Multiple Variadic Parameter Pack for Template Class

Question

I am using variadic parameter packs for policy based class design.

template 
class IShader : public Policies... {

};
         


        

Answer 1

I think the simplest answer is to create template type wrappers for your parameter packs. For example:

template 
struct Attributes {};

template 
struct Policies {};

Then you can declare your IShader type:

template 
class IShader;

Create your implementation as a specialization. Note that in a specialization, you can have multiple parameter pack arguments.

template 
class IShader, ApiType, Policies> 
    : public PolicyList...
{
    ...
};

Then you can even allow the user to specify the arguments in different orders (make sure you forward the constructors if doing it this way via inheritance):

template 
struct IShader, Attributes
    : public IShader, ApiType, Policies>
{
    using IShader, ApiType, Policies>::IShader;
};

If you're being really fancy, you can even use metaprogramming tricks to allow the arguments in any order without enumerating all orders. This is left as an exercise to the reader. :)