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

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() << '\n';
}

Answer 2

In deed, nanoseconds since Epoch = nanoseconds since midnight + 1e9 * calendar time of last midnight. You lack the latter value. You can built it with a conjunction of ::gmtime() and ::mktime().
#include <ctime>
std::time_t last_midnight()
{
// compute "now"
std::tm& tm = *::gmtime(NULL);
// move it to "last midnight"
tm.tm_hour = 0;
tm.tm_min = 0;
tm.tm_sec = 0;
// get time_t back
return ::mktime(&tm);
}
You may find interest in using ::gmtime_r() (re-entrant version) instead of ::gmtime().