为什么需要在类外部定义静态数据成员？ [英] Why does a static data member need to be defined outside of the class?

问题描述

根据静态数据成员：

静态数据成员的声明在类的成员列表中不是定义。您必须在类声明之外的命名空间范围中定义静态成员。

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.

为什么？

推荐答案

这是语言的规则，被称为一个定义规则。在一个程序中，每个静态对象（如果使用）必须定义一次，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.

类定义通常放在头文件中，包含在多个翻译单元即从多个源文件）。如果头中的静态对象的声明是一个定义，那么你最终会得到多个定义，每个单元中包含一个头，这将破坏规则。

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，语言可以做它做什么具有内联函数，允许将多个定义合并为单个定义。但它没有，所以我们坚持这个规则。

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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