std::format with std::chrono poor performance · Issue #3575 · microsoft/STL (original) (raw)

Is this a known issue or am I doing something sub-optimal?

I'm simply calling

std::format_to( std::back_inserter( buffer ), "[{:%H:%M:%S}] ", time );

where buffer is a std::string and time is of type std::chrono::system_clock::duration
and this is about 4-5 times slower than 'rolling my own' e.g.

static constexpr auto h = 3600'000'000'000LL;
static constexpr auto min = 60'000'000'000LL;
static constexpr double sec = 1'000'000'000;
auto elapsed_ns = time.count();
const auto hours = elapsed_ns / h; elapsed_ns -= hours * h;
const auto minutes = elapsed_ns / min; elapsed_ns -= minutes * min;
const double seconds = elapsed_ns / sec;

std::format_to( std::back_inserter( buffer ), "[{}:{:02}:{:010.7f}] ", hours, minutes, seconds );

Not sure where this huge difference can come from. Allocations?