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Qt/C++ - Tutorial 073. Signals and slots. Connecting Slots to Overloaded Signals in the Qt5 Syntax

Signal, Slot, Qt, Qt5

Quite a frequent problem when working with signals with slots in Qt5, according to my observations on the forum, is the connection of slots in the syntax on the pointers to signals having an overload of the signature. The same applies to slots that have an overload.

Let's take a test class that has overloaded signals.

#include <QObject>

class TestClass : public QObject
{
    Q_OBJECT
public:
    explicit TestClass(QObject *parent = nullptr);

signals:
    void testSignal(int arg1);
    void testSignal(int arg1, int arg2);
};

Here there is a signal, with an overload of the signature. Connect this signal will also be to the slots that are declared in the Widget class, and which also have an overload of the signature.

#include <QWidget>
#include "testclass.h"

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private slots:
    void onTestSlot(int arg1);
    void onTestSlot(int arg1, int arg2);

private:
    Ui::Widget *ui;
    TestClass* m_testClass;
};

How it was in Qt4

Within Qt4, everything was solved quite simply by specifying the signature of the signal and the slot in the SIGNAL and SLOT macros.

connect(m_testClass, SIGNAL(testSignal(int,int)), this, SLOT(onTestSlot(int,int)));
connect(m_testClass, SIGNAL(testSignal(int)), this, SLOT(onTestSlot(int)));

How it became in Qt5

But in Qt5, when writing in the new syntax of signals and slots, there are some problems. Because you need to make the static_cast of the method signature.

    connect(m_testClass, static_cast<void(TestClass::*)(int)>(&TestClass::testSignal),
            this, static_cast<void(Widget::*)(int)>(&Widget::onTestSlot));
    connect(m_testClass, static_cast<void(TestClass::*)(int, int)>(&TestClass::testSignal),
            this, static_cast<void(Widget::*)(int, int)>(&Widget::onTestSlot));

By the way, the new syntax also allows you to connect signals to slots with a smaller signature, as it was in Qt4.

connect(m_testClass, static_cast<void(TestClass::*)(int, int)>(&TestClass::testSignal),
        this, static_cast<void(Widget::*)(int)>(&Widget::onTestSlot));

Advantages of the new syntax

And now a stumbling block. Why use the new syntax of signals and slots? I still hear this question from time to time. Especially when people see such terrible castes of signatures.

  1. Therefore, I will list potential advantages:The ability to track errors in the connection of signals and slots at the compilation stage, rather than in the runtime
  2. Reducing compilation time by excluding macros from the code
  3. The ability to connect lambda functions, it's quite an important bun
  4. We protect ourselves from errors when we try to connect from the outside to a private slot. Yes!! Yes!! The SIGNAL and SLOT macros ignore the access levels of methods, violating OOP.

In general, for me this is enough, but for you?

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МБ
April 21, 2019, 9:40 a.m.
Моисей Бушуев

Qt - Test 001. Signals and slots

  • Result:0points,
  • Rating points-10
AA
April 17, 2019, 7:40 p.m.
Anton Ablin

Qt - Test 001. Signals and slots

  • Result:73points,
  • Rating points1
E
April 17, 2019, 6:16 p.m.
Evgeny

Qt - Test 001. Signals and slots

  • Result:100points,
  • Rating points10
Last comments
April 21, 2019, 4:22 p.m.
Евгений Легоцкой

Через метод setIcon table.horizontalHeaderItem(0).setIcon("qrc://path/to/icon.png")
April 21, 2019, 3:48 p.m.
Евгений Легоцкой

Добрый день! Спасибо за комментарий. Там действительно лучше будет сделать с инициализацией по умолчанию.
U
April 18, 2019, 3:37 p.m.
Unreal_man

А как иконку в хедер задать?
u
April 18, 2019, 2:15 a.m.
uaa

доброго времени,большое спасибо за пример для начинающего)при адаптации к своему проекту столкнулся с таким ньансом:в vepolyline.h в 47 строке нужна инициализация по умолчанию: int m_pointF...
E
April 11, 2019, 12:49 p.m.
Evgeny

Спасибо за ответ) У меня компоновщик на нее ругался просто. Оказалось, просто забыл Q_OBJECT в начале класса указать.
Now discuss on the forum
April 24, 2019, 11:22 a.m.
Ruslan Polupan

Согласен. но ситуация не поменялась. Такое чуство что данные не записываются в модель.
April 24, 2019, 6:20 a.m.
Ruslan Polupan

я так понимаю надо инфорация об устройствах.Я бы пробовал так rust@suse:~> lsblk -PNAME="sda" MAJ:MIN="8:0" RM="0" SIZE="111,8G" RO="0" TYPE="disk" MOUNTPOINT=""NAME="sda1" MAJ:MIN="8...
April 21, 2019, 4:16 p.m.
Евгений Легоцкой

Приветствую Нужно сохранять где-то выбранное значение, а потом восстанавливать его. Или использовать QSettings или добавить метод open(), в который передавать начальные значения для того...
R
April 19, 2019, 9:55 a.m.
RED_Spider

мені важко це зараз навіть перевірити, тому що знайшов коміт, це ще було в 2016 році, і цей код не буде працювати коректно зараз, єдине скажу що це були QThread
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