c++ - 在 C++ 中修改静态成员变量

Question

我正在尝试定义一个带有静态成员变量 MAX_DIST 的类 Util，在以下意义上，

class Util{

 public:
    static double MAX_DIST;
    Util():MAX_DIST(400.0){}
};

并能够在其他类中更新它，例如

Util::MAX_DIST = 387.98;

这给了我一个错误:

‘double Util::MAX_DIST’ is a static data member; it can only be initialized at its definition

但是，如果我在其定义处初始化 MAX_DIST，例如

class Util{

 public:
    static const double MAX_DIST = 400;
    Util();
};

(我必须按照编译器的指示添加“const”，否则我会收到“ISO C++ 禁止非 const 静态成员的类内初始化”错误)现在我不能在其他地方修改 MAX_DIST，因为它现在只准备好了:

error: assignment of read-only variable ‘Util::MAX_DIST’

在网上搜索无果后，我找不到解决这个悖论的方法。有人可以帮帮我吗？

Answer 1

将其放入您的 Util.cpp(或任何文件名)文件中:

double Util::MAX_DIST = 0;

静态变量需要初始化。

长答案，引用标准 9.4.2 $2:

The declaration of a static data member in its class definition is not a definition and may be of an incomplete type other than cv-qualified void. The definition for a static data member 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 (3.3.7).