C ++类静态结构成员定义

Question

I have not found a way to initialize the class member succesfully inside the constructor and I can not figure out why.

I have a header file:

#pragma once

struct STATE_MOUSE {
  bool moving;
  int left_button;
  int right_button;
  int middle_button;
  bool scroll_up;
  bool scroll_down;
};

class Message {
private:
  static STATE_MOUSE state_mouse;
public:
  Message();
  ~Message();
};

Then I have a source file:

#include "message.hpp"

STATE_MOUSE Message::state_mouse = {false, 0, 0, 0, false, false};

Message::Message() {
  //Would like to initialize state_mouse here somehow.
}

Message::~Message() {

}

Now to the issue. This set up seems to work. However I am used to initialize members inside a constructor and I have not found a way to do that with this static struct member.

The following method does not work, could someone explain why?

state_mouse.moving = false;

Answer 1

When you declare a member as static it will belong to the class with only one instance and not to the objects of the class , therefore you cannot initialize it inside the constructor. The constructor is a special member function which mainly exists to initialize the non static members of a new object.

Note that a static member is shared by all objects of the class and when an object changes it, the change can be seen from all other objects of the same class . If this is what do you want to achieve, then the method you shown is good.

Answer 2

Static member variables are not associated with each object of the class. It is shared by all objects.

If you declare a static variable inside the class then you should define it in the cpp file, otherwise, you can get error undefined reference .

Note that if the static member variable is of const int type (eg int , bool , char ), you can then declare and initialize the member variable directly inside the class declaration in the header file.