Type-checked units in C

asm_diff.diff

--- units.asm	2023-06-18 12:02:32.313569952 +1200
+++ units_unsafe.asm	2023-06-18 12:02:32.313569952 +1200
@@ -1,7 +1,7 @@
-metres_from_feet(feet):
+metres_from_feet(double):
         divsd   xmm0, QWORD PTR .LC0[rip]
         ret
-feet_from_metres(metres):
+feet_from_metres(double):
         mulsd   xmm0, QWORD PTR .LC0[rip]
         ret
 .LC3:

units.c

#include <stdio.h>

typedef struct {
	double m;
} metres;

typedef struct {
	double ft;
} feet;

metres
metres_from_feet(feet x)
{
	const feet m_to_ft = {3.28084};
	return (metres){x.ft / m_to_ft.ft};
}

feet
feet_from_metres(metres x)
{
	const feet m_to_ft = {3.28084};
	return (feet){x.m * m_to_ft.ft};
}

int
main(void)
{
	metres h = {2.4};
	printf("%lf metres = %lf feet\n", h.m, feet_from_metres(h).ft);

	return 0;
}

units_unsafe.c

#include <stdio.h>

double
metres_from_feet(double x)
{
	const double m_to_ft = 3.28084;
	return x / m_to_ft;
}

double
feet_from_metres(double x)
{
	const double m_to_ft = {3.28084};
	return x * m_to_ft;
}

int
main(void)
{
	double h = 2.4;
	printf("%lf metres = %lf feet\n", h, feet_from_metres(h));

	return 0;
}