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Const-ref when sending signals in Qt. Ask Question Asked 7 years, 7 months ago. Active 4 years, 4 months ago. Argument type for Qt signal and slot, does const reference qualifiers matters? Passing QVariant from QML to C-1. Best optimisation for Signals/Slots with Qt. Stack object Qt signal and parameter as reference (3). UPDATE 20-APR-2015. Originally I believed that passing a reference to a stack-allocated object would be equivalent to passing the address of that object. Hence in the absence of a wrapper that would store a copy (or a shared pointer), a queued slot connection could wind up using the bad data. The QObject QObject is the base class to most Qt classes. Examples of exceptions are: Classes that need to be lightweight such as graphical primitives Data containers (QString, QList, QChar, etc)Classes that needs to be copyable, as QObject s cannot be copied. Copied or Not Copied: Arguments in Signal-Slot Connections? The conclusion from the above results is that we should pass arguments to signals and slots by const reference and not by value. This advice is true for both direct and queued connections. We should still pass arguments by const reference. Qt takes care of copying the arguments. Qt – emit a signal When you trying to link the slots and signals together you need to have a QObject::connect and also a emitting, I have created a basic demonstration of this with a QPushButton ( link to qt signal and slots, and a QT link for the signal and slots ).

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Subclassing QDialog

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This chapter will teach you how to create dialog boxes using Qt.

This chapter is from the book 

C++ GUI Programming with Qt4, 2nd Edition

This chapter is from the book

This chapter is from the book 

2. Creating Dialogs

Subclassing QDialog
Signals and Slots in Depth
Rapid Dialog Design
Shape-Changing Dialogs
Dynamic Dialogs
Built-in Widget and Dialog Classes

This chapter will teach you how to create dialog boxes using Qt. Dialog boxes present users with options and choices, and allow them to set the options to their preferred values and to make their choices. They are called dialog boxes, or simply 'dialogs', because they provide a means by which users and applications can 'talk to' each other.

Most GUI applications consist of a main window with a menu bar and toolbar, along with dozens of dialogs that complement the main window. It is also possible to create dialog applications that respond directly to the user's choices by performing the appropriate actions (e.g., a calculator application).

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We will create our first dialog purely by writing code to show how it is done. Then we will see how to build dialogs using Qt Designer, Qt's visual design tool. Using Qt Designer is a lot faster than hand-coding and makes it easy to test different designs and to change designs later.

Subclassing QDialog

Our first example is a Find dialog written entirely in C++. It is shown in Figure 2.1. We will implement the dialog as a class in its own right. By doing so, we make it an independent, self-contained component, with its own signals and slots.

Figure 2.1 The Find dialog

The source code is spread across two files: finddialog.h and finddialog.cpp. We will start with finddialog.h.

Lines 1 and 2 (and 27) protect the header file against multiple inclusions.

Line 3 includes the definition of QDialog, the base class for dialogs in Qt. QDialog is derived from QWidget.

Lines 4 to 7 are forward declarations of the Qt classes that we will use to implement the dialog. A forward declaration tells the C++ compiler that a class exists, without giving all the detail that a class definition (usually located in a header file of its own) provides. We will say more about this shortly.

Next, we define FindDialog as a subclass of QDialog:

The Q_OBJECT macro at the beginning of the class definition is necessary for all classes that define signals or slots.

The FindDialog constructor is typical of Qt widget classes. The parent parameter specifies the parent widget. The default is a null pointer, meaning that the dialog has no parent.

Different classes of slot machines. The signals section declares two signals that the dialog emits when the user clicks the Find Free online poker machines no download. button. If the Search backward option is enabled, the dialog emits findPrevious(); otherwise, it emits findNext().

The signals keyword is actually a macro. The C++ preprocessor converts it into standard C++ before the compiler sees it. Qt::CaseSensitivity is an enum type that can take the values Qt::CaseSensitive and Qt::CaseInsensitive.

Qt Signal Slot With 2 Arguments

In the class's private section, we declare two slots. To implement the slots, we will need to access most of the dialog's child widgets, so we keep pointers to them as well. The slots keyword is, like signals, a macro that expands into a construct that the C++ compiler can digest.

For the private variables, we used forward declarations of their classes. This was possible because they are all pointers and we don't access them in the header file, so the compiler doesn't need the full class definitions. We could have included the relevant header files (<QCheckBox>, <QLabel>, etc.), but using forward declarations when it is possible makes compiling somewhat faster.

We will now look at finddialog.cpp, which contains the implementation of the FindDialog class.

First, we include <QtGui>, a header file that contains the definition of Qt's GUI classes. Qt consists of several modules, each of which lives in its own library. The most important modules are QtCore, QtGui, QtNetwork, QtOpenGL, QtScript, QtSql, QtSvg, and QtXml. The <QtGui> header file contains the definition of all the classes that are part of the QtCore and QtGui modules. Including this header saves us the bother of including every class individually.

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In finddialog.h, instead of including <QDialog> and using forward declarations for QCheckBox, QLabel, QLineEdit, and QPushButton, we could simply have included <QtGui>. However, it is generally bad style to include such a big header file from another header file, especially in larger applications.

On line 4, we pass on the parent parameter to the base class constructor. Then we create the child widgets. The tr() function calls around the string literals mark them for translation to other languages. The function is declared in QObject and every subclass that contains the Q_OBJECT macro. It's a good habit to surround user-visible strings with tr(), even if you don't have immediate plans for translating your applications to other languages. We cover translating Qt applications in Chapter 18.

In the string literals, we use ampersands ('&') to indicate shortcut keys. For example, line 11 cre

Qt Signal Slot Const Reference