CODEWITHSUNDEEP
c++multithreadingc++11promisefuture
Andrey Rubliov
Andrey Rubliov

Reputation: 1597

std::future_error: No associated state

I have a problem with C++ 11 future/promise. The following code works fine:

string s = "not written";

void write(promise<void>&& writePromise)
{
    cout << "\nwrite()" << endl;
    this_thread::sleep_for(chrono::seconds(1));
    s = "written";
    writePromise.set_value();
}

void read(future<void>&& readFut)
{
    cout << "read(), waiting..." << flush;
    readFut.wait();
    cout << "\ns: '" << s << "'" << endl;
}

int main()
{
    promise<void> writePromise;
    future<void> writeFuture = writePromise.get_future();

    thread tWrite(write, move(writePromise));
    thread tRead(read, move(writeFuture));
    
    tWrite.join();
    tRead.join();
}

But once I change main() to this:

int main()
{
    promise<void> writePromise;

    thread tWrite(write, move(writePromise));
    thread tRead(read, move(writePromise.get_future()));
    
    tWrite.join();
    tRead.join();
}

I get the error: terminate called after throwing an instance of 'std::future_error'
what(): std::future_error: No associated state
Aborted (core dumped)

This exception is supposed to be thrown if you call get_future() twice on the same promise.

But all I did was just passing writePromise.get_future() to a function, so I don;t see how I call it twice.

Can someone please help?

Upvotes: 10

Views: 13619

Answers (2)

juanba
juanba

Reputation: 51

My first oops is related with your move semantics approach

Once you writes move(writePromise) the promise access should be avoided because the local var has been "transferred" to the thread according my understanding

My main lesson with the std::move could be summarized as

since now i pass my enclosed var ownership to the called symbol

So, any posterior reference it's usually marked as invalid

IHTH

Upvotes: 0

ALX23z
ALX23z

Reputation: 4703

You moved writePromise before generating future from it. This is because the thread takes ownership over the promise by value.

To fix it - first obtain future and only then forward the promise.

Upvotes: 5

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