共享锁，也叫读写锁，主要应用与读多写少的场景。

比如，在多线程环境下，多个线程操作同一个文件，其中读文件的操作比写文件的操作更加频繁，那么在进行读操作时，不需要互斥，线程间可以共享这些数据，随意的读取。但是一旦有写操作，那么一定要进行互斥操作，否则读取到的数据可能存在不一致。

C++14 共享超时互斥锁 shared_timed_mutex

读线程 调用 lock_shared()获取共享锁，写线程 调用 lock() 获取互斥锁。

• 当调用lock()的时候，如果有线程获取了共享锁，那么写线程会等待，直到所有线程将数据读取完成释放共享锁，再去锁定资源，进行修改；
• 当调用lock_shared()时，如果有写线程获取了互斥锁，那么需要等待
• 当调用lock_shared()时，如果有读线程获取共享锁，也会直接返回，获取成功。

代码示例：

std::shared_timed_mutex stmtx;void ReadThread()
{while (true){stmtx.lock_shared();std::cout << "thread id = " << std::this_thread::get_id() << " lock shared  read" << std::endl;std::this_thread::sleep_for(std::chrono::milliseconds(200));stmtx.unlock_shared();std::this_thread::sleep_for(std::chrono::milliseconds(1));}
}void WriteThread()
{while (true){stmtx.lock();std::cout << "thread id = " << std::this_thread::get_id() << " lock  write" << std::endl;std::this_thread::sleep_for(std::chrono::milliseconds(500));stmtx.unlock();std::this_thread::sleep_for(std::chrono::milliseconds(1));}
}int main()
{for(int i = 0; i < 3; ++i){std::thread t(ReadThread);t.detach();}for(int i = 0; i < 2; ++i){std::thread t(WriteThread);t.detach();}getchar();return 0;
}

执行结果：

thread id = 139663449360128 lock  write
thread id = 139663457752832 lock shared  read
thread id = 139663474538240 lock shared  read
thread id = 139663466145536 lock shared  read
thread id = 139663440967424 lock  write
thread id = 139663474538240 lock shared  read
thread id = 139663466145536 lock shared  read
thread id = 139663457752832 lock shared  read
thread id = 139663449360128 lock  write
thread id = 139663457752832 lock shared  read
thread id = 139663466145536 lock shared  read
thread id = 139663474538240 lock shared  read
thread id = 139663440967424 lock  write
thread id = 139663466145536 lock shared  read
thread id = 139663457752832 lock shared  read
thread id = 139663474538240 lock shared  read
thread id = 139663449360128 lock  write
thread id = 139663457752832 lock shared  read
thread id = 139663466145536 lock shared  read
thread id = 139663474538240 lock shared  read
thread id = 139663440967424 lock  write
thread id = 139663457752832 lock shared  read
thread id = 139663466145536 lock shared  read
thread id = 139663474538240 lock shared  read

C++17 共享互斥 shared_mutex

shared_mutex与shared_timed_mutex基本一致，shared_timed_mutex较shared_mutex多了一些与时间相关的接口，如：

try_lock_for(...);
try_lock_shared_for(...);
try_lock_shared_until(...);
try_lock_until(...);

用法和共享超时互斥锁一致，如

std::shared_mutex smtx;
smtx.lock_shared();
smtx.unlock_shared();
smtx.lock();
smtx.unlock();