尽管包含正确的c ++ / winrt标头，但仍然出现“在定义前不能使用返回'auto'的函数”错误

Question

这个问题已经在这里有了答案 ：

3个月前关闭。

在C ++ / WinRT项目中使用CoreDispatcher :: RunAsync时，获取“在定义之前不能使用返回'auto'的函数” （1个答案）

我正在尝试转换一个小型的win32桌面应用程序，以使用c ++ / winrt组件和XAML孤岛。 我关注了有关该主题的大量文章，并且确实能够使用一些基本的XAML控件来编译和运行该应用程序。 使用FileOpenPicker时出现我的问题，并且随后出现错误消息，指示“在定义前不能使用返回'auto'的函数”。 我已经看到其他人通过包含相关的头文件来解决此问题，但是我已经做到了，但仍然遇到错误。

我已经包含了头文件； -我尝试将其包含在pch文件和源文件中。

#include "pch.h"

#include <winrt/Windows.Storage.h>
#include <winrt/Windows.Storage.Pickers.h>

#include "360MediaPlayer.h"

using namespace winrt;
using namespace Windows::UI;
using namespace Windows::UI::Composition;
using namespace Windows::UI::Xaml::Hosting;
using namespace Windows::Foundation::Numerics;
using namespace Windows::Storage;
using namespace Windows::Storage::Pickers;
using namespace Windows::Media::Playback;


LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

#define MAX_LOADSTRING 100

// Global Variables:
HWND _hWnd;
HWND _childhWnd;
HINSTANCE _hInstance;

HINSTANCE hInst;                                // current instance
WCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
WCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.

    _hInstance = hInstance;

    // The main window class name.
    const wchar_t szWindowClass[] = L"Win32DesktopApp";
    WNDCLASSEX windowClass = { };

    windowClass.cbSize = sizeof(WNDCLASSEX);
    windowClass.lpfnWndProc = WndProc;
    windowClass.hInstance = hInstance;
    windowClass.lpszClassName = szWindowClass;
    windowClass.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);

    //windowClass.hIconSm = LoadIcon(windowClass.hInstance, IDI_APPLICATION);

    if (RegisterClassEx(&windowClass) == NULL)
    {
        MessageBox(NULL, L"Windows registration failed!", L"Error", NULL);
        return 0;
    }

    _hWnd = CreateWindow(
        szWindowClass,
        L"Windows c++ Win32 Desktop App",
        WS_OVERLAPPEDWINDOW | WS_VISIBLE,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        NULL,
        NULL,
        hInstance,
        NULL
    );
    if (_hWnd == NULL)
    {
        MessageBox(NULL, L"Call to CreateWindow failed!", L"Error", NULL);
        return 0;
    }



/*    // Initialize global strings
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_MY360MEDIAPLAYER, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }
    */

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_MY360MEDIAPLAYER));


    // The call to winrt::init_apartment initializes COM; by default, in a multithreaded apartment.
    winrt::init_apartment(apartment_type::single_threaded);

    // Initialize the Xaml Framework's corewindow for current thread
    WindowsXamlManager winxamlmanager = WindowsXamlManager::InitializeForCurrentThread();

    // This DesktopWindowXamlSource is the object that enables a non-UWP desktop application 
    // to host UWP controls in any UI element that is associated with a window handle (HWND).
    DesktopWindowXamlSource desktopSource;
    // Get handle to corewindow
    auto interop = desktopSource.as<IDesktopWindowXamlSourceNative>();
    // Parent the DesktopWindowXamlSource object to current window
    check_hresult(interop->AttachToWindow(_hWnd));

    // This Hwnd will be the window handler for the Xaml Island: A child window that contains Xaml.  
    HWND hWndXamlIsland = nullptr;
    // Get the new child window's hwnd 
    interop->get_WindowHandle(&hWndXamlIsland);
    // Update the xaml island window size becuase initially is 0,0
    SetWindowPos(hWndXamlIsland, 0, 200, 100, 800, 200, SWP_SHOWWINDOW);

    //Creating the Xaml content
    Windows::UI::Xaml::Controls::StackPanel xamlContainer;
    xamlContainer.Background(Windows::UI::Xaml::Media::SolidColorBrush{ Windows::UI::Colors::LightGray() });

    Windows::UI::Xaml::Controls::TextBlock tb;
    tb.Text(L"Hello World from Xaml Islands!");
    tb.VerticalAlignment(Windows::UI::Xaml::VerticalAlignment::Center);
    tb.HorizontalAlignment(Windows::UI::Xaml::HorizontalAlignment::Center);
    tb.FontSize(48);

    MediaPlayer mpSphere = MediaPlayer();


    FileOpenPicker foPicker = FileOpenPicker();
    StorageFile file(foPicker.PickSingleFileAsync().get());

    mpSphere.SetFileSource(file);
    mpSphere.Play();

//  xamlContainer.Children().Append(tb);
    xamlContainer.UpdateLayout();
    desktopSource.Content(xamlContainer);

    //End XAML Island section

    ShowWindow(_hWnd, nCmdShow);
    UpdateWindow(_hWnd);


    MSG msg;

    // Main message loop:
    while (GetMessage(&msg, nullptr, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}

因此，我收到的完整错误消息如下：

错误C3779'winrt :: impl :: consume_Windows_Foundation_IAsyncOperation <winrt :: Windows :: Foundation :: IAsyncOperation <winrt :: Windows :: Storage :: StorageFile>，TResult> :: get'：返回'auto'的函数不能是在定义之前使用”

尽管存在头文件，如上所示。 如果删除与媒体文件有关的代码，仅保留XAML内容，则它将运行。 有人知道我想念什么吗？ 如果需要，我可以提供完整的项目。

Answer 1

auto关键字简单地要求编译器在编译时使用您要用作类型的类型来推断类型，例如从函数返回的值或像int这样的硬编码POD类型。

代码的问题是您缺少Winrt Foundation的包含，这是您的代码所必需的，因为它访问Async函数，说实话，大多数WinRT代码都需要include作为基础

#include <winrt/Windows.Foundation.h>

using namespace winrt;
using namespace Windows::Foundation;

由于您忘记包括基础include，因此编译器无法推断出引发错误的类型.....

请注意，auto关键字非常适合尾随返回类型形式的函数，但是我强烈建议您出于多种原因不要将它们用于变量，一些值得注意的是带有矢量迭代器的奇怪错误，即使包含在内正确地，在模板函数中，auto关键字将无法推断出类型，并会导致错误，

template <typename T>
auto findSomething(:std::string name)
    -> T*
{
    // the code here causes an error since we are trying to deduce the var inside a 
    // template function using auto
    auto _Found = ::std::find_if(somevector.begin(), somevector.end(), [&](::std::pair<::std::string, ::std::unique_ptr<class Someclass>>& pair){ return (pair.first = name) ? true : false; });

    if(_Found != somevector.end())
    {
        return static_cast<T*>(_Found->second.get());
    }
    return nullptr;
}

已更正

template <typename T>
auto findSomething(:std::string name)
    -> T*
{
    ::std::vector<::std::pair<::std::string, ::std::unique_ptr<class Someclass>>>::iterator _Found = ::std::find_if(somevector.begin(), somevector.end(), [&](::std::pair<::std::string, ::std::unique_ptr<class Someclass>>& pair){ return (pair.first = name) ? true : false; });

    if(_Found != somevector.end())
    {
        return static_cast<T*>(_Found->second.get());
    }
    return nullptr;
}

如果您不想输入::std::vector<::std::pair<::std::string, ::std::unique_ptr<class someclass>>>::iterator每次只需键入别名即可using关键字using myvectortype = ::std::vector<::std::pair<::std::string, ::std::unique_ptr<class someclass>>>这样，您只需要键入myvectortype::iterator ....

希望这有助于您修复代码。