Re: gcc 3.x custom allocators

[Benjamin Kosnik <bkoz at redhat dot com>]

>  Howdy. I'm trying to grok the new gcc 3.2 style of writing a custom
>allocator. I have a C++ based memory manager I use for a shared memory segment
>so I can instantiate STL containers in the shared memory segment for
>passing data around. This I guess now is the old style, that looks something
>like this:
>
>// code guts remove for clarity
>template < typename T, typename CTASallocType = allocator<T> >
>class ShmMem
>{
>public:
>    CTASallocType alloc;
>    typedef typename CTASallocType::size_type size_type;
>    typedef typename CTASallocType::difference_type difference_type;
>    typedef typename CTASallocType::pointer pointer;
>    typedef typename CTASallocType::const_pointer const_pointer;
>    typedef typename CTASallocType::reference reference;
>    typedef typename CTASallocType::value_type value_type;
>    typedef typename CTASallocType::const_reference const_reference;
>    template <typename U> struct rebind {
>        typedef ShmMem<U, 
>                       typename CTASallocType::template rebind<U>::other>
>other;
>    };
>
>    ShmMem(void) {  }
>    ~ShmMem() {}
>
>    ShmMem(const ShmMem& x)
>        : alloc(x.alloc) {}
>
>    template <typename U>
>    ShmMem(const ShmMem<U, 
>        typename CTASallocType::template rebind<U>::other>& x)
>        : alloc(x.alloc) {}
>  
>    void construct(pointer p, const value_type &val) { }
>    void destroy(pointer p) { }
>    pointer allocate(size_type sz, const void* vp = 0) { }
>    void deallocate(pointer p, size_type sz) { }
>};

I've enclosed some weak example code, based on the custom allocators a
la the pool allocator in TCPL 19.4.2 A User-Defined Allocator.

>  All I really want to do is hook into allocate and deallocate to use my
>custom allocator. I've been reading libstdc++ docs and stl_alloc.h, but
>so far, I just don't get it. Plus I gather this is all gonna change again 
>soon ?

I thought the only change was the removal of the "reallocate" member.

>  My code still works fine with gcc 2.95.x, but I'm trying to work using
>gcc 3.2 as much as I can these days.

I don't suppose you can post a full example of what you are trying to
do, one that works with 2.95.3? Then I could try to port it for you.

It would be really nice to have a complete, usable example in the
libstdc++ docs for this. It would be really, really nice to have docs
for custom allocators that used: 1) libhoard, 2) persistent allocation,
3) shared segments like you are trying to do.

-benjamin