It's really like a worldmap: I think it's better to have a good overall view, then dive only when needed. You can literally spend weeks into a tiny part of such

It's really like a worldmap: I think it's better to have a good overall view, then dive only when needed. You can literally spend weeks into a tiny part of such codebases. What I used when dealing with very large code base (>500kloc) was putting the classes, their relationships (inheritance etc) as some kind of graph (UML, MindMap) on a large (A3 or A0) sheet of papier. To achieve that, I used either an existing tool (reverse engineering UML model in C# for instance) or a home-baked approach (use reflection or even home-made file parsing) then graphviz or similar. In any case, you'll want to have an automated way of doing this, so you can refine afterward (ie: put colors on what you already visited, add a summary extracted from comments, etc). Good luck! If it's like for me, it will become fun :)