The signature of a signal must match the signature of the receiving slot. In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments. Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax.
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. They are completely type safe. All classes that inherit from QObject or one of its subclasses e. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate.
It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component. Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it.
This ensures that truly independent components can be created with Qt. You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal.
This will emit the second signal immediately whenever the first is emitted. Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses. When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call.
When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later. If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.
Signals are automatically generated by the moc and must not be implemented in the. They can never have return types i. A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.
A slot is called when a signal connected to it is emitted. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.
Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections i.
While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete , the signals and slots overhead is only responsible for a very small proportion of the complete function call costs.
The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice. Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application.
To solve this problem, undef the offending preprocessor symbol. The QObject -based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged , and it has a slot which other objects can send signals to. They must also derive directly or indirectly from QObject. Slots are implemented by the application programmer.
Here is a possible implementation of the Counter:: The emit line emits the signal valueChanged from the object, with the new value as argument. In the following code snippet, we create two Counter objects and connect the first object's valueChanged signal to the second object's setValue slot using QObject:: Then b emits the same valueChanged signal, but since no slot has been connected to b 's valueChanged signal, the signal is ignored.
Note that the setValue function sets the value and emits the signal only if value! This prevents infinite looping in the case of cyclic connections e. By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect call. If you pass the Qt:: UniqueConnection type , the connection will only be made if it is not a duplicate.
If there is already a duplicate exact same signal to the exact same slot on the same objects , the connection will fail and connect will return false. This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject:: This means you just give it a pointer and skip the arguments.
It also means you need to know the class of the parent:. Then the line should be: Yup, except parent is a pointer to QWidget in your first code, not to Widget. If you want to use it here you would need to cast it:. This connect statement should really be taken out of myWidget to the code that sees both the objects that are being connected and their types, e.
You should really decide on your naming convention. One class is camel-case other one not, one function uses lowercase and underscores and another camel-case. It's gonna be mighty confusing what is what. Are you sure, Chris? Even if we accept, as the basic tenet of true democracy, that one moron is equal to one genius, is it necessary to go a further step and hold that two morons are better than one genius? This connect statement should really be taken out of myWidget to the code that sees both the objects.
Thank you for the hints. Just one more question. Is the commented out connect line correct in that case? The difference is that this will compile fine and only silently fail at runtime with a warning on the output , while the new syntax won't let you even compile this, which is much better because it gives you a chance to fix it right away and not after a user of your app reports a bug: This topic has been deleted. Only users with topic management privileges can see it.