> Maybe an overloaded version of the toString() accessor
> that takes a (const std::string &) parameter?

Added that. Since the virtual method LangAltValue::toString(long n)
didn't do anything at all, I changed it to return the text value
associated with the default language qualifier "x-default" (for any
n). I don't think that's particularly good, just better than nothing.
If anybody has a better idea, please let me know. In the meantime, you
can now write

std::string copyright;

const Exiv2::XmpData& xmpData = image->xmpData();
Exiv2::XmpData::const_iterator xmpIter =
xmpData.findKey(Exiv2::XmpKey("Xmp.dc.rights"));

if(xmpIter != xmpData.end() && xmpIter->typeId() == Exiv2::langAlt)
{
copyright = xmpIter->toString(0);
}

Thanks for the feedback, Marco

-ahu.