本文來自 https://stackoverflow.com/questions/1563897/c-static-constant-string-class-member
因為我用 BING 搜這個 error 搜不到，因此記錄下來，方便後人。

問題描述

錯誤的起因是我想在 C++ 的一個類中定義 static const string，並且給這個變數初始化：

class A {
   private:
      static const string RECTANGLE = "rectangle";
}

上面這個程式碼來自第一個連結。

然後就報錯

error: in-class initialization of static data member 'const string Settings::ROOT_PATH' of non-literal type|
error: call to non-constexpr function 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'|
 

解決方法

方法一

該方法來自 sof 的最高贊回答
You have to define your static member outside the class definition and provide the initializer there.
First

// In a header file (if it is in a header file in your case)
class A {   
private:      
  static const string RECTANGLE;
};

and then

// In one of the implementation files
 

const string A::RECTANGLE = "rectangle";

The syntax you were originally trying to use (initializer inside class definition) is only allowed with integral and enum types.

方法二

該方法來自 sof 的次高贊回答
In C++11 you can do now:

class A {
 private:
  static constexpr const char* STRING = "some useful string constant"
 
;
};

更多方法

更多方法請看 sof 的討論