The code you find out seems pretty old (last update 6 years ago). Old Dropbox API is implemented there (currently deprecated, although available still). I'm not sure if there is schedule for this API support dropping. From other view point this is good example of Boost library usage (very powerful C++ library). I have no idea, in this context, why libcurl is still used (everything could be done using Boost only). Also this code could be used as good (not the best, but...) example of mixing C and C++ code. May be this could be used as start point for implementing Dropbox API v2. (at least some tips)
I just took a look at that third party libray, and I can confirm it does use the old Dropbox API v1. To clarify one point, Dropbox API v1 is already retired, and so it not available. Attempting Dropbox API v1 calls will fail.
yeah @Здравко I have recently started using boost, as a matter of fact the reason I've been searching a way to use c++ to upload file to dropbox is that I have found out the way to search files recursively by their extension, so I thought of going in Disk D:\\ and uploading all the files with .jpg or .mp4 extensions. I know what you're thinking, Dropbox application can even upload a whole folder but it is a lot more fun to do that with code, specially since I am C++ beginner.
@Greg-DB ooh, thanks for this info, I will stop cracking my head with that library