如何在头文件中定义const static std :: string？ [英] How can you define const static std::string in header file?

问题描述

我有一个类，我想存储一个静态 std :: string 是真正的const或有效的const通过getter。

我尝试过几种直接方法

1.

  const static std :: string foo =bar 

2。

  const extern std :: string foo ; //定义在标题底部，如
 ... //标题中剩余的代码
}; //关闭头类声明
 std :: string MyClass :: foo =bar
 /＃endif // MYCLASS_H 
 

我也尝试过

3。

  ：
 static std :: string foo; 
 public：
 static std :: string getFoo（）{return foo; } 
  
 
 
 
这些方法分别因为这些原因而失败：
 
 
 
错误：静态数据成员的类初始化非字符类型的常量字符串MyClass :: foo  > 
 
为 foo 指定的存储类 - 似乎不喜欢组合 extern 与 const 或 static  
 
 
许多未定义的引用我的代码的其他部分甚至getter函数的返回行
 
 
 原因具有在头部而不是源文件中的声明。这是一个将被扩展的类，所有其他函数都是纯虚函数，所以我目前没有其他的理由比这些变量有一个源文件。
 
 
 
这是做什么的？
解决方案
一个方法是定义一个方法， 
 
 
 
例如：
  class YourClass 
 {
 public：
 
 //其他东西... 
 
 const std :: string& GetString（）
 {
 //初始化静态变量
 static std :: string foo（bar）; 
 return foo; 
} 
 
 //其他东西... 
}; 
  
这只会初始化静态字符串一次，每次调用函数都会返回一个常量引用变量。有用于您的目的。
 
I have a class that I would like to store a static std::string that is either truly const or effectively const via a getter.

I've tried a couple direct approaches

 1. 
const static std::string foo = "bar";


2.
const extern std::string foo; //defined at the bottom of the header like so 
...//remaining code in header
};  //close header class declaration
std::string MyClass::foo = "bar"
/#endif // MYCLASS_H


I've also tried  

3.
protected:
    static std::string foo;
public:
    static std::string getFoo() { return foo; }


These approaches fail for these reasons respectively:

error: in-class initialization of static data member const string MyClass::foo of non-literal type
storage class specified for foo -- it doesn't seem to like combining extern with const or static
many 'undefined reference to' errors generated by other parts of my code and even the return line of the getter function
The reason I would like to have the declaration within the header rather than source file. This is a class that will be extended and all its other functions are pure virtual so I currently have no other reason than these variables to have a source file.

So how can this be done?
解决方案
One method would be to define a method that has a static variable inside of it.  

For example:
class YourClass
{
public:

    // Other stuff...

    const std::string& GetString()
    {
        // Initialize the static variable
        static std::string foo("bar");
        return foo;
    }

    // Other stuff...
};
This would only initialize the static string once and each call to the function will return a constant reference to the variable.  Useful for your purpose.

                        
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