No specific APIs are currently provided for C++. As usual you can use WebDAV or XML-RPC interfaces to access OGo groupware services, and this is the way how C++ applications like KOrganizer access OGo.
Notably on MacOSX and with upcoming GCC versions on other environments, there is a new language called ObjC++ which integrates Objective-C with C++. This provides seemless access to OGo classes from C++.