From time to time, someone has a problem because their build is
grabbing an old version of qpdf header files that are installed
somewhere on their system. I am hoping this will put an end to that.
These markers are being added for nested classes that are already
marked with QPDF_DLL_CLASS. They don't make any different on Linux,
but they matter on Windows.
Replace operator== and operator!=, which were testing for the same
underlying object, with isSameObjectAs. This change was motivated by
the fact that pikepdf internally had its own operator== method for
QPDFObjectHandle that did structural comparison. I backed out qpdf's
operator== as a courtesy to pikepdf (in my own testing) but also
because I think people might naturally assume that operator== does a
structural comparison, and isSameObjectAs is clearer in its intent.
QPDFValueProxy wasn't a good name for it. We decided the evil of
having the header file be named QPDFObject_private.hh was less than
the evil of having the class be named something other than what it
should have been named.
When a QPDF is destroyed, changing indirect objects to direct nulls
makes them effectively disappear silently when they sneak into other
places. Instead, we should treat this as an error. Adding a destroyed
object type makes this possible.
* Just removing a header file would cause build errors with no hint as
to what happened. This way, people get a warning rather than error
for the life of qpdf 11, and the warning tells them what to do.
* This avoids build surprises resulting from having two versions of
QPDF headers installed at once. If you were building code out of a
checkout of qpdf but had an older version installed on your system,
if your code included <qpdf/QPDFObject.hh>, everything would work,
but then your code would break without QPDFObject.hh later.
