I've been programming for a long time, but very rarely with anything asynchronous (and not often with anything to do with multithreading, either).

Mostly for the fun of it, I'm writing a program to download song lyrics, and save them to music files (via e.g. ID3v2 tags). I have most of that in place by now: I have two classes that handles downloading (one per site the program supports), plus the code necessary to write the tags to files.
The design I wanted from the beginning was fairly simple and synchronous; I had planned to create a synchronous/blocking API to download lyrics, and start such tasks in background threads to keep the UI alive.

The idea was that the class that wants to fetch lyrics would create a LyricFetcher instance, and call string LyricFetcher::fetchLyrics(string artist, string title). That, in turn, tries fetching the lyrics from each site in order, using something like

for (Site *site : sites) {
    lyrics = site->fetchLyrics(artist, title);
    if (successfully downloaded lyrics)
        return lyrics;
return (didn't find anything);

The problems appeared when I realized that the only APIs I could use to fetch data from the internet were asynchronous; you'd set up a request, and tell it (via signals and slots) to tell you when it's done.
With that, I couldn't simply return the lyrics as a string.
As a workaround, I designed the API to use a callback, which the site-specific code calls when it has either finished the download, or failed, i.e. something like void LyricFetcher::fetchLyrics(string artist, string title, function<void(string)> callback).
The callback was then forwarded to the site-specific classes, which have the same type signature as the fetchLyrics above.

That worked fine for a single site, but I ran in to new problems trying to implement the multi-site pseudocode above.
I can't use the pseudocode, since site->fetchLyrics would return immediately.
The only "solution" I came up with was to use nested callbacks, which is horrible.

void LyricFetcher::fetchLyrics(string artist, string title, function<void(string)> callback) {
    site1->fetchLyrics(artist, title, [=](string lyrics) {
        if (did match)
        else {
            site2->fetchLyrics(artist, title, [=](string lyrics) {

Not only is this ugly, but it also means you can't iterate through the sites in a simple loop. (I'm not even sure it'd work at all.)

What's a good way to design around issues like these?
The "user facing" API of LyricFetcher::fetchLyrics should preferably either be synchronous (with the lyrics as a simple return value), or use signals/slots to signal completion (and transfer results).

FWIW, this is in C++ with Qt (C++14, Qt 5.6), but I tried to stick with a C++y pseudocode in the post.

  • have a look at stackoverflow.com/questions/19225372/…
    – Caleth
    Commented Jun 1, 2016 at 8:42
  • @Caleth Thanks, but I'm not sure that applies to this problem. The problem here isn't about starting many tasks and waiting for them, but about running callback/signal-based in a sequential manner. I'm not even sure how to use futures with them (if that's applicable).
    – exscape
    Commented Jun 1, 2016 at 15:48
  • Use promises to emulate synchronous flow in asynchronous context.
    – Basilevs
    Commented Jun 1, 2016 at 16:59
  • 1
    what @Basilevs said: futures/promises/async exist to make asynchronous code look like synchronous code
    – Caleth
    Commented Jun 1, 2016 at 17:04
  • If however you want to ask each site in sequence then you don't need any asynchronicity in processing beyond waiting the thread you start on while the fetch is occuring
    – Caleth
    Commented Jun 1, 2016 at 17:07

1 Answer 1


The problem here isn't about starting many tasks and waiting for them, but about running callback/signal-based in a sequential manner.

Use a concurrent function which takes the function which creates the ordered structure and the function which iterates through the ordered structure as arguments. For example, in the concurrent namespace, Qt has a mapReduce method with the following signature

QtConcurrent::mappedReduced(const Sequence &sequence, MapFunction mapFunction, ReduceFunction reduceFunction, QtConcurrent::ReduceOptions reduceOptions = UnorderedReduce | SequentialReduce)

Which can be used as follows:

void addToCollage(QImage &collage, const QImage &thumbnail)
   QPainter p(&collage);
   static QPoint offset = QPoint(0, 0);
   p.drawImage(offset, thumbnail);
   offset += ...;

QImage scaled(const QImage &image)
  return image.scaled(100, 100);

QList<QImage> images = ...;
QFuture<QImage> thumbnails = QtConcurrent::mapped(images, scaled);

QList<QImage> images = ...;
QFuture<QImage> collage = QtConcurrent::mappedReduced(images, scaled, addToCollage);

The reduce function will be called once for each result returned by the map function, and should merge the intermediate into the result variable. QtConcurrent::mappedReduced() guarantees that only one thread will call reduce at a time, so using a mutex to lock the result variable is not necessary. The QtConcurrent::ReduceOptions enum provides a way to control the order in which the reduction is done. If QtConcurrent::UnorderedReduce is used (the default), the order is undefined, while QtConcurrent::OrderedReduce ensures that the reduction is done in the order of the original sequence.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.