Yes, that's true. But if the concern is build-times, exposing the data layout is harmless. We don't necessarily need full PIMPL just to get improved build times. By keeping the data layout in the .hpp, you can guarantee that your class can still stack-allocate.
if the concern is build-times, exposing the data layout is harmless
Not at all! If your private member variables use any interesting types (and why shouldn't they?) you need to include all the headers for those types too. That's exactly why you get an explosion of header inclusion.
If you change the data layout of your exposed class, you must recompile anything that uses it. That increases build times and also breaks ABI. And as the other guy commented, the data itself has types that also need definitions and file inclusions. Without PIMPL, your data layout can change without touching your header, due to changes in a related header (even a 3rd party header).
