std::chrono::system_clock 与 std::chrono::high_resolution_clock 行为

Question

Consider the 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 am expecting that the difference in counts between the two values should be minimal (ideally less than a second). But with VS2015, the difference in count is of the order of Billions of seconds. How is this possible?

Answer 1

The reason it asserts for you is because high_resolution_clock is allowed to (and often does) have a different epoch than system_clock .

It is the de facto standard (not specified but portable) that system_clock 's epoch is measuring time since 1970-01-01 00:00:00 UTC, neglecting leap seconds ¹ .

There is no de facto standard for high_resolution_clock . On gcc high_resolution_clock is a typedef for system_clock , and so on gcc platforms, you'll notice perfect synchronization. On VS and libc++ high_resolution_clock is a typedef for steady_clock .

For me, the epoch of steady_clock is whenever the computer booted up.

Here is a video tutorial for <chrono> . It covers lots of issues, including this one, and is about an hour long.

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¹ The de facto standard for system_clock has been made official for C++20 .

Objects of type system_clock represent wall clock time from the system-wide realtime clock. Objects of type sys_time<Duration> measure time since 1970-01-01 00:00:00 UTC excluding leap seconds. This measure is commonly referred to as Unix time . This measure facilitates an efficient mapping between sys_time and calendar types ([time.cal]). [ Example: sys_seconds{sys_days{1970y/January/1}}.time_since_epoch() is 0s .
sys_seconds{sys_days{2000y/January/1}}.time_since_epoch() is 946'684'800s , which is 10'957 * 86'400s . — end example ]