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

Answer 1

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.

Answer 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();