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：