错误：无效使用不完整类型“类 Move”/未定义对 Move::NONE 的引用

拜托，我不知道为什么这个简单的代码被拒绝。

它给了我 2 个编译错误。 请帮帮我。 ????

I use 代码::块 20.03

我的编译器是GNU GCC

---移动.hpp---

class Move {
    public:
        Move();
        Move(int, int);  
    public:
        int from;
        int to;
    
        const static Move NONE = Move(0,-1); //error here
        //undefined reference if I use 
        //const static Move NONE;
};

---移动.cpp---

#include "move.hpp"
Move::Move() {
    this->from = 0;
    this->to = 0;
}
Move::Move(int from, int to) {
    this->from = from;
    this->to = to;
}

---主.cpp---

#include <iostream>
#include "move.hpp"

int main(){
    Move move = Move::NONE;//error here
    std::cout << move.from << std::endl;
    std::cout << move.to << std::endl;
    return 0;
}

您应该为静态数据成员提供类外定义NONE在源文件（如 move.cpp）中，它将起作用，因为此时该类已完成，如下所示：

move.hpp

#pragma once 
class Move {
    public:
        Move();
        Move(int, int);  
    public:
        int from;
        int to;
        //declaration for NONE
        const static Move NONE    ;
//------------------------------^^------>no in class initializer here
        
};

move.cpp

#include "move.hpp"
//definition for Move::NONE
const Move Move::NONE = Move(0,-1);

Move::Move() {
    this->from = 0;
    this->to = 0;
}
Move::Move(int from, int to) {
    this->from = from;
    this->to = to;
}

工作演示 https://onlinegdb.com/n4x8Rwjiv