我可以初始化 static const class 成员吗？

Question

我要运行这个代码：

struct A {
    static const string d[] = {"1", "2"};    
};

并得到一个错误：

error: in-class initialization of static data member ‘const string A::d []’ of incomplete type
     static const string d[] = {"1", "2"};
                         ^
error: non-constant in-class initialization invalid for non-inline static member ‘A::d’
     static const string d[] = {"1", "2"};
                                        ^
note: (an out of class initialization is required)

Here 找资料说现在可以初始化我想要的东西了。那么问题是什么？

如果我添加 inline 那么它将起作用：

struct A {
    static const inline string d[] = {"1", "2"};    
};

UPD：我知道解决方法，但为什么 C++ 会那样工作？

Answer 1

如您所见，您可以使用 inline 或以下内容：

struct A {
    static const string d[];    
};

和

const string A::d[] = {"1", "2"};

在专用编译单元中（仅一次）。

Live Demo

Answer 2

标准明确指出 class 的 non-inline 静态数据成员只是声明，必须跟在命名空间范围的定义之后：class.static.data#3

3: The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void. The definition for a static data member that is not defined inline in the class definition 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 ([basic.scope.class]).

然后进一步细化这些规则，以便 non-volatile non-inline 整型或枚举类型的 const 静态数据成员可以用大括号初始化（实际上是在声明点定义变量）：class.static.data#4

4: If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]). The member shall still be defined in a namespace scope if it is odr-used ([basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer. An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.

在 non-standardese 中，这表示除了整数或枚举的情况外，您必须与定义分开定义静态成员变量，这解释了您的示例失败的原因。

我可以初始化 static const class 成员吗？

Can I initialize static const class member?

c++

static

initialization

inline

c++17