A small trick for simple Rust/C++ interop
Due to the heavy use of callbacks (sigh), Rust sometimes calls C++ and C++ sometimes calls Rust. This done by having both sides expose a C API for the functions they want the other side to be able to call.
This is for functions; but what about C++ methods? Here is a trick to rewrite one C++ method at a time, without headaches. And by the way, this works whatever the language you are rewriting the project in, it does not have to be Rust!
Note: Depending on the C++ codebase you find yourself in, the first point could be either trivial or not feasible at all. It depends on the amount of virtual methods used, etc.
Here is our fancy C++ class, User. It stores a name, a uuid, and a comment count. A user can write comments, which is just a string, that we print.
So let's first ensure it is a standard layout class. We add this compile-time assertion in the constructor (could be placed anywhere, but the constructor is as good a place as any):
