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# std::shared_future::wait

From cppreference.com
< cpp‎ | thread‎ | shared future

[edit template]

void wait() const; |  | (since C++11)

Blocks until the result becomes available. valid() == true after the call.

The behavior is undefined if valid()== false before the call to this function.

### [edit] Parameters

(none)

### [edit] Return value

(none)

### [edit] Exceptions

(none)

### [edit] Notes

The implementations are encouraged to detect the case when valid == false before the call and throw a std::future_error with an error condition of std::future_errc::no_state.

Calling wait on the same `std::shared_future` from multiple threads is not safe; the intended use is for each thread that waits on the same shared state to have a copy of a `std::shared_future`.

### [edit] Example

Run this code

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

    int fib(int n)
    {
      if (n < 3) return 1;
      else return fib(n-1) + fib(n-2);
    }

    int main()
    {
        std::shared_future<int> f1 = std::async(std::launch::async, [](){
            return fib(20);
        });
        std::shared_future<int> f2 = std::async(std::launch::async, [](){
            return fib(25);
        });

        std::cout << "waiting...\n";
        f1.wait();
        f2.wait();

        std::cout << "f1: " << f1.get() << '\n';
        std::cout << "f2: " << f2.get() << '\n';
    }

Output:

    waiting...
    f1: 6765
    f2: 75025

### [edit] See also

wait_for | waits for the result, returns if it is not available for the specified timeout duration (public member function) [edit]
wait_until | waits for the result, returns if it is not available until specified time point has been reached (public member function) [edit]

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