Understanding QFontMetrics::strikeOutPos() for Strikethrough Text in Qt GUI

Purpose

The strikeOutPos() function specifically retrieves the vertical position within the font's metrics where a strikethrough line would be drawn for text rendered with that font.
In Qt, the QFontMetrics class provides information about a particular font's characteristics.

Understanding Strikethrough Positioning

strikeOutPos() returns this offset as an integer value in pixels.
The strikethrough line, used to indicate text that should be visually crossed out, is typically drawn at a specific offset from the baseline of the font.
Fonts have various metrics that define how characters are spaced and positioned.

Using strikeOutPos()

- You can create a QFontMetrics object in several ways:
  - Pass a QFont object to the constructor:
```
QFont font("Arial", 12);
QFontMetrics fm(font);
```
  - Use QWidget::fontMetrics() to get metrics for a widget's font:
```
QFontMetrics fm = widget->fontMetrics();
```
  - Use QPainter::fontMetrics() to get metrics for the current painter's font:
```
QPainter painter(widget);
QFontMetrics fm = painter.fontMetrics();
```
Call strikeOutPos()
- Once you have a QFontMetrics object, call strikeOutPos() to retrieve the strikethrough line position:
```
int strikeOutY = fm.strikeOutPos();
```

Example: Drawing Strikethrough Text

#include <QtWidgets>
#include <QtGui>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QLabel label("This text has a strikethrough.");
    QFont font("Times New Roman", 14);
    label.setFont(font);

    QFontMetrics fm(label.font());
    int strikeOutY = fm.strikeOutPos();

    QPainter painter(&label);
    painter.drawText(0, 0, label.text());

    // Draw the strikethrough line
    painter.setPen(Qt::red);
    painter.drawLine(0, strikeOutY, label.width(), strikeOutY);

    label.show();

    return app.exec();
}

In this example:

The painter.drawLine() method draws a red line at that position across the label's text.
The strikeOutY variable stores the vertical position for the strikethrough line.

Additional Considerations

The exact positioning of the strikethrough line may vary slightly depending on the specific font you're using. However, strikeOutPos() provides a good starting point for most cases.
strikeOutPos() is typically used in conjunction with other QFontMetrics functions like width() and height() to calculate the appropriate placement and dimensions of text and strikethrough lines.

Strikethrough Text with Custom Pen Style

#include <QtWidgets>
#include <QtGui>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QLabel label("This text has a custom strikethrough.");
    QFont font("Arial", 16);
    label.setFont(font);

    QFontMetrics fm(label.font());
    int strikeOutY = fm.strikeOutPos();

    QPainter painter(&label);
    painter.drawText(0, 0, label.text());

    // Draw the strikethrough line with a dashed blue pen
    QPen pen(Qt::blue, 2, Qt::DashLine);
    painter.setPen(pen);
    painter.drawLine(0, strikeOutY, label.width(), strikeOutY);

    label.show();

    return app.exec();
}

This code modifies the previous example by setting a custom pen for the strikethrough line. The QPen object allows you to control the line's color, width, and style (solid, dashed, dotted, etc.).

Strikethrough Text with Alignment

#include <QtWidgets>
#include <QtGui>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QLabel label("This text is centered with a strikethrough.");
    QFont font("Courier New", 18);
    label.setFont(font);

    QFontMetrics fm(label.font());
    int strikeOutY = fm.strikeOutPos();
    int textWidth = fm.width(label.text());

    // Center the text horizontally within the label
    label.setAlignment(Qt::AlignCenter);

    QPainter painter(&label);
    painter.drawText((label.width() - textWidth) / 2, 0, label.text());

    // Draw the strikethrough line centered vertically within the text's bounding rectangle
    int textHeight = fm.height();
    int strikeOutYOffset = (textHeight - strikeOutY) / 2;  // Center the line vertically

    painter.setPen(Qt::red);
    painter.drawLine((label.width() - textWidth) / 2, strikeOutY + strikeOutYOffset,
                     (label.width() + textWidth) / 2, strikeOutY + strikeOutYOffset);

    label.show();

    return app.exec();
}

This example demonstrates how to center both the text and the strikethrough line within the label's bounding rectangle. It calculates the text width and height using QFontMetrics::width() and QFontMetrics::height(), then adjusts the strikethrough line position based on those values.

Strikethrough Text with Rich Text Formatting

#include <QtWidgets>
#include <QtGui>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QLabel label;
    QString text = "This text has <strike>a strikethrough</strike> part.";
    label.setText(text);
    QFont font("Times New Roman", 12);
    label.setFont(font);

    // No need to call strikeOutPos() here as rich text formatting handles strikethrough

    label.show();

    return app.exec();
}

This example showcases how Qt's rich text formatting capabilities can be used to create strikethrough text. By embedding the strikethrough tag (<strike>) within the text string, Qt automatically applies the strikethrough effect without the need to manually calculate the line position.

Using Rich Text Formatting

This approach avoids the need for manual line positioning calculations and leverages Qt's built-in formatting capabilities.
As shown in the previous example, Qt supports rich text formatting using HTML-like tags. You can embed the <strike> tag within your text string to indicate strikethrough:
```
QString text = "This text has <strike>a strikethrough</strike> part.";
QLabel label(text);
```

Custom Text Rendering with QPainter

In this example, the paintEvent method is overridden to draw the text and strikethrough line within the widget. You can customize the line's color, thickness, and position based on your requirements.

You can achieve strikethrough by manually drawing the text and a line over it using the QPainter class. This method offers more control over the appearance of the strikethrough line:

void paintEvent(QPaintEvent* event) {
    QPainter painter(this);
    QFont font = painter.font();
    painter.drawText(0, 0, text());

    // Draw the strikethrough line with desired properties (color, thickness)
    painter.setPen(Qt::red);
    int lineY = font.height() * 0.6;  // Adjust Y position as needed
    painter.drawLine(0, lineY, width(), lineY);
}

This approach requires setting a stylesheet for your label or applying it globally. It offers a declarative way to style your widgets and can be useful for maintaining consistent formatting across your application.
Qt Style Sheets (QSS) allow you to apply CSS-like styles to your Qt widgets. You can define a style for the text decoration to achieve strikethrough:
```
QLabel {
    text-decoration: line-through;
}
```