C++11特性用法：原子操作

原子操作：是说其在执行过程中是不可能被其它线程打断的，像C++中的std::atomic修饰过的变量，对这类变量的操作无需传统的加锁保护，因为C++会确保在变量的修改过程中不会被打断。

1、std::atomic应用：防止多线程数据竞争
说明：Objects of atomic types contain a value of a particular type (T).
The main characteristic of atomic objects is that access to this contained value from different threads cannot cause data races (i.e., doing that is well-defined behavior, with accesses properly sequenced). Generally, for all other objects, the possibility of causing a data race for accessing the same object concurrently qualifies the operation as undefined behavior.

代码：

// constructing atomics
#include <iostream>       // std::cout
#include <atomic>         // std::atomic, std::atomic_flag, ATOMIC_FLAG_INIT
#include <thread>         // std::thread, std::this_thread::yield
#include <vector>         // std::vector

std::atomic<bool> ready (false); // 原子对象，bool类型，初始化为false
std::atomic_flag winner = ATOMIC_FLAG_INIT;

void count1m (int id) {
  while (!ready) { std::this_thread::yield(); }      // wait for the ready signal
  for (volatile int i=0; i<1000000; ++i) {}          // go!, count to 1 million
  if (!winner.test_and_set()) { std::cout << "thread #" << id << " won!\n"; }
};

int main ()
{
  std::vector<std::thread> threads; // 多线程都需要访问原子对象但不会竞争
  std::cout << "spawning 10 threads that count to 1 million...\n";
  for (int i=1; i<=10; ++i) threads.push_back(std::thread(count1m,i));
  ready = true;
  for (auto& th : threads) th.join();

  return 0;
}

参考资料：std::atomic::atomic