Convert high resolution clock time into an integer (Chrono)

Question

I want to be able to get nanosecond accuracy with the chrono library but I can't figure out how to convert std::chrono::high_resolution_clock::now() into long int. I tried this:

#include <chrono>
#include <iostream>
using namespace std;

int main() {
    typedef std::chrono::high_resolution_clock Clock;

    long int val = Clock::now();

    cout << val << endl;

    cin.ignore();
    return 0;
}

But this gave me the error: error C2440: 'initializing' : cannot convert from 'std::chrono::system_clock::time_point' to 'long' How can I convert it to a 64 bit int? If I can't then I don't see how chrono is useful.

Answer 1

The following works with GCC 4.8 on Linux:

using namespace std::chrono;
auto now = high_resolution_clock::now();
auto nanos = duration_cast<nanoseconds>(now.time_since_epoch()).count();
std::cout << nanos << '\n';

Answer 2

First, convert the time point returned by now() into the duration since a known timepoint. This can either be the clock's epoch:

auto since_epoch = Clock::now().time_since_epoch();

or some timepoint that you've chosen:

auto since_epoch = Clock::now() - my_epoch;

Then you can get the number of nanoseconds either by converting and extracting the count:

auto nanos = duration_cast<nanoseconds>(since_epoch).count();

or by dividing by whatever granularity you want:

auto nanos = since_epoch / nanoseconds(1);

_{As noted in the comments, only do this last conversion (which leaves the Chrono library's type system, losing valuable information about what the number means) if you really need a scalar quantity; perhaps because you're interacting with an API that doesn't use the standard types. For your own calcuations, the types should allow you to perform any meaningful arithmetic that you need.}