C++單嘗試：解析外部符號

Question

我交流＃developper要做C++的東西，我不明白這裏的問題：

namespace myNamespace 
{ 
    class Application 
    { 
    private: 
     Application(void); 
     ~Application(void); 

     // Not copyable 
     Application(const Application&); 
     Application& operator= (const Application&); 

     static Application _instance; 

     [...] 

    public: 
     static Application& current(void); 
    }; 
} 

（這應該是一個單身...）

，這將導致錯誤： 「錯誤LNK2001：無法解析的外部符號 」私人：靜態類了myNameSpace ::應用了myNameSpace ::應用:: _例如「（_instance @應用@了myNameSpace @@ 0V12 @阿？）」

是因爲我正在使用我在類聲明中聲明的類嗎？

非常感謝！

Answer 1

你只聲明_instance在應用類，則需要在.cpp文件中定義它：

namespace myNamespace 
{ 
    Application Application::_instance; 
} 

§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.