Not necessarily a good idea: "protocols" in plain old C.

protocol.c

#include <stdio.h>
#include <stdlib.h>


typedef void *any;
#define protocol(n) struct n
#define end(_)


// DECLARING THE SHOW PROTOCOL ///////////////////////////////////////////////

protocol (show) {
        void (*putLn) (any);
};
	#define show_putLn(v, prtcl) (prtcl)->putLn (v)
end (show);


// IMPLEMENTING SHOW ON INT //////////////////////////////////////////////////

void int_putLn (any x)
{
	printf("%i\n", *(int*)x);
}

protocol (show) int_show = {
	int_putLn
};


// IMPLEMENTING SHOW ON FLOAT ////////////////////////////////////////////////

void float_putLn (any x)
{
	printf("%f\n", *(float*)x);
}

protocol (show) float_show = {
	float_putLn
};


// TRYING IT ALL /////////////////////////////////////////////////////////////

int main (int argc, char const *argv[])
{
	int a = 42;
	float b = 3.14;

	show_putLn (&a, &int_show);
	show_putLn (&b, &float_show);

	return 0;
}