How Do You Connect One Signal to Multiple Slots in Qt?

If you are working with Qt, you may find that you need to connect one signal to multiple slots. This can be a little tricky, but fortunately, there are several ways to accomplish this.

First, let’s define what we mean by “signal” and “slot.” In Qt, a signal is an event that is emitted when some action occurs.

For example, if you have a button in your UI and the user clicks on it, the button emits a signal indicating that it has been clicked. A slot is a function that is called when a signal is emitted. So if you have connected the button’s clicked() signal to a slot function, that function will be called whenever the button is clicked.

Now let’s look at some ways to connect one signal to multiple slots.

Method 1: Using Multiple Connections

The simplest way to connect one signal to multiple slots is to simply connect the same signal to each slot separately. For example:

“`cpp
connect(myButton, &QPushButton::clicked, myObject1, &MyClass::mySlot);
connect(myButton, &QPushButton::clicked, myObject2, &MyClass::myOtherSlot);
“`

This code connects the clicked() signal of `myButton` to two different slots: `mySlot()` in `myObject1` and `myOtherSlot()` in `myObject2`.

Method 2: Using QSignalMapper

Another way to connect one signal to multiple slots is by using a QSignalMapper object. A QSignalMapper maps an integer value (or QString) to each slot function. When the connected signal is emitted, the mapper sends the mapped value(s) as arguments to each slot.

Here’s an example:

“`cpp
QSignalMapper *mapper = new QSignalMapper(this);

connect(myButton1, &QPushButton::clicked, mapper,
static_cast(&QSignalMapper::map));
connect(myButton2, &QPushButton::clicked, mapper,
static_cast(&QSignalMapper::map));

mapper->setMapping(myButton1, 1);
mapper->setMapping(myButton2, 2);

connect(mapper, static_cast(&QSignalMapper::mapped),
myObject, &MyClass::mySlot);
connect(mapper, static_cast(&QSignalMapper::mapped),
myObject, &MyClass::myOtherSlot);
“`

In this code, we create a QSignalMapper object `mapper` and connect the clicked() signals of two buttons (`myButton1` and `myButton2`) to it. We then set up the mapper to map the integer values 1 and 2 to the buttons.

Finally, we connect the mapped() signal of the mapper to both `mySlot()` and `myOtherSlot()` in `myObject`.

Method 3: Using Qt5’s New Syntax

If you are using Qt5 or later, there is a new syntax for connecting signals to slots that makes it easy to connect one signal to multiple slots:

“`cpp
connect(myButton, &QPushButton::clicked,
[this]() { mySlot(); myOtherSlot(); });
“`

This code connects the clicked() signal of `myButton` to a lambda function that calls both `mySlot()` and `myOtherSlot()`.

Conclusion

Connecting one signal to multiple slots in Qt may seem daunting at first glance, but as we have seen there are several ways to accomplish it. Whether you use multiple connections, a QSignalMapper object or Qt5’s new syntax is up to you – choose whichever method works best for your needs.