Visual C++ 2015 – Resumable Functions

Visual C++ 2015 includes a general purpose solution to implement resumable functions based on the concept of coroutines. A coroutine is a generalized routine entity which supports operations like suspend and resume in addition to the traditional invoke and return operations.
These resumable functions are being proposed for inclusion in ISO C++17.
For the VC++ 2015 Preview, the feature only works for 64-bit targets, and requires adding the /await switch to your compiler command-line.
Such resumable functions have several use cases:

  • Asynchronous operations
  • Generator pattern
  • Reactive Streams

Here is a simple example demonstrating an asynchronous operation:

#include <future>
#include <thread>
#include <experimental\resumable>

using namespace std; 
using namespace std::chrono; 
// this could be some long running computation or I/O
future<int> calculate_the_answer() 
    return async([] { 
        this_thread::sleep_for(1s); return 42; 
// Here is a resumable function
future<void> coro() { 
    printf("Started waiting... \n"); 
    auto result = __await calculate_the_answer(); 
    printf("got %d. \n", result); 
int _tmain(int argc, _TCHAR* argv[]) 

The important line here is line 17. The function calculate_the_answer() is an asynchronous function which immediately returns by returning a future. Thanks to the __await keyword on line 17, the rest of the coro() function can be implemented as if you are simply programming synchronously! No need anymore to mess around with task continuations or what not. This makes asynchronous programming much easier 😀

Read the full explanation here.


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