Zachary Turner wrote:
The most recent experience I had is in regards to providing custom validators for types in boost::program_options. <snip>
Another experience I had which I also made a thread about a week or so ago was with regards to intrusive_ptr. <snip>
OK, I understand a little better now. In each case you want to be able to provide different behaviour for different uses of the same type. I agree with other posters that perhaps this is a confusion between traits and policies. I struggle to imagine the same pseudo-pointer type requiring two different meanings of what null is, but that may be a lack of imagination on my part :).
Regarding enable_if, I didn't actually think of that. But is there a reason that if the generic class in question uses a default traits class as I suggested, that a user cannot simply provide a template specialization of that same class that uses the class template version of enable_if to achieve a similar effect?
You can, but only if the author of the class template provides a spare template parameter for this purpose defaulted to void. e.g. template<typename Pointer, typename Enabler = void> class pointer_policy { // ... }; and I've never seen any library do that. Moreover, you *can't* do that if the template is variadic. John Bytheway