What is the difference between a trait and a policy?

Question

I have a class whose behavior I am trying to configure.

template ServerTraits;

Then later on I h

Answer 1

Here are a couple of examples to clarify Alex Chamberlain's comment:

A common example of a trait class is std::iterator_traits. Let's say we have some template class C with a member function which takes two iterators, iterates over the values, and accumulates the result in some way. We want the accumulation strategy to be defined as part of the template too, but will use a policy rather than a trait to achieve that.

template 
class C{
    void foo(Iterator begin, Iterator end){
        AccumulationPolicy::Accumulator accumulator;
        for(Iterator i = begin; i != end; ++i){
            std::iterator_traits::value_type value = *i;
            accumulator.add(value);
        }
    }
};

The policy is passed in to our template class, while the trait is derived from the template parameter. So what you have is more akin to a policy. There are situations where traits are more appropriate, and where policies are more appropriate, and often the same effect can be achieved with either method leading to some debate about which is most expressive.