I am using variadic parameter packs for policy based class design.
template <APITypes APIType, class... Policies>
class IShader : public Policies... {
};
Policies are defined when called or with defaults if none are specified. The problem comes when I need to add another variadic parameter pack:
template <AttributeType... Attributes, APITypes APIType, class... Policies>
class IShader : public Policies... {
};
This results in the error "Template parameter pack must be the last template parameter". I am planning to use the attribute pack to change the behaviour of at least one of the policies. But I can't work out how to get two variadic parameter packs in one template class.
In the discussion comments you expressed a willingness to consider some kind of indirection, or "a wrapper of some kind for the attribute list".
A lightweight std::tuple-based wrapper, together with specialization, might work here:
template <typename attribute_tuple, APITypes APIType,
typename policy_tuple> class IShader;
template <AttributeType... Attributes, APITypes APIType,
class... Policies>
class IShader<std::tuple<Attributes...>, APIType,
std::tuple<Policies...>> : public Policies... {
// ...
};
The goal here is to use a template instance along the lines of:
IShared<std::tuple<Attribute1, Attribute2>, APITypeFoo,
std::tuple<Policy1, Policy2>> ishared_instance;
And cross your fingers that this is going to match the specialized template declaration, at which point both parameter packs are available for the template specialization to use, individually.