Hygenic macros in C

Question

The C FAQ declares that a generic SWAP macro in C is essentially impossible and not worth the effort. That sounds like a challenge to me.

It cites 2 main difficulties:

1. Finding a suitable type to use as a temporary variable. There are hacks involving char[] arrays and memcopy, but this sort of solution fails on things like SWAP(my_int, my_float) . A lot of compilers support decltype style operators that make this easy, but they are nonstandard. For now I will admit defeat and just ask the user to supply a suitable type as the third argument.
2. Finding a safe name for a temporary variable. This is something I think I have a solution for. I can use the stringify operator # to stringify the input arguments and compare them to prospective temporary variable names to find one that doesn't match.

#include <stdio.h>

#define SWAP(a,b,type)                          \
do {                                            \
    if ((#a[0] != 'a') && (#b[0] != 'a')){      \
        type a_temp = a;                        \
        a = b;                                  \
        b = a_temp;                             \
    }else                                       \
    if ((#a[0] != 'b') && (#b[0] != 'b')){      \
        type b_temp = a;                        \
        a = b;                                  \
        b = b_temp;                             \
    }else                                       \
    if ((#a[0] != 'c') && (#b[0] != 'c')){      \
        type c_temp = a;                        \
        a = b;                                  \
        b = c_temp;                             \
    }                                           \
} while(0)

int main()
{
    int a_temp = 10, b_temp = 20, c_temp = 30;

    printf("a_temp = %d, b_temp = %d, c_temp = %d\n", a_temp, b_temp, c_temp);
    SWAP(a_temp, b_temp, int);
    printf("a_temp = %d, b_temp = %d, c_temp = %d\n", a_temp, b_temp, c_temp);
    SWAP(a_temp, c_temp, int);
    printf("a_temp = %d, b_temp = %d, c_temp = %d\n", a_temp, b_temp, c_temp);
    SWAP(b_temp, c_temp, int);
    printf("a_temp = %d, b_temp = %d, c_temp = %d\n", a_temp, b_temp, c_temp);

    return 0;
}

I've seen the issue of accidental capture brought up in many discussions and not seen anyone mention that you could do this, either to recommend or condemn it. Is this a good idea?

Answer 1

This will fail in some cases if the variables are lvalue expressions, eg, if you have int *a_temp and int *b_temp , and you SWAP(*a_temp, *b_temp, int) . And since the macro is silently using "common" names like a_temp , it might break if there is something like #define a_temp *a .

And anyway, it is arguably not generic since you have to pass the type: SWAP(a, b, int) is not fundamentally different from SWAPINT(a, b) . If we accept passing the type, could we also accept passing the name of the temporary variable? Or even a fixed temporary variable name like _tmpswap0123456789 – after all, we are accepting the far more common name SWAP to be now defined as this macro, so worrying about the name of a temporary variable inside it seems secondary.