静态const成员变量初始化 [英] static const member variable initialization

问题描述

好像我可以初始化POD静态const成员，但不能初始化其他类型:

Looks like I can init a POD static const member, but not other types:

struct C {
  static const int a = 42;      // OK
  static const string b = "hi"; // compile error
};

为什么?

推荐答案

仅在整数和枚举类型中允许类定义中的语法 initializer .对于 std :: string ，它必须在类定义之外定义并在那里初始化.

The syntax initializer in the class definition is only allowed with integral and enum types. For std::string, it must be defined outside the class definition and initialized there.

struct C {
  static const int a = 42;     
  static const string b; 
};

const string C::b = "hi"; // in one of the .cpp files

必须在一个翻译单元中定义

静态成员才能满足一个定义规则.如果C ++允许以下定义；

static members must be defined in one translation unit to fulfil the one definition rule. If C++ allows the definition below;

struct C {
  static const string b = "hi"; 
};

b 将在包括头文件的每个翻译单元中定义.

b would be defined in each translation unit that includes the header file.

C ++仅允许在类声明中将 integral 或 enumeration 类型的 const static 数据成员定义为快捷方式.无法定义其他类型的 const static 数据成员的原因是，需要进行非平凡的初始化(需要调用构造函数).

C++ only allows to define const static data members of integral or enumeration type in the class declaration as a short-cut. The reason why const static data members of other types cannot be defined is that non-trivial initialization would be required (constructor needs to be called).

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