Consider following code
void printPromised(std::future<int> f)
{
std::cout << f.get() << std::endl;
}
int main()
{
printPromised(std::async(std::launch::async, [](){ return 8; })); // This works
auto f = std::async(std::launch::async, [](){ return 8; });
printPromised(f); // This won't work
}
It says "It is a deleted function". Why is that? Further I need to pass (share) the same promised result which std::async
generated to multiple users. That means when someone calls the get() function, I need to pass the same result (I don't need to re generate the result using std::async
if it is already generated) and also I need the blocking mechanism which std::future::get
has.
There can be only one future. You cannot have multiple copies of the same future. So you need to transfer ownership of the future to the function:
printPromised(std::move(f));
// ^^^^^^^^^^^^
If you genuinely need shared access to a future, you can construct a shared_future
from an ordinary future by calling the share()
member function; this behaves similarly to a shared pointer:
auto sf = std::async(std::launch::async, [](){ return 8; }).share();
Now sf
can be copied, and all copies wait for the same result, i.e. the wait()
calls on all copies may block and will synchronize with the becoming-ready of the result.