QLabel

QLabel serves as a versatile widget for presenting both text and images within a Qt interface.

Core Properties

Text Formatting Demonstration

// widget.cpp
#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    // Plain text formatting
    ui->plainLabel->setTextFormat(Qt::PlainText);
    ui->plainLabel->setText("# Plain text content");
    
    // Rich text with HTML markup
    ui->richLabel->setTextFormat(Qt::RichText);
    ui->richLabel->setText("<b>Bold formatted text</b>");
    
    // Markdown formatted content
    ui->mdLabel->setTextFormat(Qt::MarkdownText);
    ui->mdLabel->setText("# Markdown Content");
}

Widget::~Widget()
{
    delete ui;
}

Image Display with Dynamic Resizing

// widget.cpp
#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    QRect bounds = this->geometry();
    ui->imageLabel->setGeometry(0, 0, bounds.width(), bounds.height());
    
    QPixmap image(":/background.png");
    ui->imageLabel->setPixmap(image);
    ui->imageLabel->setScaledContents(true);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::resizeEvent(QResizeEvent *event)
{
    QWidget::resizeEvent(event);
    ui->imageLabel->setGeometry(0, 0, event->size().width(), event->size().height());
}

Setting geometry once in the constructor produces a static label. To make the label respond to window resizing, override QWidget's resizeEvent. This leverages C++ polymorphism—Qt internally calls the base class method, but derived class implementation executes instead.

Text Alignment Configuration

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    ui->labelCentered->setText("Centered Text");
    ui->labelCentered->setAlignment(Qt::AlignHCenter | Qt::AlignVCenter);
    
    ui->labelRightMid->setText("Right Middle");
    ui->labelRightMid->setAlignment(Qt::AlignRight | Qt::AlignVCenter);
    
    ui->labelRightTop->setText("Right Top");
    ui->labelRightTop->setAlignment(Qt::AlignRight | Qt::AlignTop);
    
    ui->labelDefault->setText("Default Alignment");
}

Wrapping, Indentation, and Margins

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    // Centered positioning
    ui->labelOne->setText("Sample Text One");
    ui->labelOne->setAlignment(Qt::AlignHCenter | Qt::AlignVCenter);
    
    // Word wrapping enabled
    ui->labelWrap->setText("Extended text content that spans multiple lines when displayed");
    ui->labelWrap->setWordWrap(true);
    
    // Indentation applied
    ui->labelIndent->setText("Indented text content demonstrating the indent feature");
    ui->labelIndent->setWordWrap(true);
    ui->labelIndent->setIndent(50);
    
    // Margin spacing
    ui->labelMargin->setText("Text with margin spacing");
    ui->labelMargin->setWordWrap(true);
    ui->labelMargin->setMargin(30);
}

The distinction: margin applies padding uniformly on all four sides, while indent affects at most two directions based on alignment settings.

Buddy Relationships

QLabel supports keyboard mnemonics using the ampersand character. Pressing Alt+key combination activates the associated buddy widget.

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    // Mnemonic: Alt+O activates radioOption1
    ui->labelOption1->setBuddy(ui->radioOption1);
    // Mnemonic: Alt+T activates radioOption2
    ui->labelOption2->setBuddy(ui->radioOption2);
}

In the UI designer, labels would contain text like "&Option1" and "&Option2" respectively.

QLCDNumber

QLCDNumber renders numerical displays mimicking vintage calculator aesthetics.

Key Properties

Decimal mode supports fractional display; other bases show integers only.

Countdown Timer Implementation

// widget.h
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QTimer>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

private slots:
    void onTimeout();

private:
    Ui::Widget *ui;
    QTimer *countdownTimer;
};
#endif

// widget.cpp
#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    ui->lcdNumber->display("10");
    
    countdownTimer = new QTimer(this);
    connect(countdownTimer, &QTimer::timeout, this, &Widget::onTimeout);
    countdownTimer->start(1000);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::onTimeout()
{
    int current = ui->lcdNumber->intValue();
    
    if (current <= 0) {
        countdownTimer->stop();
        return;
    }
    
    ui->lcdNumber->display(current - 1);
}

Why Not Use Loops or Threads?

A synchronous loop with sleep blocks the entire event loop, freezing the UI. Launching a separate thread to modify UI elements violates Qt's threading contract—GUI state changes must occur on the main thread to maintain consistency.

Qt widgets maintain complex internal state where modifying one element cascades through layout and positioning. Non-deterministic thread execution would corrupt this state.

The QTimer::timeout signal fires on the main thread, ensuring safe UI updates. Always prefer timers for periodic interface modifications.

QProgressBar

Progress bars indicate task completion status visually.

Properties Reference

Timed Progress Advancement

// widget.h
class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

private slots:
    void updateProgress();

private:
    Ui::Widget *ui;
    QTimer *progressTimer;
};

// widget.cpp
#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    
    progressTimer = new QTimer(this);
    connect(progressTimer, &QTimer::timeout, this, &Widget::updateProgress);
    progressTimer->start(100);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::updateProgress()
{
    int current = ui->progressBar->value();
    
    if (current >= 100) {
        progressTimer->stop();
        return;
    }
    
    ui->progressBar->setValue(current + 1);
}

For actual file operations, calculate progress as (bytesRead / totalSize) * 100 and update accordingly.

QCalendarWidget

The calendar widget provides interactive date selection functionality.

Available Properties

Relevant Signals

Date Selection Handler

// widget.h
#include <QWidget>
#include <QDate>

class Widget : public QWidget
{
    Q_OBJECT

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

private slots:
    void onDateSelected();
};

// widget.cpp
#include "widget.h"
#include "ui_widget.h"
#include <QDebug>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    connect(ui->calendarWidget, &QCalendarWidget::selectionChanged,
            this, &Widget::onDateSelected);
}

Widget::~Widget()
{
    delete ui;
}

void Widget::onDateSelected()
{
    QDate chosen = ui->calendarWidget->selectedDate();
    qDebug() << chosen;
    ui->dateLabel->setText(chosen.toString());
}

Connect the selectionChanged signal to a slot for real-time date processing. The selectedDate() method returns a QDate object convertible to various string formats via toString().