// MainWindow.h
class MainWindow : public QMainWindow
{
        Q_OBJECT

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

public Q_SLOTS:
        void button1Click();
        void reply(QNetworkReply *reply);

private:
        Ui::MainWindow *ui;
};

// MainWindow.cpp
#include <QUrl>
#include <QNetworkReply>
#include <QNetworkRequest>
#include <QNetworkAccessManager>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QObject::connect(ui->pushButton_1, SIGNAL(clicked()), this, SLOT(button1Click()));
}

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

void MainWindow::reply(QNetworkReply *reply)
{
    qDebug() << reply->readAll().size(); // 获取的网页数据大小，0表示下载失败
    reply->deleteLater();
}

void MainWindow::button1Click()
{
    QNetworkAccessManager *nm = new QNetworkAccessManager(this);
    // 必须用信号和槽连接
    connect(nm, SIGNAL(finished(QNetworkReply*)), this, SLOT(reply(QNetworkReply*)));
    // 需要下载的网页url地址，有些网址用这种简单方法无法获取数据
    QNetworkRequest request(QUrl("http://www.baidu.com"));
    nm->get(request);
}