ReVisitors DXF library is implemented in C++. It is based on wxWidgets, an open-source cross platform library.
The base class for parsing a DXF file is strategis::DXFParser. It traverses a DXF file and calls virtual handles when visiting different group codes. This is like a SAX model and is similar to how expat traverses XML files. To actually do something useful while the file is traversed, you need to sub class from strategis::DXFParser and implement functionality in the virtual handles, like e.g. strategis::DXFParser::EntityEnded().