c++ - 为什么以下代码可以正常编译，但在使用 static 时链接显示错误

Question

以下代码可以正常编译。但是什么时候去链接，

显示如下错误

Undefined symbols for architecture x86_64:
    "derived::counter", referenced from:
     derived::getAddressCounter()      in main.cpp.o 
     ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

---

我怀疑静电有问题。但不确定为什么。因为一旦我取出静态，代码就可以正常链接了。但是静态在这段代码中是如何发挥作用的呢？

#include <iostream>
#include <string>

struct base_result { };
struct result : public base_result {
    int a;
    std::string b;
};


struct base {
    static base_result counter;

};

struct derived: public base {
    static result counter;

    result * getAddressCounter(){
        counter.a = 10; 
        counter.b = "haha";
        return &counter;
    }   
 };

int main (){ 
    derived d;
    result * ptr;

    ptr = d.getAddressCounter();

    ptr->a = 20; 
    ptr->b = "baba";
    std::cout << ptr->a << std::endl;

    std::cout << ptr->b << std::endl;
    return 0;
}

Answer 1

struct base 
{ 
     static base_result counter; 
}; 

只声明静态成员，您还需要在您的cpp文件中定义一次。

好读: What is the difference between a definition and a declaration?