问题描述
对于C ++类中的静态成员变量 - 初始化在类外完成。我想知道为什么？这是什么逻辑推理/约束？还是纯粹的传统实现 - 标准不想纠正？
For static member variables in C++ class - the initialization is done outside the class. I wonder why? Any logical reasoning/constraint for this? Or is it purely legacy implementation - which the standard does not want to correct?
我认为在类中初始化是更直观，更少的混乱。它还给出了变量的静态和全局的意义。例如，如果你看到静态const成员。
I think having initialization in the class is more "intuitive" and less confusing.It also gives the sense of both static and global-ness of the variable. For example if you see the static const member.
推荐答案
基本上，这是因为静态成员必须在一个翻译单元，以便不违反。如果语言允许类似：
Fundamentally this is because static members must be defined in exactly one translation unit, in order to not violate the One-Definition Rule. If the language were to allow something like:
struct Gizmo
{
static string name = "Foo";
};
那么 name #include 这个头文件
C ++允许你定义 >静态成员在声明中，但你仍然必须在单个翻译单元中包括一个定义，但这只是一个快捷方式，或语法糖。所以，这是允许的：
C++ does allow you to define integral static members within the declaration, but you still have to include a definition within a single translation unit, but this is just a shortcut, or syntactic sugar. So, this is allowed:
struct Gizmo
{
static const int count = 42;
};
只要a）表达式 const 整数或枚举类型，b）可以在编译时评估表达式，以及c）在某处没有违反一个定义规则的定义：
So long as a) the expression is const integral or enumeration type, b) the expression can be evaluated at compile-time, and c) there is still a definition somewhere that doesn't violate the one definition rule:
file：gizmo.cpp
file: gizmo.cpp
#include "gizmo.h"
const int Gizmo::count;
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