It also depends on what functionality you want your parser to support. In theory, parsing just means "accept or reject". In practice, you need to do something

It also depends on what functionality you want your parser to support. In theory, parsing just means "accept or reject". In practice, you need to do something like build a tree, and there are many different possible tree representations depending on what you want to do. Clang aims to support very general transformation of C++ with their parser and AST. I was shocked to learn that Clang's AST has 60K lines of header files! That's not including the code, and it's not templates! That's class definitions!!! (wc -l include/clang/AST/*.h)