C++ 新旧版本两种读写锁

一、简介

读写锁（Read-Write Lock）是一种并发控制机制，用于多线程环境中实现对共享资源的高效读写操作。读写锁允许多个线程同时读取共享资源，但在有写操作时，需要互斥地独占对共享资源的访问，以确保数据的一致性。

二、shared_mutex

std::shared_mutex 是 C++17 引入的，用于实现共享/独占访问控制，以下是简单示例：

#include 
#include 
#include 

std::shared_mutex rwLock;
int sharedData = 0;  // 共享资源

void readerFunction(int id) {
    while (true) {
        rwLock.lock_shared();
        std::cout << "Reader " << id << " reads sharedData: " << sharedData << std::endl;
        rwLock.unlock_shared();
        // 模拟一些耗时操作
        std::this_thread::sleep_for(std::chrono::milliseconds(500));
    }
}

void writerFunction(int id) {
    while (true) {
        rwLock.lock();
        sharedData++;
        std::cout << "Writer " << id << " writes sharedData: " << sharedData << std::endl;
        rwLock.unlock();
        // 模拟一些耗时操作
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
}

int main() {
    const int numReaders = 3;
    const int numWriters = 3;
    std::thread readers[numReaders];
    std::thread writers[numWriters];
    // 启动线程
    for (int i = 0; i < numReaders; i++) {
        readers[i] = std::thread(readerFunction, i);
    }
    for (int i = 0; i < numWriters; i++) {
        writers[i] = std::thread(writerFunction, i);
    }
    // Join线程
    for (int i = 0; i < numReaders; i++) {
        readers[i].join();
    }
    for (int i = 0; i < numWriters; i++) {
        writers[i].join();
    }
    return 0;
}

三、自定义 ReadWriteLock

C++17前可以通过 mutex 和 condition_variable 实现一个自定义的读写锁。代码如下：

#include 
#include 

class ReadWriteLock {
public:
    ReadWriteLock() : readersCount(0), writing(false) {}

    void lockRead() {
        std::unique_lock<std::mutex> lock(mutex_);
        readCondition_.wait(lock, [this] { return !writing; });
        readersCount++;
    }

    void unlockRead() {
        std::unique_lock<std::mutex> lock(mutex_);
        readersCount--;
        if (readersCount == 0) {
            writeCondition_.notify_one();
        }
    }

    void lockWrite() {
        std::unique_lock<std::mutex> lock(mutex_);
        writeCondition_.wait(lock, [this] { return readersCount == 0 && !writing; });
        writing = true;
    }

    void unlockWrite() {
        std::unique_lock<std::mutex> lock(mutex_);
        writing = false;
        readCondition_.notify_all();
        writeCondition_.notify_one();
    }

private:
    std::mutex mutex_;
    std::condition_variable readCondition_;
    std::condition_variable writeCondition_;
    int readersCount;
    bool writing;
};

使用示例：

#include 
#include 
#include "ReadWriteLock.hpp" // 包含上面ReadWriteLock的头文件

int sharedData = 0;  // 共享数据
ReadWriteLock rwLock;

void readerFunction(int id) {
    while (true) {
        rwLock.lockRead();
		std::cout << "Reader " << id << " reads sharedData: " << sharedData << std::endl;
        rwLock.unlockRead();
        // 模拟一些耗时操作
        std::this_thread::sleep_for(std::chrono::milliseconds(500));
    }
}

void writerFunction(int id) {
    while (true) {
        rwLock.lockWrite();
        sharedData++;
        std::cout << "Writer " << id << " writes sharedData: " << sharedData << std::endl;
        rwLock.unlockWrite();
        // 模拟一些耗时操作
        std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
}

int main() {
    const int numReaders = 3;
    const int numWriters = 3;
    std::thread readers[numReaders];
    std::thread writers[numWriters];
    // 启动线程
    for (int i = 0; i < numReaders; i++) {
        readers[i] = std::thread(readerFunction, i);
    }
    for (int i = 0; i < numWriters; i++) {
        writers[i] = std::thread(writerFunction, i);
    }
    // Join线程
    for (int i = 0; i < numReaders; i++) {
        readers[i].join();
    }
    for (int i = 0; i < numWriters; i++) {
        writers[i].join();
    }
    return 0;
}