C++多線程 ：std::lock_guard

lock_guard:這個對象僅有構造函數和析構函數。沒有其他成員函數。

// lock_guard example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock_guard
#include <stdexcept>      // std::logic_error

std::mutex mtx;

void print_even (int x) {
  if (x%2==0) std::cout << x << " is even\n";
  else throw (std::logic_error("not even"));
}

void print_thread_id (int id) {
  try {
    // using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
    std::lock_guard<std::mutex> lck (mtx);
    print_even(id);
  }
  catch (std::logic_error&) {
    std::cout << "[exception caught]\n";
  }
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_thread_id,i+1);

  for (auto& th : threads) th.join();

  return 0;
}

//run 1
 1 [exception caught]
4 is even
[exception caught]
[exception caught]
2 is even
6 is even
[exception caught]
8 is even
[exception caught]
10 is even

// run 2
[exception caught]
4 is even
10 is even
[exception caught]
[exception caught]
2 is even
6 is even
[exception caught]
8 is even
[exception caught]

std::lock_guard只有構造函數和析構函數，沒有其他的成員函數，所以僅僅是上鎖和解鎖的功能

參考文檔：
http://www.cplusplus.com/reference/mutex/lock_guard/