C / C++ / MFC

// async-unwrapping.cpp
// compile with: /EHsc
#include <ppltasks.h>
#include <iostream>

using namespace concurrency;
using namespace std;

int wmain()
{
    auto t = create_task([]()
    {
        wcout << L"Task A" << endl;

        // Create an inner task that runs before any continuation
        // of the outer task.
        return create_task([]()
        {
            wcout << L"Task B" << endl;
        });
    });
  
    // Run and wait for a continuation of the outer task.
    t.then([](task<void>& t3) // or task<void> t3
    {
        wcout << L"Task C" << endl;
        t3.wait(); // or .get()
    }).wait();
}

task<wstring> my_task()
{
    auto s = make_shared<wstring>(L"First value");
    return create_task([s] {
        wcout << *s << endl;
        *s = L"Second value";
        return *s; // It fails because of this.
    }).then([s] {
        wcout << *s << endl;
        *s = L"Third value";
        return *s;
    });
}

Quote:

1>c:\program files (x86)\microsoft visual studio 14.0\vc\include\ppltasks.h(372): error C2338: incorrect parameter type for the callable object in 'then'; consider _ExpectedParameterType or task<_ExpectedParameterType> (see below)
1> c:\program files (x86)\microsoft visual studio 14.0\vc\include\ppltasks.h(402): note: see reference to class template instantiation 'Concurrency::details::_FunctionTypeTraits<_Function,_ReturnType>' being compiled
1> with
1> [
1> _Function=my_task::<lambda_fe524192daf67b16ea75ab49971b7103>,
1> _ReturnType=std::wstring
1> ]
1> c:\vc++\concurrency_program\createtask\source.cpp(41): note: see reference to class template instantiation 'Concurrency::details::_ContinuationTypeTraits<my_task::<lambda_fe524192daf67b16ea75ab49971b7103>,_ReturnType>' being compiled
1> with
1> [
1> _ReturnType=std::wstring
1> ]
1>c:\vc++\concurrency_program\createtask\source.cpp(41): error C2440: 'return': cannot convert from 'Concurrency::task<concurrency::details::_badcontinuationparamtype>' to 'Concurrency::task<std::wstring>'
1> c:\vc++\concurrency_program\createtask\source.cpp(41): note: Constructor for class 'Concurrency::task<std::wstring>' is declared 'explicit'

task<wstring> t1 = create_task([]() {
    wprintf(L"create_task\n");
    return wstring(L"create_task"); // It doesn't throw any error
});