Qt documentation: Singleshot Timer with Lambda function as slot. If a singleshot timer is required, it is quiet handy to have the slot as lambda function right in the place where the timer is declared. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. Here, I want to briefly discuss how the same effect can be achieved with Qt itself. C is not as dynamic as Python, so Python's approaches of using lambda or functools.partial won't work 1. Fortunately, the Qt folks provided a solution that can make passing extra arguments to slots relatively simple. For example, you can connect a signal with two parameters (a float and a bool) to a slot with only a float parameter; the additional bool parameter from the signal is ignored. It's important to note that if a parameter for a signal or slot is of a type that is specific to Cascades, you must use the full namespace when you specify the parameter.

1. Qt Signal Slot Lambda Parameters
2. Qt Signal Slot Lambda Parameter Chart

Signals and slots are used to connect between an event from gui and a function. In other words, you can manage situations what happens after an situations.

For example:

Qt Signal Slot Lambda Parameters

on this example, we say that, call the onButtonClicked() function after button clicked:

If you want to get signals, you must connect these to slots. Slots are functions defined as slot like this example:

this code on header file.

And last important think is that, signals and slots must have same parameters. It works:

But there is no connection in this example:

QCombobox Signals And Slots

I’ve toyed with QT connections trying to create anonymous function when connecting a slot to a signal. For instance, I am trying to do the following:

So far so good, but sometime I find it cumbersome to define all the slots. Create a declaration, a definition, etc…

I would like to declare the body right there! Here’s what I would like to do using std::function or a c++ lambda:

I’ve achieved that by creating a global object named _Q which is responsible to maintained the QT meta object information about all connection that needs to be called.

Here’s the header of the class in question:

And here’s the implementation:

Qt Signal Slot Lambda Parameter Chart

The idea here is that we maintain the QMetaObject needed to dispatch the call, when qt_metacall is called, to the proper anonymous functions. The magic is done in FillMetaStructs() to maintain the meta object structure each time a new connection is made when Call(...) is called.