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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TT
June 13, 2019, 7:01 p.m.
Taimoor Tanweer

C++ - Test 001. The first program and data types

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TT
June 13, 2019, 6:51 p.m.
Taimoor Tanweer

C++ - Test 002. Constants

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ВМ
June 13, 2019, 12:30 p.m.
Ваня Мороз

C++ - Test 001. The first program and data types

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Last comments
i
June 17, 2019, 6:10 a.m.
ingenfly

Только по осям xAxis2, уAxis2 значения начинаются с 0. Почему-то xAxis2 и xAxis не синхронизированы по данным. Ну и QCustomPlot последний.
June 16, 2019, 8:21 p.m.
Евгений Легоцкой

Добрый день. Ну точно также добавляете ту же самую информацию на ось xAxis2, только добавляете другое форматирование customPlot->xAxis2->setDateTimeFormat("hh:mm"); если я ...
EF
June 14, 2019, 1:56 p.m.
Egor Fomin

Спасибо за ваш ответ, у меня получилось реализовать это. Тем не менее появилась другая проблема, поэтому опять надеюсь на вашу помощь. Скажем, я уже выставил точки и они соеденены. Когда я нач...
d
June 13, 2019, 2:47 p.m.
damix

Можно классу, который описывает точку, добавить сигнал, который подавать (emit), когда точка перемещается (переопределить mouseMoveEvent или mouseReleaseEvent). Так вот эти сигналы у каждой из...
i
June 13, 2019, 2:09 p.m.
ingenfly

Здравствайте! Подскажите, пожалуйста: customPlot->xAxis2->setTickLabels(true); //Здесь включается отображение данных на оси xAxis2. а можно как-то продублировать информацию cus...
Now discuss on the forum
June 20, 2019, 9:30 a.m.
IscanderChe

Вернулся к этой задачке только-только, поэтому и не ответил ничего раньше.Как переопределить mouseReleaseEvent(QMouseEvent* event) у QTableView, если QTableView задан в ui? Или задавать QTabl...
I
June 19, 2019, 1:41 p.m.
Intruder

Всем добрый день. При разборе XML файла наткнулся на тег вот такого плана: <TagName attribute1="value1" attribute2="value2" /> При попытке проверить на наличие такого элеме...
June 19, 2019, 12:55 p.m.
Михаиллл

Скажите пожалуйста, как его в таком случае перемещать и удалять?
June 18, 2019, 7:50 p.m.
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Большое спасибо! SDK заработал.К сожалению удалось продвинутся только на один шаг. При сборке чистого проекта NDK выдаёт следующие ошибки C:\Android\ndk-bundle/toolchains/arm-linux-andr...
June 18, 2019, 4:59 p.m.
Михаиллл

Добрый день.В этом учебнике представлен код INSTALLED_APPS = ( ... 'rest_framework', 'snippets.apps.SnippetsConfig',) На строчке 'snippets.apps.SnippetsConf...
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