Question

Coming from C# it was quite common for me to write a class that might have a DateTime as a return type (or parameter). Now I’m programming in C++. What is the appropriate data type to use as a method return value returning a datetime value (time_t, tm struct, Boost.Date_Time, something else?) E.g.:

class Customer
{
   ...
   ? GetDateTimeCreated() const;
   void SetLastContactDateTime(? date);
   ...
}

Also, what about accepting date/times as function parameters?

Answer 1

If you’re writing a library, and the datetime value will exposed via the API, then I’d use time_t so that users of the library are not forced to use Boost.

As was pointed out, struct tm is used for formatting a time_t. You don’t normally want to be passing those around.

If your program already uses Boost, and you like the convenience of Boost.DateTime, then by all means use Boost.DateTime. If I remember correctly, boost::posix_time::ptime is just a wrapper around two 64-bit integers, so it’s lightweight enough to pass around by value.

If you’re using C++11 features, then you might want to use std::chrono::time_point from <chrono>. If you’re still on C++03, you can use Boost.Chrono, which aims to implement the C++11 time facilities. By using Boost.Chrono, you should be able to more easily make the switch to C++11 in the future. Chrono doesn’t have as many features as Boost.DateTime, but it’s a step up from plain old time_t.

If you need sub-second precision, then Boost.DateTime or Chrono is the way to go.