无法设置静态对象字段的值（错误LNK2001：未解析的外部符号） [英] Can't set value of static object field (error LNK2001: unresolved external symbol)

问题描述

我有一个看似简单和直接的代码段，这是我在写的游戏中遇到的一个问题的简化版本。我试图在一个类中的静态字段设置为另一个值从我的主要方法。
然而这个代码不会，我不明白为什么。

我收到错误

1> Source.obj：error LNK2001：未解析的外部符号public：
static class A * B :: a（？a @ B @@ 2PAVA @@ A）

$ b b

  class A 
 {
 public：
 A（）
 {
 
} $ b b}; 
 
 class B 
 {
 public：
 static A * a; 
}; 
 
 int main（）
 {
 B :: a = new A; 
} 
  

什么规则说我必须定义我的静态类成员

但是定义的规则是什么？

从 c ++参考它说

定义和ODR

定义是完全定义由声明引入的实体的声明。每个声明都是一个定义，除了以下：

...

4）在类定义内声明静态数据成员

< blockquote>

  struct S {//定义S 
 int n; // define S :: n 
 static int i; // declares，but does not define S :: i 
}; 
 int S :: i = 0; //定义和初始化S :: i 
  

作为一个额外的参考，你也可以在这里检查维基百科，一种定义规则

UPDATE：

我终于找到了目前的（2014年6月2日）最新免费提供的标准参考资料（目前发布的标准的副本可用于约30美元我认为）：

§9.4.2

2 静态数据成员在其类定义中的声明不是定义，可能是不完全的
类型，而不是cv-qualied void。静态数据成员的定义应出现在包含成员类类定义的命名空间
范围中。在命名空间范围的定义中，静态
数据成员的名称应该使用::运算符通过其类名来定义。静态数据成员的
定义中的初始化表达式在其类

的范围内

I have seemingly simple and straightforward segment of code that is a simplified version of a problem I have been having in a game I am writing. I am trying to set a static field in one class to another value from my main method. However this code will not and I don't understand why.

I get the error

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;
}

What is the rule saying that I have to define my static class member outside the class to get it linked?

解决方案

As from your comment

but what is the rule that defines that?

From the c++ reference it says

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

As an additional reference you can also check here Wikipedia, One Definition Rule

UPDATE:
I finally found the current (2nd June 2014) latest freely available standard reference (a copy of the currently released standard is available for about 30$ I think):

§ 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

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