c++ - 无法修改其他线程中 ref 传递的值

Question

我试图向一位同事演示为什么您最好将 const 引用传递给使用以下代码执行只读操作的函数。令我惊讶的是它打印出“It's safe!”，即使我在另一个线程休眠时更改了 passedBool 的值。

我试图查明我是否在某处打错了字，编译器是否优化了代码并通过复制传递 passedBool 以避免一些开销，或者是否启动另一个线程创建了 passedBool.

class myClass
{
public:
  myClass(bool& iBool)
  {
    t = thread(&myClass::myMethod,this,iBool);
  }

  ~myClass()
  {
    t.join();
  }

private:
  thread t;

  void myMethod(bool& iBool)
  {
    this_thread::sleep_for(chrono::seconds(1));

    if(iBool)
      cout << "It's safe!" << endl;
    else
      cout << "It's NOT safe!!!" << endl;
  }
};


void main()
{
  bool passedBool = true;

  cout << "Passing true" << endl;
  myClass mmyClass(passedBool);  

  cout << "Changing value for false" <<endl;
  passedBool = false;

  cout << "Expect \"It's NOT safe!!!\"" <<endl;
}

Answer 1

The arguments to the thread function are moved or copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref).

from here