Could C++ have not obviated the pimpl idiom?

Question

As I understand, the pimpl idiom is exists only because C++ forces you to place all the private class members in the header. If the header were to contain only the public interface, theoretically, any change in class implementation would not have necessitated a recompile for the rest of the program.

What I want to know is why C++ is not designed to allow such a convenience. Why does it demand at all for the private parts of a class to be openly displayed in the header (no pun intended)?

Answer 1

This has to do with the size of the object. The h file is used, among other things, to determine the size of the object. If the private members are not given in it, then you would not know how large an object to new.

You can simulate, however, your desired behavior by the following:

class MyClass
{
public:
   // public stuff

private:
#include "MyClassPrivate.h"
};

This does not enforce the behavior, but it gets the private stuff out of the .h file. On the down side, this adds another file to maintain. Also, in visual studio, the intellisense does not work for the private members - this could be a plus or a minus.