c++ - 无法设置静态对象字段的值(错误 LNK2001 : unresolved external symbol)

Question

我有一段看似简单明了的代码，它是我在编写的游戏中遇到的问题的简化版本。我试图将一个类中的静态字段设置为我的主方法中的另一个值。 但是这段代码不会，我不明白为什么。

我收到错误

1>Source.obj : error LNK2001: unresolved external symbol "public: static class A * B::a" (?a@B@@2PAVA@@A)

class A
{
public:
    A()
    {

    }
};

class B
{
public:
    static A* a;
};

int main()
{
    B::a = new A;
}

规则是什么规定我必须在类外部定义静态类成员才能链接它？

Answer 1

根据您的评论

but what is the rule that defines that?

来自c++ reference它说

Definitions and ODR

Definitions are declarations that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:

...

4. Declaration of a static data member inside a class definition

struct S {    // defines S
    int n;        // defines S::n
    static int i; // declares, but doesn't define S::i
};
int S::i = 0; // defines and initializes S::i

作为附加引用，您还可以在此处查看 Wikipedia, One Definition Rule

---

我终于找到了当前的(2014年6月2日)latest freely available standard reference (我认为当前发布的标准的拷贝售价约为 30 美元):

§ 9.4.2

2 The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member shall appear in a namespace scope enclosing the member’s class definition. In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the :: operator. The initializer expression in the definition of a static data member is in the scope of its class