generic-ish swap

meta_swap.h

#ifndef DETAIL_META_HEADER_SWAP
#define DETAIL_META_HEADER_SWAP

#include <stddef.h>
#include <string.h>

// Assigns each variable to the other's value.
// If the variables have different types, each is cast to the other's type before assignment.
#define meta_swap(A, B) \
	((void)_Generic(typeof_unqual(A), typeof_unqual(B): meta_swap(true, &(A), &(B), &(char[sizeof(A)]){}, sizeof(A)), default: _Generic(typeof_unqual(A), typeof_unqual(B): (int){}, default: (A)) = (typeof(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (A)))){ (typeof(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (A))))*(typeof(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (B)))*)meta_swap(false, &(B), &(typeof(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (B)))){ (typeof(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (B))))(_Generic(typeof_unqual(A), typeof_unqual(B): 0, default: (A))) }, &(char[sizeof(B)]){}, sizeof(B)) }))
static inline void* (meta_swap)(bool same, void* a, void* b, void* buffer, size_t size) {
	memcpy(buffer, a, size);
	memcpy(a, b, size);
	if (same) {
		memcpy(b, buffer, size);
	}
	return buffer;
}

#endif

test.c

#include <stdio.h>

typedef struct {
	int x;
	char y;
} A;

int main() {
	int x = 1;
	double y = 2.0;
	meta_swap(x, y);
	printf("%i <-> %lf\n", x, y);

	A a = { 1, 2 };
	A b = { 3, 4 };
	meta_swap(a, b);
	printf("(%i, %i) <-> (%i, %i)\n", a.x, a.y, b.x, b.y);
}