tzdb::locate_zone not exported on windows

[apmorton]

I can get the tzdb, because get_tzdb is DATE_API.

DATE_API const tzdb& get_tzdb();

If I then try to call locate_zone or current_zone on the tzdb I get link errors because they aren't exported.

[HowardHinnant]

Try compiling tz.cpp with DATE_BUILD_DLL defined. This will trigger DATE_API to be defined as __declspec(dllexport): https://github.com/HowardHinnant/date/blob/master/include/date/tz.h#L122-L123.

Then when compiling the client code define DATE_USE_DLL. This will trigger DATE_API to be defined as __declspec(dllimport): https://github.com/HowardHinnant/date/blob/master/include/date/tz.h#L124-L125.

Both locate_zone and current_zone are decorated with DATE_API:

#if HAS_STRING_VIEW
DATE_API const time_zone* locate_zone(std::string_view tz_name);
#else
DATE_API const time_zone* locate_zone(const std::string& tz_name);
#endif

DATE_API const time_zone* current_zone();

https://github.com/HowardHinnant/date/blob/master/include/date/tz.h#L283-L289

Or, use std::chrono with a recent version of MSVC. This library has been part of std::chrono starting with C++20 and MSVC has been shipping it for a couple of years now. Issues such as this are far less likely to arise when using the std::lib.

https://gcc.godbolt.org/z/YxbY9jvbf

[apmorton]

The free functions have DATE_API, but the tzdb member functions don't: https://github.com/HowardHinnant/date/blob/master/include/date/tz.h#L1208-L1214

[HowardHinnant]

I don't have Windows to test on. If you submit a patch that fixes your problem, I'll accept it.