c++ - How to get UTC seconds since epoch till midnight

Question

I have to instantiate an object of a custom library class that takes nanoseconds since 'epoch' in UTC time to construct:

class utc_time
{
public:
utc_time(TYPE nanoseconds_since_epoch):
_nanoseconds_since_epoch(nanoseconds_since_epoch){}

private:
TYPE _nanoseconds_since_epoch;
};

what I have as my input is nanoseconds since 'midnight' in UTC time. Naturally, I need to get the epoch till last midnight nanoseconds(in UTC) to add it to my input and supply it to my class constructor. I know we have gtime that may be helpful, but I dont know how to extract the required information.

I appreciate your clues

Answer 1

In C++11 and later, <chrono> can be used to do this very easily:

#include <chrono>
#include <iostream>

int
main()
{
    using namespace std::chrono;
    using namespace std;
    using days = duration<int, ratio<86400>>;
    nanoseconds last_midnight =
        time_point_cast<days>(system_clock::now()).time_since_epoch();
    cout << last_midnight.count() << '
';
}