C ++如何在cpp之间与extern共享常量-错误：指定了存储类

Question

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

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;

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

Answer 1

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.

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

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;
}

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

Answer 2

I think you either want to use static members in your class, use a namespace instead of a class, or put your constants in the global namespace.

For static members:

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

and in your source file

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

However, it looks like your class is more like a namespace than a class, since there would be no point in making instances.

But there is another option. You may not even want to use a class or a namespace, but just have them all in the global namespace:

// header
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
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;