"Peter Dimov" <pdimov@mmltd.net> wrote in message news:002e01c4b37d$cfac5c80$6401a8c0@pdimov2...
David M. Jones wrote:
I ran into a situation (see my "smart list" post on the boost users group) where I wanted to create a custom allocator whose pointer typedef is boost::shared_ptr. In other words, the code would look something like:
template <typename T> class my_allocator { public: typedef boost::shared_ptr<T> pointer; };
The problem with doing this is that boost::shared_ptr<T> does not have the same "interface" as the equalivalent raw pointer type T*. (I use the term interface in the generic programming/template sense; not in the object oriented sense.) One example is that there is no operator= on boost::shared_ptr that takes a raw pointer; instead reset() must be used.
Raw pointers don't necessarily have an operator= that takes another kind of raw pointer. For example, T __near * cannot be assigned T*, only itself. Similarly, shared_ptr<T> cannot be assigned T*, only itself.
If a component tries to assign a T* to allocator<T>::pointer, it is broken WRT nonstandard pointers.
Thank you for your reply, Peter. Let me be more specific about the problem I ran into. I am using MSVC 7.1 and the implementation of the STL that comes with it. When I tried to create and use an object of type std::list<int, my_allocator<int> >, I received a compiler error because the STL code contains _Myhead = 0; where _Myhead is of type my_allocator<node>::pointer (ie. boost::shared_ptr<node>) and node is the class that defines the nodes in the linked list. So the STL is not calling operator=() for a boost::shared_ptr with just any pointer type on the right-hand side. In fact, it's not even a T* -- it is specially setting it to null. Are you telling me that this STL implementation is (slightly) invalid? Or are you telling me that I should not expect boost::shared_ptr to be a valid type for use as a custom allocator::pointer? Or both? I have read that Scott Meyers (Effective STL; Item 10) says that custom allocators should always have typedef T* pointer but I have also read that Matt Austern (December 2000 CUJ column) says that custom allocators can sometimes be "some pointer-like class"; it seems to me that boost:shared_ptr is a "pointer-like class".