QReadWriteLock的学习

参考：

Qt线程的简单使用--QReadWriteLock的用法_qt的读写锁怎么应用-CSDN博客

应用场景：多个线程同时进行读操作。

比如：100个线程进行读操作，1个线程进行写操作。

示例1：

#include <QObject>
#include <QThread>
class Read_thread : public QThread
{
    Q_OBJECT
public:
    explicit Read_thread(QObject *parent = nullptr);
protected:
    void run();
};

class Write_thread : public QThread
{
    Q_OBJECT
public:
    explicit Write_thread(QObject *parent = nullptr);
protected:
    void run();
};

#include <QReadWriteLock>
int counter=0;
QReadWriteLock lock;

void Read_thread::run()
{
    lock.lockForRead();
    for(int i=0;i<5;i++)
    {
        this->msleep(10);
        qDebug()<<"Read_thread:"<<QThread::currentThreadId()<<"   "<<counter;
    }
    lock.unlock();
}
void Write_thread::run()
{
    lock.lockForWrite();
    for(int i=0;i<5;i++)
    {
        counter++;
        this->msleep(10);
        qDebug()<<"Write_thread:"<<QThread::currentThreadId()<<"   "<<counter;
    }
    lock.unlock();
}

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    qDebug()<<"main:"<<QThread::currentThreadId();
    connect(this,SIGNAL(destroyed()),this,SLOT(quitThreadSlot()));
    t=new Read_thread(this);
    t->start();

    t1=new Read_thread(this);
    t1->start();

    t2=new Write_thread(this);
    t2->start();

    t3=new Write_thread(this);
    t3->start();
}

void Widget::quitThreadSlot()
{
    t1->quit();
    t1->wait();

    t->quit();
    t->wait();

    t2->quit();
    t2->wait();

    t3->quit();
    t3->wait();
}

输出结果：

main: 0x5a08

Read_thread: 0x4f30 0

Read_thread: 0x62d8 0

Read_thread: 0x4f30 0

Read_thread: 0x62d8 0

Read_thread: 0x62d8 0

Read_thread: 0x4f30 0

Read_thread: 0x4f30 0

Read_thread: 0x62d8 0

Read_thread: 0x4f30 0

Read_thread: 0x62d8 0

Write_thread: 0x339c 1

Write_thread: 0x339c 2

Write_thread: 0x339c 3

Write_thread: 0x339c 4

Write_thread: 0x339c 5

Write_thread: 0x60b0 6

Write_thread: 0x60b0 7

Write_thread: 0x60b0 8

Write_thread: 0x60b0 9

Write_thread: 0x60b0 10

分析结果：

可以看到，允许不同的线程同时读counter,这里就是在交替地读counter,

之后进行写操作，不是交替进行的，

而是先执行Write_thread: 0x339c，

再执行Write_thread: 0x60b0。

即：可以同时读，不可以同时写。

?示例2：修改部分代码：

    t=new Read_thread(this);
    t->start();

    QThread::msleep(1000);
    t2=new Write_thread(this);
    t2->start();

    t3=new Write_thread(this);
    t3->start();

    t1=new Read_thread(this);
    t1->start();

输出结果：

main: 0x5c84

Read_thread: 0xb60 0

Read_thread: 0xb60 0

Read_thread: 0xb60 0

Read_thread: 0xb60 0

Read_thread: 0xb60 0

Write_thread: 0x3aa0 1

Write_thread: 0x3aa0 2

Write_thread: 0x3aa0 3

Write_thread: 0x3aa0 4

Write_thread: 0x3aa0 5

Write_thread: 0x4788 6

Write_thread: 0x4788 7

Write_thread: 0x4788 8

Write_thread: 0x4788 9

Write_thread: 0x4788 10

Read_thread: 0x38b0 10

Read_thread: 0x38b0 10

Read_thread: 0x38b0 10

Read_thread: 0x38b0 10

Read_thread: 0x38b0 10

分析：Read_thread:?0xb60先进行读操作

主线程阻塞1s

Write_thread: 0x3aa0进行写操作

Write_thread:0x4788进行写操作

Read_thread: 0x38b0进行读操作

示例3：QReadLocker和QWriteLocker的使用

void Read_thread::run()
{
    QReadLocker locker(&lock);
    for(int i=0;i<5;i++)
    {
        this->msleep(10);
        qDebug()<<"Read_thread:"<<QThread::currentThreadId()<<"   "<<counter;
    }
}

void Write_thread::run()
{
    QWriteLocker locker(&lock);
    for(int i=0;i<5;i++)
    {
        counter++;
        this->msleep(10);
        qDebug()<<"Write_thread:"<<QThread::currentThreadId()<<"   "<<counter;
    }
}

?这样写只是更简单了一些，不需要再locked(),unlocked()了。

?