Michel Fortin napisał:
> We already argument this over and over in the past. First, I totally
> acknowledge that C++ style containers have a problem: they make it
> easier to copy the content than pass it by reference. On the other side
> of the spectrum, I think that class semantics makes it too easy to have
> null dereferences, it's easy to get lost when you have a container of
> containers.
>
> I have some experience with containers having class-style semantics: in
> Objective-C, I ended up creating a set of macro-like functions which I
> use to initialize containers whenever I use them in case they are null.
> And I had to do more of these utility functions to handle a particular
> data structure of mine which is a dictionary of arrays of objects. In
> C++, I'd have declared this as a "map< string, vector< Object > >" and
> be done with it; no need for special care initializing each vector, so
> much easier than in Objective-C.
>
> I agree that defining structs to have reference semantics as you have
> done is complicated. But I like the lazy initialization, and we have a
> precedent for that with AAs (ideally, AAs would be a compatible
> container too). Can't we just use the GC instead of reference counting?
> I'd make things much easier. Here is a implementation:
>
> struct Container
> {
> struct Impl { ... }
>
> private Impl* _impl;
> ref Impl impl() @property
> {
> if (!impl) impl = new Impl;
> return *impl;
> }
>
> alias impl this;
> }
>
> I also believe reference semantics are not to be used everywhere, even
> though they're good most of the time. I'd like to have a way to bypass
> it and get a value-semantic container. With the above, it's easy as
> long as you keep Container.Impl public:
>
> void main() {
> Container lazyHeapAllocatedContainer;
> Container.Impl stackAllocatedContainer;
> }
>
> void MyObject {
> Container.Impl listOfObjects;
> }
Is there anything implementation specific in the outer struct that provides ref
semantics to Impl? If not, Container could be generic, parametrized by Impl
type.
Overall, I think a value-like implementation in a referency wrapper is a
clear-cut idiom, bringing order to otherwise messy struct-implemented
ref-semantics. Do you know of a existing collection library that exploits this
idea?
--
Tomek
