Qt Slots and Signals. Get Slot Receiver Object

Question

I have a QLineEdit which I Want to connect to a QLabel so that depending on the validity of the text entered. I have two problems while doing this.

QLineEdit *text = new QLineEdit(this);
layout->addWidget(text, rowno, 0);
QLabel *button = new QLabel(this);
button->setStyleSheet("QLabel { background-color : green; color : white; }");
button->setAlignment(Qt::AlignCenter);
button->setText("OKAY");
QObject::connect(text, SIGNAL(textEdited(const QString &)), button, SLOT(CheckValidity(const QString &)));

this does not connect any changes made in QLineEdit to my custom slot. I cannot figure out why! Also in the custom slot, I want to change the background colour of my label depending on the QString passed. How do I get a reference for the label? It is present as the receiver of the signal but I can't figure out a way to refer to it.

Mattia F. · Accepted Answer · 2016-07-04T14:42:13.377

1

CheckValidity is not a QButton's slot, it's a custom slot defined in your own class (I assume that, since you have not specified it).

So, change the last line to:

QObject::connect(text, SIGNAL(textEdited(const QString &)), this, SLOT(CheckValidity(const QString &)));

If you want to know the sender object, use qobject_cast:

QLabel *sender_label = qobject_cast<QLabel*> (sender ());

edited Jul 04 '16 at 14:42

answered Jul 04 '16 at 14:18

Mattia F.

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Can I pass the label as an argument in the custom slot? I believe QT signals and slots need to have the same number of arguments, but i need to connect a particular label with a particular textbox and they are created dynamically by the user at runtime, hence I cannot keep a reference to it in my class. – Vasundhara Mehta Jul 04 '16 at 14:24

score 0 · Answer 2 · answered Jul 04 '16 at 14:36

0

There is no CheckValidity slot in QLabel (why button is QLabel?). Check output window of debugger after connection trying.
QObject::sender() + cast. Cast may be dynamic_cast or qobject_cast, look their difference in Qt Assistant.

answered Jul 04 '16 at 14:36

ilotXXI

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score 0 · Answer 3 · answered Jul 04 '16 at 15:35

0

If you want to supply additional arguments into your slot invocation, you can use lambda instead of a slot:

QObject::connect(text, &QLineEdit::textEdited, [=](const QString &text) { checkValidity(button, text); });

answered Jul 04 '16 at 15:35

majk

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Qt Slots and Signals. Get Slot Receiver Object

3 Answers3