datap is pointer to your datablock (context), for example data table to be filled w/ exif callback.
It is passed to callback as 1st parameter (void *, you'll need to explicitly convert type before use).
Input datastream is passed as last parameter to callback as (void*). You'll need it (so, your callback should be C++ function in that case) if you plan to read tag values. If you want to only collect tags/tag types, you may ignore this last parameter.
Know nothing about CodeGear C++, so could not help with it.
datap is pointer to your datablock (context), for example data table to be filled w/ exif callback.
It is passed to callback as 1st parameter (void *, you'll need to explicitly convert type before use).
Input datastream is passed as last parameter to callback as (void*). You'll need it (so, your callback should be C++ function in that case) if you plan to read tag values. If you want to only collect tags/tag types, you may ignore this last parameter.
Know nothing about CodeGear C++, so could not help with it.