目标：实现下拉框点选，编辑框显示选择的项。如下图：

.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
public:
    void init();

private slots:

    void on_comboBox_currentIndexChanged(const QString &arg1);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "QList"
#include "QStringList"
#include "QMessageBox"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    init();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::init()
{
     QStringList list;
     list << "A" << "B" << "C" << "D" << "E";
     ui->comboBox->addItems(list);
     ui->comboBox->addItem("F");
     int icom = 0;
     icom = ui->comboBox->count();
     //ui->comboBox->setMaxCount(icom);
     ui->comboBox->addItem("G");
}
void MainWindow::on_comboBox_currentIndexChanged(const QString &arg1)
{
    ui->lineEdit->setText(arg1);
}

此为Qcombox简单操作。