Re: [Boost-users] nondynamic: A utility class that promotes safe programming

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2008-03-28 08:21:07

Sorry,

I mixed smth. That is true, that placement new can not be overridden. The
problem, is that you can not allocate for vector items with new[]. Because
std containers allocate some number of items using std::allocator class. C++
defines the following interface for the allocator class:

template <class T>
class allocator
{
public:
    typedef size_t                             size_type;
    typedef ptrdiff_t                     difference_type;
    typedef T*                                     pointer;
    typedef const T*                         const_pointer;
    typedef T&                                     reference;
    typedef const T&                         const_reference;
    typedef T                                     value_type;

    template <class U>
    struct rebind
    { typedef allocator<U> other; };

    allocator() throw();
    allocator(const allocator&) throw();
    template <class U>
    allocator(const allocator<U>&) throw();
    ˜allocator() throw();

    pointer address(reference x) const;
    const_pointer address(const_reference x) const;

    pointer allocate(size_type, allocator<void>::const_pointer hint = 0);
    void deallocate(pointer p, size_type n);

    size_type max_size() const throw();

    void construct(pointer p, const T& val);
    void destroy(pointer p);
};

size_type parameter in allcoate member function means:
    The number of elements for which sufficient storage is to be allocated.

What do you think how this allocator will allocate the number of these
element? Actually in any possible way.
It can use new[] or new char[sizeof(T)*NumElems] or use an own mem-manager.
And the worse thing you can't make
allocator friend of your class, since users of containers can pass their own
allocators.

And despite all enumerated facts you will fail to compile, since the
function address will fail to return the pointer to x, because it is
prohibited in your case.

On Fri, Mar 28, 2008 at 11:33 AM, Daniel Lidström <daniel.lidstrom_at_[hidden]>
wrote:

> Hello Ovanes,
>
>
> As I understand it, the standard containers use the global placement new,
> which cannot be overridden. These lines are actually ok to do it seems:
>
>   char dataIOBuffer[sizeof(DataIO)] = {0};
>   DataIO* inplaceCreated = ::new(dataIOBuffer)DataIO(IDataIOPtr());
>
This storage is possibly not properly aligned when used like this.

> Regardless of where I place (no pun intended) the placement new operator
> in nondynamic.
>

> --
> Daniel <http://lists.boost.org/mailman/listinfo.cgi/boost-users>
>

Sorry for misleading post before.

Ovanes