Default Title

2. Basic C++ | QT C++ Syntax and Semantics

Basic QT C++ Syntaxes

Data type represents the basic type information for a basic element. The naming convention follows. For numeric types it is FLOAT, SIGNED, or UNSIGNED followed by the _BITS where BITS is the size of the data. BOOLEAN is a true / false (1,0) represented in an 8 bit container. The UNSIGNED variants with multiple bit definitions are for packed graphical data formats and represent vectors with per vector member sizes which are treated as a single unit for packing and alignment purposes. MATRIX the three matrix types contain FLOAT_32 elements and are treated as 32 bits for alignment purposes. RS_* objects. 32 bit opaque handles.

Basic Types & Qt Core Classes

Basic data types are normal C++ data types

int, bool, double, char etc.

Structs and arrays are created normally

Qt variants of most of the containers in the C++ Standard

Library

QList<T>

QVector<T>

QMap<T,C>

For String types, Qt holds its own type : QString

QDate, QDateTime : – Can be compared, converted to strings

QChar – 16-bit Unicode character

QString – Unicode character string. Can be resized, may contain 8-bit terminating strings or binary data

QByteArray - Used instead of QString, when memory conservation is important (Qt for embedded Linux)

QEventLoop, QEvent – Used to enter and exit event loop

QHash – Template providing a hash-table-based dictionar

QQueue – Template class implementing a FIFO queue

QPoint, QRect – Rectangle is defined using the top left and bottom right

QTimer – One shot or periodic 1 ms timer (accuracy depends on platform)

QVariant – Union of the common Qt types

QVector – Template class for dynamic arrays (flat), QLinkedList more efficient, if many insertion and deletion operations needed

Iterator classes – Java (QVectorIterator) and STL-like

Identifier Naming Rules

Can consist of upper and lower case letters, digits, dollar sign ($) and the underscore ( _ ) character.
Must begin with a letter, dollar sign, or an underscore
Are case sensitive
Keywords cannot be used as identifiers
Within a given section of your program or scope, each user defined item must have a unique identifier
Can be of any length.

QObject Class

Heart of Qt's object model

Base class for all object classes

So, all QWidgets are QObjects also

Provides object trees and object ownership

QObject's responsibility is to provide a central location for the most important concepts in Qt

Has three major responsibilities

Memory Management

Introspection (runtime identification of object types)

Event handling

Parent/Child Relationship

Each QObject instance may take a parent argument

Child informs its parent about its existence, upon which the parent adds it to its own list of children

If a widget object does not have a parent, it is a window

The parent does the following for its children: Hides and shows children, when hidden/shown itself

Enables and disables children when enabled or disabled itself

Note that a child may be explicitly hidden, although the parent is shown

Creating Objects

Objects inheriting from QObject are allocated on the heap using new

If a parent object is assigned, it takes ownership of the newly created object – and eventually calls delete

QLabel *label = new QLabel(”Hello World”, parent);

Objects not inheriting QObject are allocated on the stack, not the heap

QStringList list;

QColor color;

Exceptions

QFile and QApplication (inheriting QObject) are usually allocated on the stack
Modal dialogs are often allocated on the stack, too

Signals and Slots

Observer pattern

Type-safe callbacks

More secure than callbacks, more flexible than virtual

methods

Many-to-many relationship

Implemented in QObject

Signals

A signal is a way to inform a possible observer that something of interest has happened inside the observed class

A QPushButton is clicked
An asynchronous service handler is finished
Value of QSlider is changed

Signals are member functions that are automatically implemented in the meta-object

Only the function declaration is provided by the developer

Signal is sent, or emitted , using the keyword emit

emit clicked();
emit someSignal(7, “Hello”);

Slots

A slot is a function that is to be executed when a signal has been emitted.

(When QPushButton is pressed), close QDialog
(When service is ready), ask for the value and store it
(When QSlider value is changed), show a new value in QLCDNumber

A Slot function is a normal member function implemented by the developer

Signals and Slots

The signals and slots mechanism is fundamental to Qt programming. It enables the application programmer to bind objects together without the objects knowing anything about each other.

Slots are almost identical to ordinary C++ member functions.

The main difference is that they can be connected to a signal - in which case it is automatically called when the signal is emitted.

To setup the signal-slot connection, we must first define the connection.

Signals and Slots

The connect() statement looks like this:

connect(sender, SIGNAL(signal), receiver, SLOT(slot));

Main features:

One signal can be connected to many slots

connect(slider, SIGNAL(valueChanged(int)),

spinBox, SLOT(setValue(int)));

Many signals can be connected to the same slot
connect(lcd,,SIGNAL(overflow()))
this, SLOT(handleMathError()));

connect(calculator, SIGNAL(divisionByZero()),

this, SLOT(handleMathError()));

A signal can be connected to another signal
connect(lineEdit, SIGNAL(textChanged(const QString &)),

this, SIGNAL(updateRecord(const QString &)));
.

Connections can be removed
disconnect(lcd, SIGNAL(overflow()),

this, SLOT(handleMathError()));

Exceptionally if a signal has more parameters than the slot it is connected to, the additional parameters are ignored

connect(ftp, SIGNAL(rawCommandReply(int, const QString &)),