QPushButton does not honor horizontal expanding size policy - c++

Question

I'm trying to put several QPushButton entities inside a QVBoxLayout such that they are centered and expanding. The expanding tag works fine until I tell the QVBoxLayout to use AlignHCenter, after which the QPushButton's all jump to the minimum size and stay there. What am I doing wrong?
QVBoxLayout *vBoxLayout = new QVBoxLayout(this);
setLayout(vBoxLayout);
//Create title and add to layout
QLabel *titleLabel = new QLabel(this);
titleLabel->setText(menuTitle);
titleLabel->setAlignment(Qt::AlignHCenter | Qt::AlignTop);
titleLabel->setMaximumHeight(35);
titleLabel->setStyleSheet(QString("QLabel { font-size: 16pt; }"));
vBoxLayout->addWidget(titleLabel);
vBoxLayout->setStretchFactor(titleLabel, 1);
//Create buttons and add to layout
QMap<int, QString>::const_iterator it;
for (it = m_buttonMapping.cbegin(); it != m_buttonMapping.cend(); ++it)
{
QPushButton *button = new QPushButton(it.value(), this);
connect(button, SIGNAL(clicked()), sigMapper, SLOT(map()));
sigMapper->setMapping(button, it.key());
button->setMinimumHeight(40);
button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
button->setMaximumWidth(800);
button->setMinimumWidth(300);
vBoxLayout->addWidget(button);
vBoxLayout->setAlignment(button, Qt::AlignHCenter); //<-- without this, expanding works fine!
vBoxLayout->setStretchFactor(button, 1);
}
vBoxLayout->setContentsMargins(10, 0, 10, 0);

Answer 1

By specifying the alignment on the layout, you keep your QPushButtons from being able to expand. Available new space will be used to keep the QPushButtons centered, instead of allowing them to resize and for an amount of space around them to be utilized for centering. Stretch factors fulfill your requirement for a proportional resizing and centering of a layout's contents.
To get around this, create a wrapper widget and layout (or just a layout), and add the widget that is laid out by your vBoxLayout to the wrapper layout with a stretch factor applied. Before and after adding your widget, you'll add QSpacerItems to the wrapper layout with QHBoxLayout::addStretch. You can then adjust the stretch factors of your widget and the spacers to get the effect you want.
Here's some sample code that should solve your problem:
MainWindow.cpp
#include "MainWindow.hpp"
#include <QPushButton>
#include <QLabel>
#include <QBoxLayout>
MainWindow::MainWindow(QWidget* parent)
: QMainWindow(parent) {
QWidget* centralWidget = new QWidget(this);
QVBoxLayout* layout = new QVBoxLayout(centralWidget);
// Create a wrapper widget that will align horizontally
QWidget* alignHorizontalWrapper = new QWidget(centralWidget);
layout->addWidget(alignHorizontalWrapper);
// Layout for wrapper widget
QHBoxLayout* wrapperLayout = new QHBoxLayout(alignHorizontalWrapper);
// Set its contents margins to 0 so it won't interfere with your layout
wrapperLayout->setContentsMargins(0, 0, 0, 0);
wrapperLayout->addStretch(1);
QWidget* widget = new QWidget(alignHorizontalWrapper);
wrapperLayout->addWidget(widget, 3);
wrapperLayout->addStretch(1);
QVBoxLayout* vBoxLayout = new QVBoxLayout(widget);
QLabel* titleLabel = new QLabel(this);
titleLabel->setText(QStringLiteral("Menu"));
titleLabel->setAlignment(Qt::AlignHCenter | Qt::AlignTop);
titleLabel->setMaximumHeight(35);
titleLabel->setStyleSheet(QStringLiteral("QLabel { font-size: 16pt; }"));
vBoxLayout->addWidget(titleLabel);
vBoxLayout->setStretchFactor(titleLabel, 1);
for (int i = 0; i < 3; ++i) {
const QString& value = QStringLiteral("Button ") + QString::number(i);
QPushButton* button = new QPushButton(value, this);
button->setMinimumHeight(40);
button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
//button->setMaximumWidth(800);
button->setMinimumWidth(300);
vBoxLayout->addWidget(button);
//vBoxLayout->setAlignment(button, Qt::AlignHCenter); // without this, expanding works fine!
vBoxLayout->setStretchFactor(button, 3);
}
vBoxLayout->setContentsMargins(10, 0, 10, 0);
this->setCentralWidget(centralWidget);
}
MainWindow.hpp
#ifndef MAINWINDOW_HPP
#define MAINWINDOW_HPP
#include <QMainWindow>
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget* parent = nullptr);
};
#endif // MAINWINDOW_HPP
main.cpp
#include "MainWindow.hpp"
#include <QApplication>
int main(int argc, char* argv[]) {
QApplication app(argc, argv);
MainWindow window;
window.show();
return app.exec();
}