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 — Toby Speight
– Toby Speight, Commented May 18, 2016 at 15:13

Jablonski · Accepted Answer · 2015-07-10 09:24:23Z

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, 2015 at 9:24

answered Jul 10, 2015 at 4:41

Jablonski

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3 Comments

To마SE Over a year ago

see also stackoverflow.com/a/16795664/846250 for a small snippet to make the overload declaration "automatic"

Neurotransmitter Over a year ago

Finding that static_cast requirement was a tricky one, thanks.

rbaleksandar Over a year ago

Just a side-note: same applies to the finished signal: static_cast<void (QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished). Gotta love C++ spaghetti syntax. LOL

kefir500 · Accepted Answer · 2021-09-22 10:12:06Z

2

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, 2021 at 10:12

answered Sep 22, 2021 at 7:43

kefir500

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