Slot never called

[Helson]

Hello everyone.
I'm trying to make this code works
http://www.bogotobogo.com/Qt/Qt5_Downloading_Files_QNetworkAccessManager_QNetworkRequest.php

I'm put a qDebug() << "running"; inside the slot to test.
But the slot "**replyFinished **" is never called.

The only difference is the main function that i'm not using.

The below code is inside another method, and works.

Downloader d;
d.doDownload();

What is wrong?!

Thanks in advance!

[Chris Kawa]

The only difference is the main function that i'm not using.

And that's the problem.
Since you put the downloader in some method it goes out of scope and is destroyed, ie.

void someFunction() {
   Downloader d;       //d is created here
   d.doDownload();
}                      //d is destroyed here

The difference is that in the example downloader is created in the main() and right after doDownload() is called the appplication enters an event loop: return a.exec();, which blocks until the download is finished, so the downloader does not exit the scope.

To fix this either put Downloader instance in a scope that is wide enough or create it on the heap and delete after the download is finished.

[Helson]

@Chris-Kawa

Great explanation!

But if I want use within this function?
Create it on the heap is the solution?
How can I do this?

Thanks.

[Chris Kawa]

How can you create an instance on the heap? Just as with any other c++ class: with the new keyword.

    Downloader* d = new Downloader();
    d->doDownload();

but that is leaking memory, so you need to make sure the downloader is deleted when it finishes. You could, for example, emit a signal when it's done:

class Downloader : public QObject
{
   ...
signals:
   void finished() const;
   ...

Emit it in the replyFinished():

void Downloader::replyFinished (QNetworkReply *reply)
{
   ...
    reply->deleteLater();
    emit finished(); 
}

and then you can use it like this:

void someFunction()
{
    Downloader* d = new Downloader();
    d->doDownload();
    connect(d, &Downloader::finished, d, &Downloader::deleteLater);
}

[Helson]

@Chris-Kawa
Yes! Now it is being called!

Now i'm getting a compile error:

C2248: 'Downloader::finished' : cannot access protected member declared in class 'Downloader'

signals:
void finished() const;

[A Former User]

Hi,
please post the full source code.

[Chris Kawa]

@Helson said:

C2248: 'Downloader::finished' : cannot access protected member declared in class 'Downloader'

Looks like you misplaced something. finished() should be a public method (a signal). Posting your code is a good idea as @Wieland suggested.

[Helson]

Here is the code:

http://pastebin.com/H9dVD3QZ

[mrjj]

Hi
note:
its Qt4

[kshegunov]

@Chris-Kawa

Looks like you misplaced something. finished() should be a public method (a signal).

Chris, in Qt4 signals used to be protected, so it might be simply that he's using a bit older version than we assumed. ;)

[Chris Kawa]

awww, ok. Good catch guys.

@Helson so just replace

connect(d, &Downloader::finished, d, &Downloader::deleteLater);

with

connect(d, SIGNAL(finished()), d, SLOT(deleteLater()));

and think about upgrading to Qt5. Especially since you're following tutorials there's no point in learning obsolete stuff.

[mrjj]

@Chris-Kawa said:
connect(d, SIGNAL(finished()), d, SLOT(deleteLater()));

we did. still said it was protected ?!
so we moved to ctor in Download class

[Chris Kawa]

@mrjj

so we moved to ctor in Download class

Don't do that. It makes your class dependent on being created on the heap, which you can't guarantee. If someone creates it on the stack you will crash on double delete.

[mrjj]

@Chris-Kawa
Ah yes I see. Luckily we did not do it, ;)
we moved to dodownload member function and
used old syntax.

Im not used to Qt4 so the whole protected signals were
confusing.

[kshegunov]

Im not used to Qt4 so the whole protected signals were
confusing.

If you think about it, semantically it's more correct that signals are protected. A signal is raised from the object itself to notify others and in principle shouldn't be available for others to emit it! My suspicion is that they're now public because of the new way of connecting, which by my opinion is not such an improvement over the old one.

PS.
Is it me, or the forum is going nuts? I have the distinct feeling that I see Helson's posts as mrjj's?

[mrjj]

hi, I talked with him in chat also, so I did post here.
Forums crash every 2 minutes for me though.

[kshegunov]

@mrjj
Yes, but who are you? Are you indeed mrjj, or there's some bug in the forum and you're in fact Helson? :)

[mrjj]

@kshegunov
Heh. Im mrjj
(i hope)

[kshegunov]

@mrjj
Ok, because I would have expected this:

Ah yes I see. Luckily we did not do it, ;)
we moved to dodownload member function and
used old syntax.

to be written by Helson. So it's my mistake, sorry for the misunderstanding. :)

[mrjj]

@kshegunov
well You should have the credits as you solved it :)
and sorry for writing a bit unclear :)

[kshegunov]

@mrjj
Thanks for the credits, although I'm not sure how much I care about them, still I appreciate the sentiment. :)