c++ - std::chrono::system_clock vs std::chrono::high_resolution_clock behavior -

consider following code snippet:

#include <chrono> #include <cassert>  int main() {      auto result1 = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::high_resolution_clock::now().time_since_epoch());      auto result2 = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch());      assert((result2.count() - result1.count()) < 10); } 

i expecting difference in counts between 2 values should minimal (ideally less second). vs2015, difference in count of order of billions of seconds. how possible?

the reason asserts because high_resolution_clock allowed (and does) have different epoch system_clock.

it de facto standard (not specified portable) system_clock's epoch measuring time since 1970-01-01 00:00:00 utc, neglecting leap seconds.

there no de facto standard high_resolution_clock. on gcc high_resolution_clock typedef system_clock, , on gcc platforms, you'll notice perfect synchronization. on vs , libc++ high_resolution_clock typedef steady_clock.

for me, epoch of steady_clock whenever computer booted up.

here video tutorial <chrono>. covers lots of issues, including one, , hour long.