单例模式来管理QNetworkAccessManager对象的创建和销，举一个完整的http多线程例子，包括调用

下面是一个完整的使用单例模式来管理QNetworkAccessManager对象的HTTP多线程例子：

// Singleton.h
class Singleton {
public:
    static Singleton& getInstance() {
        static Singleton instance;
        return instance;
    }

    QNetworkAccessManager* getNetworkAccessManager() {
        if (!m_networkAccessManager) {
            m_networkAccessManager = new QNetworkAccessManager(this);
        }
        return m_networkAccessManager;
    }

private:
    Singleton() {}
    QNetworkAccessManager* m_networkAccessManager = nullptr;
};

// HttpWorker.h
class HttpWorker : public QObject {
    Q_OBJECT
public:
    HttpWorker(QNetworkAccessManager* networkAccessManager, QObject* parent = nullptr)
        : QObject(parent)
        , m_networkAccessManager(networkAccessManager)
    {}

public slots:
    void doWork(const QUrl& url) {
        QNetworkRequest request(url);
        QNetworkReply* reply = m_networkAccessManager->get(request);

        QObject::connect(reply, &QNetworkReply::finished, [=]() {
            qDebug() << reply->readAll();
            reply->deleteLater();
            emit finished();
        });
    }

signals:
    void finished();

private:
    QNetworkAccessManager* m_networkAccessManager = nullptr;
};

// main.cpp
#include "Singleton.h"
#include "HttpWorker.h"
#include <QCoreApplication>
#include <QThread>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // 创建多个HttpWorker对象，并且在不同的线程中执行
    QNetworkAccessManager* networkAccessManager = Singleton::getInstance().getNetworkAccessManager();
    HttpWorker worker1(networkAccessManager);
    HttpWorker worker2(networkAccessManager);
    QThread thread1;
    QThread thread2;
    worker1.moveToThread(&thread1);
    worker2.moveToThread(&thread2);
    QObject::connect(&thread1, &QThread::started, [&]() {
        worker1.doWork(QUrl("https://www.baidu.com"));
    });
    QObject::connect(&thread2, &QThread::started, [&]() {
        worker2.doWork(QUrl("https://www.google.com"));
    });
    QObject::connect(&worker1, &HttpWorker::finished, &thread1, &QThread::quit);
    QObject::connect(&worker2, &HttpWorker::finished, &thread2, &QThread::quit);
    QObject::connect(&thread1, &QThread::finished, &a, &QCoreApplication::quit);
    QObject::connect(&thread2, &QThread::finished, &a, &QCoreApplication::quit);
    thread1.start();
    thread2.start();

    return a.exec();
}

上面的代码中，首先通过Singleton::getInstance().getNetworkAccessManager()方法获取QNetworkAccessManager对象，然后在main函数中创建两个HttpWorker对象，分别在不同的线程中执行，分别发送HTTP GET请求获取百度首页和谷歌首页的内容。请求完成后，通过HttpWorker的finished信号槽和QThread的finished信号槽来退出线程，并最终调用QCoreApplication的quit()方法退出程序。

通过使用单例模式来管理QNetworkAccessManager对象的创建和销毁，避免了重复创建对象造成资源浪费和内存泄漏的问题，同时也提高了代码的可维护性和可扩展性。通过在不同的线程中执行HTTP GET请求，可以提高程序的并发处理能力，从而提高程序的性能和响应速度。