consume_t template

Question

what's the purpose of the consume_t template class? I don't understand how it works

that's the first things I found and copied out...

template <typename T>
struct consume;

template <typename D>
struct consume_IInterface
{
    [[nodiscard]] WINRT_IMPL_AUTO(hstring) Verb() const;
};

template <> struct consume<IInterface>
{
    template <typename D> using type = consume_IInterface<D>;
};

template <typename D, typename I = D>
using consume_t = typename consume<I>::template type<D>;

template <typename D, typename I>
struct require_one : consume_t<D, I>

template <typename D, typename... I>
struct __declspec(empty_bases) require : require_one<D, I>...
{};

...
now, I see multiple classes defined as

struct something : require<something, IInterface>
{}

so, let's analyze it... first, require does derive from every passed interface by using require_one... that's the easy part. Then consume_t is used and here I don't see what the purpose is... it's inheriting from a inner template class of the "consume<I>" template class? ... why? what's the purpose of it? ... I didn't find the corresponding code that explains this.
And then... what's the purpose of the finally inherited class? Does it do anything? What's this "Verb" function doing they added?

Answer 1

Hi @Rudolf Meier ,

About alias templates:

template < template-parameter-list >  
using identifier attr(optional) = type-id ;  

attr - optional sequence of any number of attributes
identifier - the name that is introduced by this declaration, which becomes either a type name or a template name.
template-parameter-list - template parameter list, as in template declaration.
type-id - abstract declarator or any other valid type-id (which may introduce a new type, as noted in type-id). The type-id cannot directly or indirectly refer to identifier. Note that the point of declaration of the identifier is at the semicolon following type-id.

About Inheritance I suggest you to read this document.

 template <typename D, typename I>  
 struct require_one : consume_t<D, I>  

In my opinion, the code you uploaded should come from a tutorial, the point of this code is to show you inheritance between class templates. You may be confused due to lack of application of class templates.

Best regards,

Elya

---

If the answer is the right solution, please click "Accept Answer" and upvote it.If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.