c++ - co_await 不工作

Question

我尝试尝试本文中提到的 co_await https://blogs.msdn.microsoft.com/vcblog/2016/04/04/using-c-coroutines-to-simplify-async-uwp-code/但是有奇怪的编译错误:

C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2825: '_Ret': must be a class or namespace when followed by '::' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2825: '_Ret': must be a class or namespace when followed by '::' (compiling source file MainPage.xaml.cpp) 
MainPage.xaml.cpp(44): note: see reference to class template instantiation 'std::experimental::coroutine_traits<void,::MainPage ^,Windows::UI::Xaml::Navigation::NavigationEventArgs ^>' being compiled
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2510: '_Ret': left of '::' must be a class/struct/union (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2510: '_Ret': left of '::' must be a class/struct/union (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2061: syntax error: identifier 'promise_type' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44): error C2061: syntax error: identifier 'promise_type' (compiling source file MainPage.xaml.cpp)
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\experimental\resumable(44,0): Error C2238: unexpected token(s) preceding ';' (compiling source file MainPage.xaml.cpp)

本来，我把下面的代码放在构造函数中

#include <experimental\resumable>
#include <pplawait.h>

using namespace concurrency;

MainPage::MainPage()
{
    InitializeComponent();

    auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");

    // Preparing app data structures
}

这是行不通的。我的猜测是这在构造函数中是不可能的(原因很明显)所以我将 co_await 行移动到

void MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
    auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
}

导致上述编译错误。

Answer 1

我的猜测是，第一个问题是您无法返回 void自 void 以来的可恢复函数不填充 co_await 的任何协程特征期待(如 get_return_object ， set_result 等)

如果您已经在使用 PPL，请返回 task<void> :

task<void> MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
    auto my_data_file = co_await Windows::ApplicationModel::Package::Current->InstalledLocation->GetFileAsync("samples.txt");
}