Re: public headers

iesvs@free.fr wrote:


I haven't read this whole back-and-forth, but I totally understand where
iesvs is coming from here. I've often wished I could declare just the public
part of a class in a header, and the private part elsewhere. And there's no
deep reason I shouldn't be able to do that; it's just a missing language
feature. C++ already supports incomplete struct/class types, which
correspond to the special case where everything is hidden. It could just as
well support partially complete types. I can even think of a reasonably
intuitive syntax for it that doesn't require any new keywords:

     // header

     class A {
     public:
         float f;
         void method();
         ... // <- that's a literal '...'
     };

     // private implementation

     class A : public Base {
     public:
         float f; // must match (enforced by implementation)
         void method();
     private:
         int x;
     };

     void A::method() { ... }

The current syntax "class A;" would be identical to "class A { ... };".

I suspect, though, that if I proposed this to the standards committee,
they'd say "use virtual functions". And indeed you can get exactly the same
effect with virtual functions:

     // header

     class A {
     public:
         float f;
         virtual void method() = 0;
     };

     // private implementation

     class A_impl : public A, public Base {
     public:
         void method();
     private:
         int x;
     };

     void A_impl::method() { ... }

So this is what I recommend to the original poster. As far as I can tell, it
does exactly what you want. The only advantage of incomplete class
declarations would be efficiency: in a typical implementation they'd save
you a machine word per object and a hard-to-predict indirect jump per method
call.

-- Ben