These documents describes some C++ classes that may be of use if you want to write LV2 plugins in C++. They implement most of the boilerplate code so you only have to write the parts that matter, and hide the low-level C API.
The classes are separated into two libraries. One, called liblv2-plugin, contains the classes LV2::Plugin and LV2::Synth, defined in the files lv2plugin.hpp and lv2synth.hpp. They are base classes that you can inherit in order to create new LV2 plugins. The other library, called liblv2-gui, contains the class LV2::GUI, defined in the file lv2gui.hpp, which you can use in a similar way to create new LV2 plugin GUIs.
These libraries are still in development - the API may change in future versions. However, if you only link to the static libraries you should at least not get any runtime problems with plugins that are already built.
1.5.4