Re: [boost] [intrusive] Comments on interface changes and stateful value_traits (long)

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2007-08-25 04:26:03

Two more comments on extrusive containers:

-> Instead of changing functions passing the address of the container, 
the traits can define a function called "get_external_traits()" or 
similar that the container will call before using the trait if the 
corresponding option is activated (code not compiled, so it might 
contain errors):

//Predeclaration
struct external_value_traits;

//Internal traits
struct internal_value_traits
{
    typedef external_value_traits external_traits;

    //Suppose external_value_traits is derived
    //from the container
    template<class C>
    static const external_traits *get_external_traits(const C &c)
    {  return static_cast<RealTraits>(c); }
};

so we can have something like this:

typedef list_options
   < value_traits<internal_value_traits>
   , external_value_traits<true>
   >::type options;

typedef list<T, options> MyList;

struct external_value_traits
    : public MyList
{
    //...
    snode_ptr to_node_ptr (value_type &value)
    {
       //whatever conversion using any external auxiliary data
    }
}

-> If external traits are used (for example, external value traits) 
iterator should be also modified adding an additional pointer pointing 
to the container (so that the iterator can have access to the external 
traits to make conversions). I still don't know how can this impact some 
operations like merging, etc...

So external traits look exciting but they might require a bit of work to 
avoid any overhead to users not activating those advanced options.

Regards,

Ion