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?

We recommend hosting TIMEWEB
We recommend hosting TIMEWEB
Stable hosting, on which the social network EVILEG is located. For projects on Django we recommend VDS hosting.
Support the author Donate
AK

Отличная необходимая статья.

Comments

Only authorized users can post comments.
Please, Log in or Sign up
How to become an author?

Contribute to the evolution of the EVILEG community.

Learn how to become a site author.

Learn it
Donate

Good day, Dear Users!!!

I am Evgenii Legotckoi, developer of EVILEG. And it is my hobby project, which helps to learn programming another programmers and developers

If the site helped you, and you want also support the development of the site, than you can donate by following ways

PayPalYandex.Money
Timeweb

Let me recommend you the excellent hosting on which EVILEG is located.

For many years, Timeweb has been proving his stability.

For projects on Django I recommend VDS hosting

View Hosting Timeweb
AS
May 26, 2020, 11:29 a.m.
Artem Sun-Dun-Chan

C ++ - Test 004. Pointers, Arrays and Loops

  • Result:50points,
  • Rating points-4
MN
May 25, 2020, 11:33 a.m.
Mitja Nagibin

C ++ - Test 004. Pointers, Arrays and Loops

  • Result:50points,
  • Rating points-4
f
May 25, 2020, 5:05 a.m.
falcon

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

  • Result:66points,
  • Rating points-1
Last comments
AL
May 29, 2020, 5:24 a.m.
Aleksandr Leonidov

Django - Tutorial 036. How to add authentication through social networks. VKontakte

Здравствуйте! После клика на ссылку авторизации выдает json словарь с ошибкой: {"error":"invalid_request","error_description":"redirect_uri is incorrect, check application redirect uri in …
May 28, 2020, 3:14 p.m.
Evgenij Legotskoj

Qt/C++ - Lesson 039. How to paint stroke in QSqlTableModel by value in the column?

Ну в моём примере, который в статье сработало так model->setData(model->index(1, 1), 7); Поскольку model->index(1, 0) - это индекс колонки id, которая скрыта, поэтому…
MA
May 28, 2020, 3:08 p.m.
Mihail A

Qt/C++ - Lesson 039. How to paint stroke in QSqlTableModel by value in the column?

Спасибо, завтра првоерю. А model->setData(model->index(1, 0), 7); Тоже заработало?
May 28, 2020, 3:06 p.m.
Evgenij Legotskoj

Qt/C++ - Lesson 039. How to paint stroke in QSqlTableModel by value in the column?

Да, метод data всё-таки влиял, я переписал его так и заработало удаление QVariant TableModel::data(const QModelIndex &idx, int role) const{ if (role == Qt::BackgroundColorRole) {…
May 28, 2020, 2:49 p.m.
Evgenij Legotskoj

Django - Tutorial 011. Adding comments to the site based on Django

Он более функциональный и его функционал объективно лучше поддерживается Django. Из первого, что приходит на ум: Это наличие полей типа Array Поддержка полей для JSON …
Now discuss on the forum
IP
May 29, 2020, 1:55 a.m.
Igor' Poroshin

QTablwView + QSqlQueryModel скрыть пустой столбец

Да, понятно. В данном случае лучше использовать серверную процедуру (если такие поддерживаются), в которой будет проверяться наличие всех пустых строк у нужного столбца и вызываться соответ…
RG
May 28, 2020, 6:21 p.m.
Rovshan Gurbanov

Сборка под старые версии Android

У меня SDK почти все версии есть, NDK есть версии 10, 17, 21. Но собирается приложение только с NDK v21 под Android версии 7.0 и выше Версия Qt у меня 5.14.2
May 28, 2020, 7:58 a.m.
Evgenij Legotskoj

Освобождение памяти QMainWindow::setCentralWidget

Да, соглашусь. Просто удаление происходит позже, а не сразу.
May 28, 2020, 5:43 a.m.
Mihailll

При подключении к git как указать пароль?

Нужно сделать ssh-keygen и потом полученый из файла код скопировать в ssh ключ в бикбакете
F
May 28, 2020, 1:42 a.m.
Fidan

QML

Да, проблема ушла, спасибо.
About
Services
© EVILEG 2015-2020
Recommend hosting TIMEWEB