I started using Boost's approach, that is keep those things public but in their own clearly-named internal namespace (be it an actual namespace or otherwise).

I started using Boost's approach, that is keep those things public but in their own clearly-named internal namespace (be it an actual namespace or otherwise). This way users can get to them if they really need to, say for a crucial bug fix, but they're also clearly an implementation detail so you're free to change it without users getting surprised when things break etc.