This is literally spelled out for you in the docs. you just need to copy-paste that code into your code. I can't think of a better way the docs could make this clear

the new syntax needs the address of a class member:
connect(comboBox, &QComboBox::activated ,this, [](QString text){
cout << text.toUtf8().constData() << endl;
});

won't compile, since there are more than one activated() methods with different signatures. Thus the compiler won't know which one to link to.

@SGaist said in Adding Lambda Expression as Slot:

qOverload requires C++14.

Just a note:
the function qOverload<>() requires C++14
the helper class QOverload<>::of() also works since C++11

Because you are trying to use an overload that doesn't exists:

connect(comboBox, qOverload<QString>(&QComboBox::activated), this, [](const QString &text){
        cout << text.toUtf8().constData() << endl;
    });

qOverload requires C++14.

You don't even need the this parameter.

For more information, see here

connect(comboBox, &QComboBox::activated ,this,  [](QString text){
        cout << text.toUtf8().constData() << endl;
    });

But I always get as result : No matching member function for call to 'connect'

connect(comboBox, QOverload<QString>::of(&QComboBox::activated) , this,  [](const QString & text) {
        cout << text.toUtf8().constData() << endl;
    });

connect(comboBox, SIGNAL(activated(QString)) ,[](QString text){
        cout << text.toUtf8().constData() << endl;
    });