This is a text-only version of the following page on https://raymii.org:
---
Title : Qt Property Macro (Q_PROPERTY with 95% less code)
Author : Remy van Elst
Date : 29-05-2024 22:00
URL : https://raymii.org/s/blog/Qt_Property_Macro_Q_PROPERTY_with_95_percent_less_code.html
Format : Markdown/HTML
---
Adding [properties to QObject based classes](https://web.archive.org/web/20240529180113/https://doc.qt.io/qt-6/properties.html) is cumbersome. Although [the property system](https://web.archive.org/web/20240529180113/https://doc.qt.io/qt-6/properties.html) and [Signals and Slots](https://web.archive.org/web/20240529180002/https://doc.qt.io/qt-6/signalsandslots.html) are great to use, especially with QML, it takes a lot of boilerplate code to add such properties to a class, at least 15 to 20 lines for each property. I've cobbled up a macro that saves you about 95% of lines when using a `Q_PROPERTY`. (22 lines to 1 line).
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### Adding a Q_PROPERTY
To add a [property to a QObject based class](https://web.archive.org/web/20240529180113/https://doc.qt.io/qt-6/properties.html),
you have to add the following items to the class:
- `Q_PROPERTY` statement
- Declaration of getter and setter methods
- Definition of getter and setter methods
- Value changed signal definition
- Member variable
Recent versions of Qt Creator have made this more efficient by auto-filling
the `Q_PROPERTY` statement and via a right-click you can now go
to `Refactor` then `Add missing Q_PROPERTY members`, but that is still
more work and more typing than using a macro that does it all for you.
With this macro, `QObject::connect` still works as you would expect just as
accessing the property from QML.
There also is the `MEMBER` Qt property type but that is still more boilerplate
than this because you still need to write the entire line including notify
signal and member variables.
### QObject Property Macro
This is the macro I've defined:
/* Macro to define Q_PROPERTY backed by a regular value
* QP_V = Q_Property, value (not reference) */
#define QP_V(Type, Name) \
private: \
Q_PROPERTY(Type Name READ Name WRITE set##Name NOTIFY Name##Changed FINAL) \
Type _##Name; \
public: \
Type Name() const { return _##Name; } \
public Q_SLOTS: \
void set##Name(Type value) { if (_##Name != value) { _##Name = value; Q_EMIT Name##Changed(value); } } \
Q_SIGNALS: \
void Name##Changed(Type)
// end macro
Due to macro expansion combined with `moc` I'm using `Q_SIGNALS`, `Q_SLOTS`
and `Q_EMIT`. The regular `signals`, `slots` and `emit` keywords do not work
in this construction. These macro's can also be used [if you want to use a
third-party signals and slots system]
(https://doc.qt.io/qt-6/signalsandslots.html#using-qt-with-3rd-party-signals-and-slots).
You can use it like this:
QP_V(property Type, property Name);
Example:
QP_V(bool, myBool);
The only thing I haven't got working is capitalization of function names.
Convention is to use `setMyThingChanged`. With the macro, that becomes `setmyThingChanged`.
You can capitalize you variable to fix that.
The macro is simple enough that you can easily update it for for example
`CONSTANT` properties or read-only properties (without a `WRITE` method).
### Usage example:
In [my monitoring app](https://leafnode.nl) there is an update check, which
uses a `Q_PROPERTY`. That is now just one line:
```
QP_V(bool, newVersionAvailable);
```
Instead of all of these lines in the header file:
private:
Q_PROPERTY(bool newVersionAvailable READ newVersionAvailable WRITE setNewVersionAvailable NOTIFY newVersionAvailableChanged FINAL)```
public:
bool newVersionAvailable() const;
signals:
void newVersionAvailableChanged(bool newVersionAvailable);
public slots:
setNewVersionAvailable(bool newNewVersionAvailable);
private:
bool _newVersionAvailable = false;
And in the implementation (`.cpp`) file:
bool MyClass::newVersionAvailable() const
{
return _newVersionAvailable;
}
void MyClass::setNewVersionAvailable(bool newNewVersionAvailable)
{
if (_newVersionAvailable == newNewVersionAvailable)
return;
_newVersionAvailable = newNewVersionAvailable;
emit newVersionAvailableChanged(_newVersionAvailable);
}
Saving all that boilerplate code makes it easier to read the source and figure
out what actually happens and matters instead of hundreds and hundreds of
lines of boilerplate.
---
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