Converting from std::chrono::system_clock::time_point to b_date and back

[Ofer Reichman]

I've spent some time trying to figure out how to store a time_point using the C++ driver.

I'm posting my code for review and to help anyone who might need it (until the driver supports it).

Storing:

using std::chrono::system_clock;
using std::chrono::duration_cast;
using std::chrono::milliseconds;

system_clock::time_point tp = system_clock::now(); // for example

document << "someTimePoint" << b_date{duration_cast<milliseconds>(tp.time_since_epoch()).count()};

Retrieving:

system_clock::time_point tp(duration_cast<system_clock::duration>(milliseconds{doc->view()["someTimePoint"].get_date().value}));

Note: Different clocks have different epochs, so this code may not work correctly if the machine that stores the time_point is different than the one that retrieves it.

Hope this is helpful.

Any reviews are welcome!

Ofer

[Adam Midvidy]

Hi Ofer,

Thanks for sharing this. If you'd like to add a constructor to b_date that takes a std::chrono::system_clock::time_point, we'd be happy to take a pull request for it. We have an outstanding ticket for this: https://jira.mongodb.org/browse/CXX-626.

Thanks,

Adam

[Ofer Reichman]

Would be happy to do so, once I get some reviews. I'm not sure I'm doing it right.

According to the manual, "BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970)."  So I think I got that right.

Is using the type system_clock::time_point correct?

Is the C++ code optimal? (especially the 2nd one - retrieval)

[Ofer Reichman]

done. added constructor and casting operator to b_date.