示例代码创建了一个名为ComboBoxWidget的自定义QWidget类，并在initUI方法中创建了一个垂直布局。然后将一个只读的QLineEdit和一个QPushButton添加到布局中。当按钮被点击时，会调用showMenu方法，该方法创建一个QMenu并添加选项。每个选项连接到自定义的onOptionSelected方法，该方法在选择一个选项时被调用，并将选项的文本设置到QLineEdit中

#pragma execution_character_set("utf-8")#include <QApplication>
#include <QWidget>
#include <QHBoxLayout>
#include <QLineEdit>
#include <QPushButton>
#include <QMenu>
#include <QAction>class ComboBoxWidget : public QWidget 
{
public:ComboBoxWidget(QWidget* parent = nullptr) : QWidget(parent) {initUI();}private:void initUI() {QHBoxLayout* layout = new QHBoxLayout(this);layout->setSpacing(0);lineEdit = new QLineEdit(this);lineEdit->setReadOnly(true);lineEdit->setAlignment(Qt::AlignCenter);button = new QPushButton("▼", this);button->setFixedWidth(30);layout->addWidget(lineEdit);layout->addWidget(button);connect(button, &QPushButton::clicked, this, &ComboBoxWidget::showMenu);setLayout(layout);setGeometry(300, 300, 200, 100);setWindowTitle("ComboBox Example");show();}void showMenu() {QMenu* menu = new QMenu(this);menu->setFixedWidth(lineEdit->width() + button->width());QAction* action1 = new QAction("Option 1", this);connect(action1, &QAction::triggered, [this, action1]() { onOptionSelected(action1->text()); });QAction* action2 = new QAction("Option 2", this);connect(action2, &QAction::triggered, [this, action2]() { onOptionSelected(action2->text()); });QAction* action3 = new QAction("Option 3", this);connect(action3, &QAction::triggered, [this, action3]() { onOptionSelected(action3->text()); });QAction* action4 = new QAction("Option 4", this);connect(action4, &QAction::triggered, [this, action4]() { onOptionSelected(action4->text()); });menu->addAction(action1);menu->addAction(action2);menu->addAction(action3);menu->addAction(action4);menu->exec(lineEdit->mapToGlobal(lineEdit->rect().bottomLeft()));}void onOptionSelected(const QString& text) {lineEdit->setText(text);}private:QLineEdit* lineEdit;QPushButton* button;
};int main(int argc, char* argv[]) 
{QApplication app(argc, argv);ComboBoxWidget widget;widget.show();return app.exec();
}

效果展示：