Re: [boost] BGL adjacency list container selectors

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2002-01-14 13:29:38

The change you suggest is in fact the current approach we use to work
around VC++. As to whether that approach is simpler is not so clear to me.
For example, the 2nd approach requires a nested class, which could be
regarded as a complication. Overall, I'd classify this NAD ;)

On Tue, 8 Jan 2002, David Abrahams wrote:

david.> The scheme used by BGL for selecting the component containers of adjacency_list seems to have an extra, needless level of indirection. Why bother with the traits at all? It seems to me that VecS et. al. could be generators themselves.
david.>
david.> For example, the docs say:
david.>
david.>   There may also be situations when you want to use a container that has more template parameters than just ValueType. For instance, you may want to supply the allocator type. One way to do this is to hard-code in the extra parameters within the specialization of container_gen. However, if you want more flexibility then you can add a template parameter to the selector class. In the code below we show how to create a selector that lets you specify the allocator to be used with the std::list.
david.>
david.>
david.>   template <class Allocator>
david.>   struct list_with_allocatorS { };
david.>
david.>   namespace boost {
david.>     template <class Alloc, class ValueType>
david.>     struct container_gen<list_with_allocatorS<Alloc>, ValueType>
david.>     {
david.>       typedef typename Alloc::template rebind<ValueType>::other Allocator;
david.>       typedef std::list<ValueType, Allocator> type;
david.>     };
david.>   }
david.> With the change I am proposing, it could read as follows:
david.>   There may also be situations when you want to use a container that has more template parameters than just ValueType. For instance, you may want to supply the allocator type. In the code below we show how to create a selector that lets you specify the allocator to be used with the std::list.
david.>  template <class Alloc>
david.>  struct list_with_allocatorS {    template <class ValueType>    struct apply    {
david.>       typedef typename Alloc::template rebind<ValueType>::other Allocator;
david.>       typedef std::list<ValueType, Allocator> type;
david.>     }; };
david.>
david.> IMO this is a considerable simplification, because it can be done entirely in the user's namespace.
david.>

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 Jeremy Siek                          http://php.indiana.edu/~jsiek/
 Ph.D. Student, Indiana Univ. B'ton   email: jsiek_at_[hidden]
 C++ Booster (http://www.boost.org)   office phone: [phone]
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