如何创建一个可变结构，它采用与 ctor 参数相同的 "variadic invocable"？

Question

我正在尝试推广此 COM 对象自动管理 class，但我什至不确定是否可行。目前，我已经为 1、2 和 3 个指针定义了它，但我想让它成为所有情况下的单一实现。

手动实施：

#include <concepts>
#include <type_traits>

#include <Unknwn.h>

template <std::derived_from<IUnknown> COMInterface>
struct AutoManagedCOMObj {
    COMInterface* ptr;

    template<std::invocable<COMInterface**> Invocable>
    AutoManagedCOMObj(Invocable initializer) {
        HRESULT hr = initializer(&ptr);
        if (FAILED(hr)) exit(hr);
    }
    ~AutoManagedCOMObj() {
        ptr->Release();
    }
};

template <
    std::derived_from<IUnknown> COMInterface1,
    std::derived_from<IUnknown> COMInterface2
>
struct AutoManaged2COMObjs {
    COMInterface1* ptr1;
    COMInterface2* ptr2;

    template<std::invocable<COMInterface1**, COMInterface2**> Invocable>
    AutoManaged2COMObjs(Invocable initializer) {
        HRESULT hr = initializer(&ptr1, &ptr2);
        if (FAILED(hr)) exit(hr);
    }
    ~AutoManaged2COMObjs() {
        ptr2->Release();
        ptr1->Release();
    }
};

template <
    std::derived_from<IUnknown> COMInterface1,
    std::derived_from<IUnknown> COMInterface2,
    std::derived_from<IUnknown> COMInterface3
>
struct AutoManaged3COMObjs {
    COMInterface1* ptr1;
    COMInterface2* ptr2;
    COMInterface3* ptr3;

    template<std::invocable<COMInterface1**, COMInterface2**, COMInterface3**> Invocable>
    AutoManaged3COMObjs(Invocable initializer) {
        HRESULT hr = initializer(&ptr1, &ptr2, &ptr3);
        if (FAILED(hr)) exit(hr);
    }
    ~AutoManaged3COMObjs() {
        ptr3->Release();
        ptr2->Release();
        ptr1->Release();
    }
};

初步尝试一般解决方案：

template<
    std::derived_from<IUnknown> ... T
>
struct AutoManagedCOMObjs {};

template<
    std::derived_from<IUnknown> COMInterface,
    std::derived_from<IUnknown>... Rest
>
struct AutoManagedCOMObjs<COMInterface, Rest ...> {
    COMInterface* first;
    AutoManagedCOMObjs<Rest ...> rest;

    template<std::invocable<COMInterface**, Rest** ...> Invocable>
    AutoManagedCOMObjs(Invocable initializer) {
        HRESULT hr = initializer(&first, &rest...);
        if (FAILED(hr)) exit(hr);
    }
    ~AutoManagedCOMObjs() {
        // the dtor is an unsolved problem as well
        //ptr2->Release();
        //ptr1->Release();
    }
};

我想不出将结构声明中的 ... Rest 与 invocable 参数签名联系起来的方法。

如果有帮助的话。 VS告诉我

Error '...': there are no parameter packs available to expand (referring to the first line of the ctor).

对于上下文，这就是我使用这些 classes 之一的方式：

AutoManagedCOMObj<ID3D11Texture2D> back_buffer_interface([](ID3D11Texture2D** ptr){
    return swap_chain->GetBuffer(0, uuid(ptr), (void**)ptr);        
});

Answer 1

std::tuple (with std::apply) 可能有帮助：

template <std::derived_from<IUnknown>... Ts>
struct AutoManagedCOMObj
{
    std::tuple<Ts*...> tuple_ptrs;

    template<std::invocable<Ts**...> Invocable>
    AutoManagedCOMObj(Invocable initializer)
    {
        HRESULT hr = std::apply([&](auto*&... ptrs){ return initializer(&ptrs...);},
                                tuple_ptrs);
        if (FAILED(hr)) exit(hr);
    }
    ~AutoManagedCOMObj()
    {
        std::apply([](auto*... ptrs){ (ptrs->Release(), ...); }, tuple_ptrs);
    }
};

如何创建一个可变结构，它采用与 ctor 参数相同的 "variadic invocable"？

How to create a variadic struct that takes a "variadic invocable" of the same arity as the ctor parameter?

c++

variadic-templates

c++-concepts