How do I get a QLabel to expand to full width?

Question

I want a QLabel to expand to full width of the container regardless of the contents. (I want this because I dynamically set the text and add widgets later which cause it to cut off part of the text)

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    this->setFixedSize(100,100);
    QHBoxLayout *layout = new QHBoxLayout;
    this->setLayout(layout);
    QLabel *label = new QLabel;
    label->setStyleSheet("background-color:blue");
    label->setSizePolicy(QSizePolicy::MinimumExpanding, 
                         QSizePolicy::MinimumExpanding);
    label->setText(tr("test"));
    layout->addWidget(label, 0, Qt::AlignTop | Qt::AlignLeft);
}

This code shows that the blue box does not expand to the entire width, why?

Answer 1

You must set:

layout->setContentsMargins(0,0,0,0);

By default every QWidget or QFrame add 15 pixels of margin in every direction.

The main problem is with setting the alignment when you add the widget to the layout. Use label->setAlignment instead.

layout->addWidget(label);

I compiled your code, it works with those changes.

Here is the minimal example:

#include <QApplication>
#include <QtGui>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget* w = new QWidget;
    w->setFixedSize(100,100);
    QHBoxLayout* layout = new QHBoxLayout;
    layout->setContentsMargins(0,0,0,0);
    w->setLayout(layout);
    QLabel* label = new QLabel;
    label->setAlignment(Qt::AlignTop | Qt::AlignLeft);
    label->setContentsMargins(0,0,0,0);
    label->setStyleSheet("background-color:blue");
    label->setSizePolicy(QSizePolicy::MinimumExpanding,
                     QSizePolicy::MinimumExpanding);
    label->setText("test");
    layout->addWidget(label);
    w->show();
    return a.exec();
}