C++ Floating Point Double 655

main.cpp

#include <cstdio>
#include <string>

__declspec(noinline) double get_delta(double lhs, double rhs) {
	return lhs - rhs;
}

void print_dbl(std::string const& what, double val) {
	const int dig14 = DBL_DIG - 1;
	printf("'%s'@%d: %.*e\n", what.c_str(), dig14, dig14, val);
	std::string s14(15 + 1, '.');
	printf("' '@__: %s\n", s14.c_str());
	printf("'%s'@%d: %.*e\n", what.c_str(), 18, 18, val);
	uint64_t bits;
	memcpy(&bits, &val, sizeof(uint64_t));
	printf("'%s'@XX: 0x%016llx\n", what.c_str(), bits);
	printf("\n");
}

int main()
{
	double a = 655.36;
	double b = 655.34;
	double d = get_delta(a, b);
	double zz2 = 0.02;

	print_dbl("a", a);
	print_dbl("b", b);
	print_dbl("d", d);
	print_dbl("2", zz2);
}

Output:

'a'@14: 6.55360000000000e+02
' '@__: ................
'a'@18: 6.553600000000000136e+02
'a'@XX: 0x40847ae147ae147b

'b'@14: 6.55340000000000e+02
' '@__: ................
'b'@18: 6.553400000000000318e+02
'b'@XX: 0x40847ab851eb851f

'd'@14: 1.99999999999818e-02
' '@__: ................
'd'@18: 1.999999999998181011e-02
'd'@XX: 0x3f947ae147ae0000

'2'@14: 2.00000000000000e-02
' '@__: ................
'2'@18: 2.000000000000000042e-02
'2'@XX: 0x3f947ae147ae147b