Question

On my platform this prints 9223372036854775808.

double x = 1e19;
std::cout << static_cast<unsigned __int64>(x) << '\n';

I tried Boost.NumericConversion, but got the same result.

Splitting x into 2 equal part, then adding together converted halves give the correct result. But I need a generic solution to use in a template code.

Thank you in advance.

EDIT:
This problem shows up on Visual Studio 2008, but not MinGW. Casting 4.0e9 into unsigned long works fine.

Answer 1

Seems like it works well with gcc, but it is problematic in Visual Studio. See Microsoft’s answer regarding this issue:

Our floating-point to integer
conversions are always done to a
signed integer. In this particular
case we use FIST instruction which
generates 800..00 as you described.
Therefore, there is no defined
behavior for converting to unsigned
64-bit integer values which are
larger than largest 64-bit signed
integer.

So you can only convert the numbers in the signed 64-bit integer range: −9,223,372,036,854,775,808 to +9,223,372,036,854,775,807 (-2^63~2^63-1).