code I need to compose function "test_1" using "test" and the VER. However compiler reported error since it was the "test_((u_long)1)" instead of "test_1" generated.

My question is: How to write macro so that it can generate "test_1"?

Thanks in advance!

#define VER ((u_long)1)      /* This is defined in some other header file which I can't change*/

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, VER)

int test_1() {
  return 0;
}

int main() {
  fcall( test )();
  return 0;
}

Upvotes: 4

Views: 2225

Answers (3)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

I can't guarantee it's 100% portable, but it should work:

#define VER ((u_long)1)

#define STRIP1(x) STRIP2 x

#define STRIP2(x) STRIP3 x

#define STRIP3(x)

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, STRIP1(VER))

Live example

It works by interpreting the parentheses inside the definition of VER as macro invocation. Here's the individual expansions, as they occur:

STRIP1(VER) // STRIP1 with argument VER

STRIP2 ((u_long)1) // STRIP2 with argument (u_long)1

STRIP3 (u_long)1  // STRIP3 with argument u_long, followed by 1

1

If we re-arrange the whitespace (it's insignificant anyway), we get this:

STRIP1(VER)

STRIP2((u_long) 1)

STRIP3(u_long) 1

1

Upvotes: 6

BLUEPIXY
BLUEPIXY

Reputation: 40145

As modified Angew version

#define VER ((u_long)1)

#define _(x) x
#define id(x) _(x)

#define STRIP3(x)
#define STRIP2(x) id(STRIP3 x)
#define STRIP1(x) id(STRIP2 x)

#define paste(x, y, z) x ## y ## z
#define paste2(x, y, z) paste(x, y, z)
#define fcall(fname) paste2(fname, _, STRIP1(VER))

Upvotes: 0

Mats Petersson
Mats Petersson

Reputation: 129344

With that source, you probably can't, since macro expansion is textual, so the ((u_long)1) can not possibly become 1.

One solution would of course be to actually write some code to print the actual value of VER. The other alternative is to convince the people that own the file you can't change that they should change it for you...

Upvotes: 0

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