A suggestion about: "Method bodies may only appear in the class definition if they are very short, no more than five lines. Otherwise the method body should be defined outside of the class definition."
To avoid dependency explosions that increase compile times and allow for link-time substitutability, I would suggest that method bodies should very rarely appear in header files. In my experience, this dramatically improves the likelihood that complex and/or user-defined types can be forward-declared and therefore can improve encapsulation and lower compilation times. This is also critical for link-time injection of mocks for unit testing, which is often the first step when transitioning a non-OO codebase toward OO. Classically, the main reason to put small functions into header files was to allow for inlining of small functions. With LTO, this point should be somewhat moot. In our internal testing of mega-compilation (a poor man's LTO where source files for a given binary module are #included into a single compilation unit), clearing header files of almost all method bodies resulted in a 20% decrease in compilation times and an 11% performance improvement at the system level. These improvements were icing on the cake, as our primary goal was to improve unit-testability by way of better encapsulation (and other OO principles).
