How to print prefixed unit without changing base unit?

Question

If I define a value in bytes and print it:

using namespace boost::units;
using namespace boost::units::information;
quantity<info> nbytes = 1234 * bytes;
std::cout << nbytes << std::endl; // 1234 B

Ok, that's fine, but what I really want is "1.234 kB". There's an auto-scaling prefix for that:

std::cout << engineering_prefix << nbytes << std::endl; // 9.872 kb

It printed in units of the base unit, bits. How can I get it to auto-scale, but preserve the base unit of bytes?

What does "preserve the base unit of bytes" mean? Can you give an example? — tadman, May 23 '23 at 00:41
The example is in the question. I want the output to be "1.234 kB" (that is kilobytes), not "9.872 kb" (which is kilobits). — John Freeman, May 23 '23 at 00:48
Ah, it wasn't clear because the difference in `b` vs. `B` wasn't obvious. — tadman, May 23 '23 at 00:49

score 0 · Answer 1 · answered May 23 '23 at 01:54

One way (the only way I've found so far) is to define my own base unit that does not scale down to bits. It appears the implementation of do_print_prefixed deep in the bowels of Boost.Unit unscales the value and there is no way to stop it.

struct byte_unit
    : public boost::units::base_unit<byte_unit, boost::units::information_dimension, 1>
{};
using storage = boost::units::make_system<byte_unit>::type;
using byte = boost::units::unit<boost::units::information_dimension, storage>;

BOOST_UNITS_STATIC_CONSTANT(bytes, byte);

namespace boost {
namespace units {

template <>
struct base_unit_info<ripple::CommunicationMeter::byte_unit>
{
    static BOOST_CONSTEXPR const char* name() { return ("byte"); }
    static BOOST_CONSTEXPR const char* symbol() { return ("B"); }
};

}  // namespace units
}  // namespace boost

How to print prefixed unit without changing base unit?

1 Answers1