c++ - 错误 C2664 'HRESULT IUnknown::QueryInterface(const IID &,void **)' : cannot convert argument 1 from 'const winrt::guid' to 'const IID &'

Question

当我使用来自 microsoft docs to migrate to winrt from cx 的辅助函数时，这个错误发生在我身上.我看到一个类似的问题 here ，但提到的解决方案似乎对我不起作用。此处提到的解决方案在文件中出现此错误的任何其他 winrt header 之前添加#include 。

template <typename T>
T from_cx(Platform::Object ^ from) {
T to{nullptr};

winrt::check_hresult(reinterpret_cast<::IUnknown*>(from)->QueryInterface(
    winrt::guid_of<T>(), reinterpret_cast<void**>(winrt::put_abi(to))));

return to;
}

这是整个文件:

#pragma once

#include <Unknwn.h>
#include <winrt/Windows.Foundation.h>

namespace x {
namespace y {

template <typename T>
T from_cx(Platform::Object ^ from) {
    T to{nullptr};

    winrt::check_hresult(reinterpret_cast<::IUnknown*>(from)->QueryInterface(
        winrt::guid_of<T>(), reinterpret_cast<void**>(winrt::put_abi(to))));

    return to;
}

template <typename T>
    T ^
    to_cx(winrt::Windows::Foundation::IUnknown const& from) {
        return safe_cast<T ^>(reinterpret_cast<Platform::Object ^>(winrt::get_abi(from)));
    }
}
}

Answer 1

winrt::guid_of()返回 winrt::guid。每What's new in C++/WinRT :

• Breaking change. GUID is now projected as winrt::guid. For APIs that you implement, you must use winrt::guid for GUID parameters. Otherwise, winrt::guid converts to GUID, as long as you include unknwn.h before you include any C++/WinRT headers. See Interoperating with the ABI's GUID struct.

根据 Interoperating with the ABI's GUID struct :

GUID is projected as winrt::guid. For APIs that you implement, you must use winrt::guid for GUID parameters. Otherwise, there are automatic conversions between winrt::guid and GUID as long as you include unknwn.h (implicitly included by <windows.h> and many other header files) before you include any C++/WinRT headers.

If you don't do that, then you can hard-reinterpret_cast between them.

因此，要么确保 unknwn.h 包含在 WinRT header 之前，要么您可以显式地reinterpret_cast，例如:

template <typename T>
T from_cx(Platform::Object ^ from) {
    T to{nullptr};
    winrt::guid iid = winrt::guid_of<T>();

    winrt::check_hresult(
        reinterpret_cast<::IUnknown*>(from)->QueryInterface(
            reinterpret_cast<GUID&>(iid),
            reinterpret_cast<void**>(winrt::put_abi(to)))
    );

    return to;
}