shared_mutex

• shared_mutex::lock()用法同mutex::lock()
• shared_mutex::lock_shared()允许多线程同时进入临界区，只用用于只读场景，不然是线程不安全的
• shared_mutex::lock_shared()与shared_mutex::lock()互斥，不能同时上锁

#include <shared_mutex>
#include <iostream>
#include <thread>
using namespace std;

shared_mutex m;
int share_counter{0};

void increment1(int target)
{
    for (int i = 0; i < target; i++)
    {
        m.lock();
        share_counter++;
        m.unlock();
    }
}

void increment2(int target)
{
    for (int i = 0; i < target; i++)
    {
        m.lock_shared();
        share_counter++;
        m.unlock_shared();
    }
}

using incre_fun = void(int);
void test_increment(int target, incre_fun fun)
{
    share_counter = 0;
    thread t1(fun, target);
    thread t2(fun, target);
    t1.join();
    t2.join();
    cout << "Count result: " << share_counter << endl;
}

void test_increment2(int target, incre_fun fun1, incre_fun fun2)
{
    share_counter = 0;
    thread t1(fun1, target);
    thread t2(fun2, target);
    t1.join();
    t2.join();
    cout << "Count result: " << share_counter << endl;
}

void SharedMutexTest()
{
    test_increment(1000000, increment1);              // Count result: 2000000
    test_increment(1000000, increment2);              // Count result: 1906613
    test_increment2(1000000, increment1, increment2); // Count result: 2000000
}