How to check if a std::async task is finished?

Question

In my graphics application I want to generate a batch meshes in another thread. Therefore I asynchrony call the member function using std::async.

task = async(launch::async, &Class::Meshing, this, Data(...)); 

In my update loop I try to check if the thread is ready. If yes, I will send the mesh to the video card and start the next thread. If not, I will skip these operations.

#include <future> using namespace std;  class Class { public:     void Update()     {         if(task.finished()) // this method does not exist         {             Data data = task.get();             // ...             task = async(launch::async, &Class::Meshing, this, Data(/* ... */));         }     }  private:     struct Data     {         // ...     };     future<Data> task;     Data Meshing(Data data)     {         // ...     } }; 

How can I check if the asynchrony thread finished without stucking in the update function?

Answer 1

Use future::wait_for(). You can specify a timeout, and after that, get a status code.

Example:

task.wait_for(std::chrono::seconds(1)); 

This will return future_status::ready, future_status::deferred or future_status::timeout, so you know the operation's status. You can also specify a timeout of 0 to have the check return immediately as soon as possible.