C++ Futures and Promises std::future_error and std::future_errc
Example
If constraints for std::promise and std::future are not met an exception of type std::future_error is thrown.
The error code member in the exception is of type std::future_errc and values are as below, along with some test cases:
enum class future_errc {
broken_promise = /* the task is no longer shared */,
future_already_retrieved = /* the answer was already retrieved */,
promise_already_satisfied = /* the answer was stored already */,
no_state = /* access to a promise in non-shared state */
};
Inactive promise:
int test()
{
std::promise<int> pr;
return 0; // returns ok
}
Active promise, unused:
int test()
{
std::promise<int> pr;
auto fut = pr.get_future(); //blocks indefinitely!
return 0;
}
Double retrieval:
int test()
{
std::promise<int> pr;
auto fut1 = pr.get_future();
try{
auto fut2 = pr.get_future(); // second attempt to get future
return 0;
}
catch(const std::future_error& e)
{
cout << e.what() << endl; // Error: "The future has already been retrieved from the promise or packaged_task."
return -1;
}
return fut2.get();
}
Setting std::promise value twice:
int test()
{
std::promise<int> pr;
auto fut = pr.get_future();
try{
std::promise<int> pr2(std::move(pr));
pr2.set_value(10);
pr2.set_value(10); // second attempt to set promise throws exception
}
catch(const std::future_error& e)
{
cout << e.what() << endl; // Error: "The state of the promise has already been set."
return -1;
}
return fut.get();
}
