How Best to Incorporate Traits and a Policies

I see the STL using class templates accepting a traits and a policy.
To me, the traits are the "what" and the policy is the "how". How can
a policy know what to do without the traits of the topic? Wouldn't it
be better for containers to take a policy template argument and for
the policy to take a traits template argument? Consider an allocator
(policy) that is going to be used in a multi-threaded environment.
The programmer could use something like the following (simplified)
traits.

template<typename TYPE>
struct NormalTraits
{
typedef TYPE value_type;
typedef value_type *pointer;
typedef std::size_t size_type;
};
template<>
struct NormalTraits<void>
{
private: typedef void TYPE;
typedef TYPE value_type;
typedef value_type *pointer;
typedef std::size_t size_type;
};

template<typename TYPE>
struct VolatileTraits
{
typedef TYPE value_type;
typedef volatile value_type *pointer;
typedef std::size_t size_type;
};
template<>
struct VolatileTraits<void>
{
private: typedef void TYPE;
typedef TYPE value_type;
typedef volatile value_type *pointer;
typedef std::size_t size_type;
};

Now, a programmer could specify the VolatileTraits<to an allocator
policy for a multi-threaded container and the policy would know what
types should be given to the signatures of methods like 'pointer
allocate( size_type, typename
TRAITS::rebind<void>::other::const_pointer )'.

Milburn Young


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In article <1184740202.232783.131560@z24g2000prh.googlegroups .com>,
Milburn Young <Milburn.Young@gmail.comwrote:seems that you have no traits publically accessable if type is void:)

boost and tr1 provide add_volatile and is_volatile templates
which are simple to write if you don't have either.

template <class A,class Bool = is_volatile<A class Allocator;

template<class Aclass Allocator<A,true_type>
{
// volatile allocator
};

template <class Aclass Allocator<A,false_type>
{
// non volatile allocator
};
less is more:)

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On Jul 18, 3:59 pm, Milburn Young <Milburn.Yo...@gmail.comwrote:Any distinction is really arbitrary. In the STL, they are
called traits. In other, more recent literature, they are
called policy. Same thing in the end.

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