c++ - 在静态函数中使用静态变量的问题

Question

我有一个完整的静态类，使用 std::map

这是简化的情况

.h

#ifndef KEYBOARD_H
#define KEYBOARD_H

#include <map>

class Keyboad {

    static std::map<char, bool> pressed;    

    static void keyPressedEvent(char _key); 

};

#endif

.cpp

#include "Keyboard.h"

void Keyboard :: keyPressedEvent(char _key) {

    Keyboard :: pressed[_key] = true;

}

但是静态成员变量有问题，因为我得到

Undefined symbols:
"Keyboard::pressed", referenced from:
__ZN15Keyboard7pressedE$non_lazy_ptr in Keyboard.o
(maybe you meant: __ZN15Keyboard7pressedE$non_lazy_ptr)
ld: symbol(s) not found
collect2: ld returned 1 exit status

当我删除它时，它运行正常

为什么会出现这个问题，使用静态变量应该没有问题:/

谢谢

Answer 1

您需要在 .cpp 文件中定义 pressed map :

#include "Keyboard.h"
std::map<char, bool> Keyboard::pressed;

// The rest as before