C ++如何在cpp之间与extern共享常量-错误：指定了存储类

问题描述

我在头文件中用extern声明了一些常量：

I've the header file in which I declare some constants with extern:

#ifndef CONSTANTS_H
#define CONSTANTS_H

#include <string>

class Constants
{
    public:
        Constants(){}
        extern const std::string DELIMITER;
        extern const std::string FILENAME;
        extern const int BLUCAR;
        extern const int REDCAR;
        extern const int EMPTY;
        extern const int SPARSE_LIM;
    protected:
    private:
};

#endif // CONSTANTS_H

然后在源代码中将它们定义为

Then in the source I define them like this:

#include "constants.h"

extern const std::string DELIMITER = ",";
extern const std::string FILENAME = "cars.csv";
extern const int BLUCAR = 1;
extern const int REDCAR = 2;
extern const int EMPTY = 0;
extern const int SPARSE_LIM = 5;

编译器为什么给我错误：为 DELIMITER指定存储类？

Why the compiler gives me Error: storage class specified for 'DELIMITER'?

推荐答案

首先，这些似乎不是班级成员。您使用 extern 的方式看起来像是免费的。也许在命名空间中。

Firstly, these don't seem to be class members. The way you're using extern it looks like you intended for these to be free. Perhaps in a namespace. Take them out of that class.

然后，在定义它们时，省去 extern 。

Then, when you define them, leave out the extern.

在这种情况下，它的意思是在其他位置查找此变量。您不想在其他地方找到它。它在此处！

In this context it means "find this variable elsewhere". You don't want to find it elsewhere. It's here!

// Header
namespace Constants {
   extern const std::string DELIMITER;
   extern const std::string FILENAME;
   extern const int         BLUCAR;
   extern const int         REDCAR;
   extern const int         EMPTY;
   extern const int         SPARSE_LIM;
}


// Source
namespace Constants {
   const std::string DELIMITER  = ",";
   const std::string FILENAME   = "cars.csv";
   const int         BLUCAR     = 1;
   const int         REDCAR     = 2;
   const int         EMPTY      = 0;
   const int         SPARSE_LIM = 5;
}

请记住，对于 static，您也可以这样做对象定义！

Remember, you'd do the same for static object definitions!

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