如何从boost :: chrono :: hight_resolution_clock :: time_point获得纳秒？

Question

I am new to boost and chrono. I am writing a logger that logs the timestamps of API calls, entry and exit. I tried using boost::xtime first, but it wasn't giving the high resolution value I needed. Hence was thinking about using Chrono. I declared a boost::chrono::hight_resolution_clock::time_stamp x; variable for getting the timestamp and assigned it to boost::chrono::hight_resolution_clock::now ();. Now, I need to get the nanoseconds from this variable and put it in my log file (thats the requirement). So I cast it boost::chrono::duration_cast (x). But it just wouldn't let me do that. It needs 2 parameters apparently, and I only have one. Is there a way to get around this?. Is it possible to create another time_stamp variable and assign zero to it and use that variable?. I tried assigning zero, but its not working. Kindly help me out.

Thanks, Sam

Answer 1

If tagged c++11 , any reason why not to use std::chrono ?

// Using std::chrono
auto start = std::chrono::high_resolution_clock::now();  // start timer

/* do some work */

auto diff = std::chrono::high_resolution_clock::now() - start; // get difference 
auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(diff);
std::cout << "it took: " << nsec.count() << " nanoseconds" << std::endl;

Answer 2

boost::chrono::duration_cast converts a duration into the specified units, but you've given it a boost::chrono::time_point , not a duration.

There's really no such thing as " the current time in nanoseconds ". To get a duration, you need to specify the time since which you want to know how many nanoseconds have elapsed (an "epoch"). Different clocks will measure their time based on different epochs.

boost::chrono::system_clock (currently) uses the Unix epoch (midnight Jan 1, 1970) as its epoch, but it's not steady and it may not have the resolution you need (it's in nanoseconds on my Ubuntu box, but in 1/10,000,000ths of a second on my Windows box).

boost::chrono::high_resolution_clock uses boot up as its epoch, is steady, and measures time in nanoseconds on both boxes I tested on.

Boost also provides other clocks like process_cpu_clock that use other epochs and count in other units.

Thus you can get nanos since Jan 1, 1970 using system_clock , but it may not actually be nanosecond-accurate, and it may go backwards if the user changes the system time or the computer syncs with network time, or you can get nanos since some other point in time using high_resolution_clock .