Variadic recursive preprocessor macros - is it possible?

Question

I\'ve run into a little theoretical problem. In a piece of code I\'m maintaining there\'s a set of macros like

#define MAX_OF_2(a, b)       (a) > (b) ? (a         


        

Answer 1

I think that, even if you could expand macros recursively, there would be one little problem with your approach in terms of efficiency... when the macros are expanded, if the MAX_OF_[N-1] is greater, then you have to evaluate it again from scratch.

Here is a silly and stupid answer that probably no one will like xD

file "source.c"

#include "my_macros.h"
...

file "Makefile"

myprogram: source.c my_macros.h
 gcc source.c -o myprogram

my_macros.h: make_macros.py
 python make_macros.py > my_macros.h

file "make_macros.py"

def split(l):
    n = len(l)
    return l[:n/2], l[n/2:]

def gen_param_seq(n):
    return [chr(i + ord("A")) for i in range(n)]

def make_max(a, b):
    if len(a) == 1:
        parta = "("+a[0]+")"
    else:
        parta = make_max(*split(a))

    if len(b) == 1:
        partb = "("+b[0]+")"
    else:
        partb = make_max(*split(b))

    return "("+parta +">"+partb+"?"+parta+":"+partb+")"

for i in range(2, 9):
    p = gen_param_seq(i)
    print "#define MAX_"+str(i)+"("+", ".join(p)+") "+make_max(*split(p))

then you'll have those pretty macros defined:

#define MAX_2(A, B) ((A)>(B)?(A):(B))
#define MAX_3(A, B, C) ((A)>((B)>(C)?(B):(C))?(A):((B)>(C)?(B):(C)))
#define MAX_4(A, B, C, D) (((A)>(B)?(A):(B))>((C)>(D)?(C):(D))?((A)>(B)?(A):(B)):((C)>(D)?(C):(D)))
#define MAX_5(A, B, C, D, E) (((A)>(B)?(A):(B))>((C)>((D)>(E)?(D):(E))?(C):((D)>(E)?(D):(E)))?((A)>(B)?(A):(B)):((C)>((D)>(E)?(D):(E))?(C):((D)>(E)?(D):(E))))
#define MAX_6(A, B, C, D, E, F) (((A)>((B)>(C)?(B):(C))?(A):((B)>(C)?(B):(C)))>((D)>((E)>(F)?(E):(F))?(D):((E)>(F)?(E):(F)))?((A)>((B)>(C)?(B):(C))?(A):((B)>(C)?(B):(C))):((D)>((E)>(F)?(E):(F))?(D):((E)>(F)?(E):(F))))
#define MAX_7(A, B, C, D, E, F, G) (((A)>((B)>(C)?(B):(C))?(A):((B)>(C)?(B):(C)))>(((D)>(E)?(D):(E))>((F)>(G)?(F):(G))?((D)>(E)?(D):(E)):((F)>(G)?(F):(G)))?((A)>((B)>(C)?(B):(C))?(A):((B)>(C)?(B):(C))):(((D)>(E)?(D):(E))>((F)>(G)?(F):(G))?((D)>(E)?(D):(E)):((F)>(G)?(F):(G))))
#define MAX_8(A, B, C, D, E, F, G, H) ((((A)>(B)?(A):(B))>((C)>(D)?(C):(D))?((A)>(B)?(A):(B)):((C)>(D)?(C):(D)))>(((E)>(F)?(E):(F))>((G)>(H)?(G):(H))?((E)>(F)?(E):(F)):((G)>(H)?(G):(H)))?(((A)>(B)?(A):(B))>((C)>(D)?(C):(D))?((A)>(B)?(A):(B)):((C)>(D)?(C):(D))):(((E)>(F)?(E):(F))>((G)>(H)?(G):(H))?((E)>(F)?(E):(F)):((G)>(H)?(G):(H))))

and the best thing about it is that... it works ^_^