Ever since C++20 introduced coroutine support, I was wondering how this could integrate with Qt. Apparently I wasn’t the only one: before long, QCoro popped up. A really cool library! But it doesn’t use the existing future and promise types in Qt; instead it introduces its own types and mechanisms to support coroutines. I kept wondering why no-one just made QFuture and QPromise compatible – it would certainly be a more lightweight wrapper then.

With a recent project at work being gargantuan mess of QFuture::then() continuations (ever tried async looping constructs with continuations only? 🥴) I had enough of a reason to finally sit down and implement this myself. The result: https://gitlab.com/pumphaus/qawaitablefuture.

Example

#include <qawaitablefuture/qawaitablefuture.h>

QFuture<QByteArray> fetchUrl(const QUrl &url)
{
    QNetworkAccessManager nam;
    QNetworkRequest request(url);

    QNetworkReply *reply = nam.get(request);

    co_await QtFuture::connect(reply, &QNetworkReply::finished);
    reply->deleteLater();

    if (reply->error()) {
        throw std::runtime_error(reply->errorString().toStdString());
    }
    co_return reply->readAll();
}

It looks a lot like what you’d write with QCoro, but it all fits in a single header and uses native QFuture features to – for example – connect to a signal. It’s really just syntax sugar around QFuture::then(). Well, that, and a bit of effort to propagate cancellation and exceptions. Cancellation propagation works both ways: if you co_await a canceled QFuture, the “outer” QFuture of coroutine will be canceled as well. If you cancelChain() a suspended coroutine-backed QFuture, cancellation will be propagated into the currently awaited QFuture.

What’s especially neat: You can configure where your coroutine will be resumed with co_await continueOn(...). It supports the same arguments as QFuture::then(), so for example:

QFuture<void> SomeClass::someMember()
{
    co_await QAwaitableFuture::continueOn(this);
    co_await someLongRunningProcess();
    // Due to continueOn(this), if "this" is destroyed during someLongRunningProcess(),
    // the coroutine will be destroyed after the suspension point (-> outer QFuture will be canceled)
    // and you won't access a dangling reference here.
    co_return this->frobnicate();
}

QFuture<int> multithreadedProcess()
{
    co_await QAwaitableFuture::continueOn(QtFuture::Launch::Async);

    double result1 = co_await foo();
    // resumes on a free thread in the thread pool
    process(result1);

    double result2 = co_await bar(result1);
    // resumes on a free thread in the thread pool
    double result3 = transmogrify(result2);

    co_return co_await baz(result3);
}

See the docs for QFuture::then() for details.

Also, if you want to check the canceled flag or report progress, you can access the actual QPromise that’s backing the coroutine:

QFuture<int> heavyComputation()
{
    QPromise<int> &promise = co_await QAwaitableFuture::promise();
    promise.setProgressRange(0, 100);

    double result = 0;

    for (int i = 0; i < 100; ++i) {
        promise.setProgressValue(i);
        if (promise.isCanceled()) {
            co_return result;
        }
        frobnicationStep(&result, i);
    }
    co_return result;
} 

Outlook

I’m looking to upstream this. It’s too late for Qt 6.11 (already in feature freeze), but maybe 6.12? There have been some proposals for coroutine support on Qt’s Gerrit already, but none made it past the proof-of-concept stage. Hopefully this one will make it. Let’s see.

Otherwise, just use the single header from the qawaitablefuture repo. It an be included as a git submodule, or you just vendor the header as-is.

Happy hacking!

Caveat: GCC < 13

There was a nasty bug in GCC’s coroutine support: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101367 It affects all GCC versions before 13.0.0 and effectively prevents you from writing co_await foo([&] { ... }); – i.e. you cannot await an expression involving a temporary lambda. You can rewrite this out as auto f = foo([&] { ... }); co_await f; and it will work. But there’s no warning at compile time. As soon as the lambda with captures is a temporary expression inside the co_await, it will crash and burn at runtime. Fixed with GCC13+, but took me a while to figure out why things went haywire on Ubuntu 22.04 (defaults to GCC11).