c++11使用atomic多线程同步3种方法

// 使用CAS实现多线程无锁访问共享变量，达到线程同步
# include <iostream>
# include <thread>
# include <mutex>
# include <atomic>
using namespace std;

std::atomic<int> var(0);

void add() {
    int expect = var;

    while (!atomic_compare_exchange_weak(&var, &expect, var + 1));
}

int main(int argc, char *argv[]) {
    int count = 2000;
    std::thread threads[count];

    for (int i = 0; i < count; ++i) threads[i] = std::thread(add);

    for (int i = 0; i < count; ++i) threads[i].join();

    cout << var << endl; // var 为 2000，无误！！

    return 0;
}

// 使用原子数据类型的原子方法操作，达到同步线程
# include <iostream>
# include <thread>
# include <mutex>
# include <atomic>
using namespace std;

std::atomic<int> var(0);


void add() {
    var.fetch_add(1);

    // cout << std::boolalpha ;
    // cout << "is_lock_free:" << var.is_lock_free() << endl;  // true
}

int main(int argc, char *argv[]) {
    int count = 2000;
    std::thread threads[count]; // default-constructed threads

    for (int i = 0; i < count; ++i) threads[i] = std::thread(add);

    for (int i = 0; i < count; ++i) threads[i].join();

    cout << var << endl;

    return 0;
}

// 使用RAII的mutex，同步线程
# include <iostream>
# include <thread>
# include <mutex>
# include <atomic>
using namespace std;

std::mutex mtx;


void add(int *var) {
    std::lock_guard<std::mutex> l(mtx);

    (*var)++;
}

int main(int argc, char *argv[]) {
    int count = 2000;
    std::thread threads[count]; // default-constructed threads


    int *var = new int [1];

    for (int i = 0; i < count; ++i) threads[i] = std::thread(add, var);

    for (int i = 0; i < count; ++i) threads[i].join();

    cout << *var << endl;

    return 0;
}