# std::promise<R>::set_exception

From cppreference.com
< cpp‎ | thread‎ | promise

void set_exception( std::exception_ptr p ); |  | (since C++11)

Atomically stores the exception pointer `p` into the shared state and makes the state ready.

The operation behaves as though set_value, `set_exception`, set_value_at_thread_exit, and set_exception_at_thread_exit acquire a single mutex associated with the promise object while updating the promise object.

An exception is thrown if there is no shared state or the shared state already stores a value or exception.

Calls to this function do not introduce data races with calls to get_future (therefore they need not synchronize with each other).

### Parameters

p | - | exception pointer to store. The behavior is undefined if `p` is null.

### Return value

(none)

### Exceptions

std::future_error on the following conditions:

- *this has no shared state. The error code is set to `no_state`.

- The shared state already stores a value or exception. The error code is set to `promise_already_satisfied`.

### Example

Run this code

    #include <thread>
    #include <iostream>
    #include <future>

    int main()
    {
        std::promise<int> p;
        std::future<int> f = p.get_future();

        std::thread t([&p]{
            try {
                // code that may throw
                throw std::runtime_error("Example");
            } catch(...) {
                try {
                    // store anything thrown in the promise
                    p.set_exception(std::current_exception());
                    // or throw a custom exception instead
                    // p.set_exception(std::make_exception_ptr(MyException("mine")));
                } catch(...) {} // set_exception() may throw too
            }
        });

        try {
            std::cout << f.get();
        } catch(const std::exception& e) {
            std::cout << "Exception from the thread: " << e.what() << '\n';
        }
        t.join();
    }

Output:

    Exception from the thread: Example

### See also

set_exception_at_thread_exit | sets the result to indicate an exception while delivering the notification only at thread exit (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/thread/promise/set_exception&oldid=132952"