Error while connecting lambda function to QProcess::error

Question

In following code I want to connect lambda function to QProcess::error signal:

void Updater::start() {
    QProcess process;
    QObject::connect(&process, &QProcess::error, [=] (QProcess::ProcessError error) {
        qWarning() << "error " << error;
    });
    process.start("MyProgram");
    process.waitForFinished();
}

But I get strange error:

error: no matching function for call to 'Updater::connect(QProcess* [unresolved overloaded function type], Updater::start()::)' });

What I do wrong here? The code executes inside method of class derived from QObject. The project configured to work with c++11.

I use Qt 5.3.1 on Linux x32 with gcc 4.9.2

Possible duplicate of [Connecting overloaded signals and slots in Qt 5](http://stackoverflow.com/questions/16794695/connecting-overloaded-signals-and-slots-in-qt-5) — Toby Speight, May 18 '16 at 15:13

Jablonski · Accepted Answer · 2015-07-10T09:24:23.690

8

Problem is that the QProcess has another error() method, so compiler just doesn't know which method use. If you want to deal with overloaded methods, you should use next:

QProcess process;
connect(&process, static_cast<void (QProcess::*)(QProcess::ProcessError)>
(&QProcess::error), [=](QProcess::ProcessError pError) {
    qWarning() << "error " << pError;
});
process.start("MyProgram");
process.waitForFinished();

Yes, it looks ugly, but there is no another way (only old syntax?).

This special line tells compiler that you want to use void QProcess::error(QProcess::ProcessError error), so now there is no any ambiguity

More information you can find here.

edited Jul 10 '15 at 09:24

answered Jul 10 '15 at 04:41

Jablonski

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Thanks, @Chernobyl,that helped me! – folibis Jul 10 '15 at 10:06
1

see also http://stackoverflow.com/a/16795664/846250 for a small snippet to make the overload declaration "automatic" – To마SE Sep 10 '15 at 10:43
Finding that `static_cast` requirement was a tricky one, thanks. – Neurotransmitter Sep 01 '16 at 14:49
Just a side-note: same applies to the `finished` signal: `static_cast(&QProcess::finished)`. Gotta love C++ spaghetti syntax. LOL – rbaleksandar Jul 24 '18 at 08:02

kefir500 · Answer 2 · 2021-09-22T10:12:06.130

1

For those who are using Qt 5.6 or later, the QProcess::error signal is deprecated. You can use the QProcess::errorOccurred signal instead to avoid the naming ambiguity and complicated casting.

QProcess process;
connect(&process, &QProcess::errorOccurred, [=](QProcess::ProcessError error) {
    qWarning() << error;
});
process.start("MyProgram");
process.waitForFinished();

edited Sep 22 '21 at 10:12

answered Sep 22 '21 at 07:43

kefir500

4,184
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Error while connecting lambda function to QProcess::error

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