Question

I use the following code:

struct WorkData
{
    std::string name;
    std::function<void(std::string)> Callback;

    WorkData(){};
    WorkData(const WorkData& other)
    {
        name = other.name;
        Callback = std::ref(other.Callback);
    }
};

WorkData data; // this is the data to pass to queue_task() function bellow
data.Callback = std::bind(&ResultProcessor::Handler, resProc, std::placeholders::_1);

template <typename Functor>
void queue_task(Functor& fn, WorkData& workData )
{
    group.run([&fn, workData](){
          workData.Callback("resultComming"); // runtime ERROR- access violation
    });
}

queue_task function queues work to be done asynchronously on another thread (by calling group.run(lambda) from above). The problem that I’m experiencing is that I get an access violation when trying to call workData.Callback().

The reason I make a copy of workData inside group.run() is because I want to capture workData by value so that when the group.run() lambda executes it has a copy of the state when queue_task() was called. I would expect that workData.Callback() would execute on the instance of object passed on line:

data.Callback = std::bind(&ResultProcessor::Handler, resProc, std::placeholders::_1);

EDIT: resProc from above is alive (not destroyed) when the crash line is called

Answer 1

The use of std::ref in your copy constructor means that you keep a reference to the old WorkData‘s Callback member, not a copy. You want Callback = other.Callback to make a copy to avoid an access violation (probably from accessing the old callback after it has been freed). In order to keep a reference to resProc in the std::function, you need to use std::ref(resProc) in the call to std::bind.