秒后的 system_clock::now() 值在 C++ 20 中代表什么？

Question

I'm exploring the timestamp in C++ 20 returned from system_clock::now() and when I print out the returned std::chrono::time_point it prints the date and time in the format YYYY-MM-DD HH:MM:SS.xxxxxxx .

Any idea what the xxxxxxx value is? I assumed microseconds initially but I realised microseconds are to six decimal places whereas this value is always to seven.

I'm running this in VS 2022.

#include <iostream>
#include <chrono>

int main()
{
    using namespace std::chrono;
    auto timeDate = zoned_time{ current_zone(), system_clock::now() };
    std::cout << timeDate << std::endl;  // Output: 2022-07-29 08:55:22.8582577 BST
}

Answer 1

It's the fractional part of a second.

The output you're getting comes from operator<<(zoned_time) . This outputs the time in the format "{:L%F %T %Z}" (see cppreference.com ). The %T part of the format is equivalent to "%H:%M:%S" ( reference ) and the %S specifies the seconds as a decimal floating-point number with a precision matching that of the precision of the input (ie the precision of system_clock::now() in your case).