You’re probably familiar with const member functions such as:

   class A  {  public:    int get_x() const; // can't change A's state  };

C++ also supports volatile member functions. You declare a member function with the volatile specifier to ensure that it can be called safely for a volatile object:

   class B  {    int x;  public:    void f() volatile; // volatile member function  };  int main()  {    volatile B b;  // b is a volatile object    b.f();  // call a volatile member function safely  }

The object b is declared volatile. Calling a non-volatile member function from this object is unsafe, because b’s state might have been changed by a different thread in the meantime. To ensure that f() can be called safely for a volatile object, it’s declared volatile too.