Re: [Boost-users] intrusive list with pointer_plus_bit

From: Christian Holmquist (c.holmquist_at_[hidden])
Date: 2008-05-16 20:42:07

2008/5/15 Ion Gaztañaga <igaztanaga_at_[hidden]>:
> Christian Holmquist wrote:
>> Hi,
>>
>> I have a class X : public intrusive::list_base_hook<> and would like
>> to embed an additional member bool using the pointer_plus_bit.
>>
>> Can this be achieved using the value_traits somehow, and have the list
>> working 'appropriately'?
>
> It can be achieved using value_traits. Just repeat the following example:
>
> http://www.boost.org/doc/libs/1_35_0/doc/html/intrusive/value_traits.html#intrusive.value_traits.value_traits_example
>
> but instead of defining this value_traits:
>
> //Define the node traits. A single node_traits will be enough.
> struct simple_node_traits
> {
>    typedef simple_node    node;
>    typedef node *         node_ptr;
>    typedef const node *   const_node_ptr;
>
>    static node *get_next(const node *n)
>    {  return n->next_;  }
>    static void set_next(node *n, node *next)
>    {  n->next_ = next;  }
>    // ...
> };
>
> Use this (supposing you want to embed the bit in the next pointer):
>
> struct simple_node_traits
> {
>    // ...
>
>    static node *get_next(const node *n)
>    {  return pointer_plus_bit<node*>::get_pointer(n->next_);  }

fails to compile due to n->next_ points to void* and not node*.
is it safe to use static_cast<node*>(n->next_) here?

>
> FYI, I've just erased pointer_plus_bit.hpp and substituted it with a
> more generic pointer_plus_bits.hpp in Trunk. Now that I see that it's
> being used, I will need to provide backwards compatibility.

Ah well,it's not production code, so to me it's not important with
backward compatibility.

What I'm trying to do is to build a rollback mechanism that's more
robust to exceptions due to failing memory allocations. since i'm
doing some optimizations on the way, it just seemed as a good idea
using the pointer_plus_bit feature.

BI is a great lib by the way, the most inspiring addition to boost in
1.35 (although I've been using it from trunk for quite a while)!

>
> I hope it works,
>
> Ion
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