interface.c

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

struct float_interface {
	void *value;

	void (*add) (void *rhsp, void *lhsp);
	void (*print) (void *rhsp);
	void (*scan) (void *rhsp);
};

void float_interface_add(struct float_interface fi1,
				struct float_interface fi2)
{
	fi1.add(fi1.value, fi2.value);
}

void float_interface_print(struct float_interface fi1)
{
	fi1.print(fi1.value);
}

void float_interface_scan(struct float_interface fi1)
{
	fi1.scan(fi1.value);
}

static void float_add(void *rhsp, void *lhsp)
{
	float rhs = *(float *)rhsp;
	float lhs = *(float *)lhsp;
	rhs += lhs;
	*(float *)rhsp = rhs;
}

static void float_print(void *rhsp)
{
	float rhs = *(float *)rhsp;
	printf("%.9f", rhs);
}

static void float_scan(void *rhsp)
{
	scanf("%f", (float *)rhsp);
}

struct float_interface new_float(void)
{
	float *fp = malloc(sizeof(float));
	struct float_interface fi = {
		.value = (void *)fp,
		.add = float_add,
		.print = float_print,
		.scan = float_scan,
	};
	return fi;
}

static void double_add(void *rhsp, void *lhsp)
{
	double rhs = *(double *)rhsp;
	double lhs = *(double *)lhsp;
	rhs += lhs;
	*(double *)rhsp = rhs;
}

static void double_print(void *rhsp)
{
	double rhs = *(double *)rhsp;
	printf("%.9lf", rhs);
}

static void double_scan(void *rhsp)
{
	scanf("%lf", (double *)rhsp);
}

struct float_interface new_double(void)
{
	double *fp = malloc(sizeof(double));
	struct float_interface fi = {
		.value = (void *)fp,
		.add = double_add,
		.print = double_print,
		.scan = double_scan,
	};
	return fi;
}

int main(void)
{
	int type = 0;
	printf("enter type (0 = float, 1 = double): ");
	scanf("%d", &type);
	struct float_interface fi1, fi2;
	if (type == 0) {
		fi1 = new_float();
		fi2 = new_float();
	} else {
		fi1 = new_double();
		fi2 = new_double();
	}

	printf("enter two numbers: ");
	float_interface_scan(fi1);
	float_interface_scan(fi2);

	float_interface_add(fi1, fi2);
	printf("sum is: ");
	float_interface_print(fi1);
	printf("\n");

	free(fi1.value);
	free(fi2.value);
	return 0;
}