C Macro expansion into multiple function calls

5
votes

Is it possible to write a preprocessor macro such that would transform a variable number of arguments into successive function calls, such as

MAP(f, 1, 2, 3, ..., n)

into 

f(1); f(2); f(3); ... f(n);

So far, I've got following, which seems to work:

#define MAP(f, t, ...) \
{\
    t __varlist[] = {__VA_ARGS__};\
    for(int i = 0; i < sizeof(__varlist) / sizeof(t); i++)\
        f(__varlist[i]);\
}

Note that this macro takes a type parameter so that it can be a bit more useful.

Is there a way to do it without declaring a temporary? Or does it not matter, because the compiler is so smart that it figures out everything? I'm kind of new to C.

2
cmacrosc-preprocessor
If there's no way to do it in one pass you'll need multiple pre-processor passes. en.wikipedia.org/wiki/M4_(computer_language) - John Smith
@JohnSmith it's obviously possible if you generate some more clutter... ;) - Gleno
It's not obvious to me as I don't know everything about the pre-processor, hence the if qualifier. Looks like both ways make a reasonable mess though :P - John Smith

2 Answers

1
votes

In gcc, you can avoid the type with typeof:

#define MAP(f, a1, ...) \
{\
    typeof(a1) __varlist[] = {a1, __VA_ARGS__};\
    for(int i = 0; i < sizeof(__varlist) / sizeof(t); i++)\
        f(__varlist[i]);\
}

Doing it without a temporary, i.e. really building N calls if there are N parameters, is possible, but rather complicated, and will be limited to some maximum (to support up to N, you'll need to define N macros).
To do this, you'll need a macro that counts its arguments (search this site), and use this number, plus concatenation, to choose one of N macros (e.g. MAP1(f, a), MAP2(f, a, b) and so on).

2
votes

use boost.

note:limit size 256. BOOST_PP_LIMIT_SEQ

#include <stdio.h>
#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/tuple/size.hpp>
#include <boost/preprocessor/tuple/to_seq.hpp>

#define PROC(r, f, elem) f(elem);
//#define MAP(f, ...) BOOST_PP_SEQ_FOR_EACH(PROC, f, BOOST_PP_TUPLE_TO_SEQ(BOOST_PP_TUPLE_SIZE((__VA_ARGS__)),(__VA_ARGS__)))
#define MAP(f, ...) BOOST_PP_SEQ_FOR_EACH(PROC, f, BOOST_PP_TUPLE_TO_SEQ((__VA_ARGS__)))

void f(int data){
    printf("%d\n", data);
}

int main(){

    MAP(f, 1, 2, 3);
    return 0;
}