What is the precision of long double in C++?

Question

Does anyone know how to find out the precision of long double on a specific platform? I appear to be losing precision after 17 decimal digits, which is the same as when I just use double. I would expect to get more, since double is represented with 8 bytes on my platform, while long double is 12 bytes.

Before you ask, this is for Project Euler, so yes I do need more than 17 digits. :)

EDIT: Thanks for the quick replies. I just confirmed that I can only get 18 decimal digits by using long double on my system.

Answer 1

You can find out with std::numeric_limits:

std::cout << std::numeric_limits<long double>::digits10 << std::endl;

Answer 2

You can use

sizeof(long double)

to find out how many bytes are used to store the long double.

Also this has nothing to do with platforms but with compilers.

Answer 3

You can use <cfloat>. Specifically:

LDBL_DIG