Re: [Boost-users] intrusive list with pointer_plus_bit

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2008-05-15 14:32:35

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_);  }

    static void set_next(node *n, node *next)
    {  return pointer_plus_bit<node*>::set_pointer(n->next_, next);  }
    //...
};

I haven't compiled it, so it might contain errors. To access to the set 
bit you should define a function that accesses next pointer and extracts 
the bit, something like:

pointer_plus_bit<node*>::set_bit(x.next_, true);
bool ret = pointer_plus_bit<node*>::get_bit(x.next_);

Take in care that you should make sure that

has_pointer_plus_bit<void *, boost::alignment_of<node>::value>::value;

is true, otherwise, you won't have space to store the bit, because the 
pointed type has not even alignment.

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.

I hope it works,

Ion