Hey there! Let's work through why your QHBoxLayout isn't working and clear up those questions you have about Qt widget behavior.

The Core Problem: QMainWindow's Special Structure #

QMainWindow is a special widget with a predefined layout structure (it includes dedicated spots for menus, toolbars, status bars, etc.). You can't directly call setLayout() on a QMainWindow — that's exactly why your layout wasn't having any effect. Instead, you need to use a central widget as the container for your custom content, then apply your layout to that widget.

Modified Working Code #

Here's how to adjust your mainwindow.cpp to get the layout working correctly:

#include "mainwindow.h"
#include <QPushButton>
#include <QHBoxLayout>
#include <QWidget>

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent) {
    setWindowTitle(tr("Two Button Example"));
    resize(400,300);

    // Step 1: Create a central widget to hold your layout and buttons
    QWidget *centralWidget = new QWidget(this);
    setCentralWidget(centralWidget);

    // Step 2: Create your buttons (no need for setGeometry or explicit setParent)
    button1 = new QPushButton(tr("Bye!"));
    connect(button1, SIGNAL(clicked()), this, SLOT(close()));

    button2 = new QPushButton(tr("Hide B1!"));
    // Note: I adjusted this to hide button1 instead of button2 (matches your button text)
    connect(button2, SIGNAL(clicked()), button1, SLOT(hide()));

    // Step 3: Create the layout and attach it to the central widget
    QHBoxLayout *layout = new QHBoxLayout(centralWidget);
    layout->addWidget(button1);
    layout->addWidget(button2);

    // No need for individual show() calls — the central widget will display all its children
}

MainWindow::~MainWindow() { }

Answering Your Specific Questions #

Let's break down each of your issues one by one:

1. Layout not taking effect: As mentioned, you were calling setLayout() directly on QMainWindow, which ignores the call entirely. Using a central widget and applying the layout to it fixes this.
2. setGeometry behavior: When a widget has no parent, setGeometry() uses screen coordinates. When it has a parent, it uses the parent's local coordinate system. But when using layouts, you shouldn't need setGeometry() at all — the layout manager handles positioning and sizing automatically.
3. Parent widget confusion: When you add a widget to a layout, the layout automatically sets the widget's parent to the widget that the layout is attached to (in our case, the central widget). That's why the examples you saw didn't use explicit setParent() calls — it's handled behind the scenes.
4. Button overlap/ oversized buttons: Without a proper layout, Qt uses each widget's sizeHint() as its default size, but there's no logic to position them side by side. The layout will arrange them horizontally and adjust their sizes to fit the available space.

Understanding sizeHint() #

sizeHint() is a virtual function every Qt widget implements to return its "preferred" size. This size is calculated based on the widget's content (like button text, icons, or nested widgets).

• Layout managers use sizeHint() as a guide when arranging widgets. For example, a QPushButton's sizeHint() is determined by its text length and any icon it has.
• You don't usually need to call sizeHint() directly, but you can override it in custom widgets to define your own preferred size.
• If you want to enforce a fixed size, you can use setFixedSize() instead of relying on sizeHint(), or adjust layout properties like setStretchFactor() to control how widgets grow/shrink with the window.

内容的提问来源于stack exchange，提问作者HWerneck