在线求助，自己定义了类成员链接时却提示无法找到

[贰到不行]

#include <iostream>

class screen
{
public:
	screen &up()
	{
		std::cout << "up" << std::endl;
		return *this;
	}
	screen &down()
	{
		std::cout << "down" << std::endl;
		return *this;
	}
	screen &left()
	{
		std::cout << "left" << std::endl;
		return *this;
	}
	screen &right()
	{
		std::cout << "right" << std::endl;
		return *this;
	}
	enum direction{
		UP, DOWN, LEFT, RIGHT, COUNT
	};
public:
	typedef screen &(screen::*action)();
	static action Menu[screen::COUNT];/* =
	{
		&screen::up, &screen::down, &screen::left, &screen::right
	};*/

};

int main()
{
	screen::Menu[screen::UP] = &screen::up;
	screen::Menu[screen::DOWN] = &screen::down;
	screen::Menu[screen::LEFT] = &screen::left;
	screen::Menu[screen::RIGHT] = &screen::right;
	screen scr;
	for(int i=0; i<screen::COUNT; i++)
	{
		(scr.*(screen::Menu[i]))();
	}
	return 0;
}

[贰到不行]

找不到错误，编译指令为：g++ -Wall -std=c++11 -o proc proc2.cpp 把静态设置为私有的化就没有问题了

[贰到不行]

错误提示为： lifei@lifei-Inspiron-N4050:~/C++/19$ g++ -Wall -std=c++11 -o proc proc2.cpp /tmp/cc3VwBq5.o: In function `main': proc2.cpp:(.text+0x17): undefined reference to `screen::Menu' proc2.cpp:(.text+0x1d): undefined reference to `screen::Menu' proc2.cpp:(.text+0x2d): undefined reference to `screen::Menu' proc2.cpp:(.text+0x33): undefined reference to `screen::Menu' proc2.cpp:(.text+0x43): undefined reference to `screen::Menu' /tmp/cc3VwBq5.o:proc2.cpp:(.text+0x49): more undefined references to `screen::Menu' follow collect2: error: ld returned 1 exit status

[贰到不行]

已经解决了，跟static的作用有关： 即静态数据成员必须在类外进行有关的定义和初始化，在类体中只是完成了声明。