[ryl00]

> I want it all, I want to have my cake and eat it too.

Conceptually this should be possible with some preprocessor/header tinkering; something like:

private.h:

  struct Bar {
    double x;
    double y;
  };
  #define BAR_DEFINED

public.h:

  #ifndef BAR_DEFINED
  struct Bar {
    char opaque[16];
  };
  #endif

  struct Foo {
    Bar bar;
  };

consumer.cpp:

  #include "public.h"

private_implementation.cpp:

  #include "private.h"
  #include "public.h"

[blt]

this code can crash on platforms where unaligned access is not allowed. you need an alignas on public Bar. and of course the catastrophic bugs if the size of opaque is not kept in sync properly. the point is: the module system should be doing something like this for you behind the scenes.