根据 静态数据成员在 IBM C++ 知识中心:
According to Static data members on the IBM C++ knowledge center:
类的成员列表中静态数据成员的声明不是定义.您必须在类声明之外的命名空间范围内定义静态成员.
The declaration of a static data member in the member list of a class is not a definition. You must define the static member outside of the class declaration, in namespace scope.
这是为什么?关于内存分配的原理图是什么?
Why is that? What's the schematic behind that regarding the memory allocation?
这是语言的一条规则，称为一个定义规则.在一个程序中，每个静态对象(如果使用的话)必须定义一次，而且只能定义一次.
It's a rule of the language, known as the One Definition Rule. Within a program, each static object (if it's used) must be defined once, and only once.
类定义通常在头文件中，包含在多个翻译单元中(即来自多个源文件).如果标题中的静态对象声明是一个定义，那么您最终会得到多个定义，每个包含标题的单元中都有一个定义，这会破坏规则.因此，它不是一个定义，您必须在其他地方提供一个定义.
Class definitions typically go in header files, included in multiple translation units (i.e. from multiple source files). If the static object's declaration in the header were a definition, then you'd end up with multiple definitions, one in each unit that includes the header, which would break the rule. So instead, it's not a definition, and you must provide exactly one definition somewhere else.
原则上，该语言可以对内联函数执行它所做的事情，允许将多个定义合并为一个.但事实并非如此，所以我们坚持这条规则.
In principle, the language could do what it does with inline functions, allowing multiple definitions to be consolidated into a single one. But it doesn't, so we're stuck with this rule.
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