stringstream causes link error?

I have the following class:

#ifndef CGE_NET_MESSAGE_PARSER_HPP
#define CGE_NET_MESSAGE_PARSER_HPP
#include "Game/platform.hpp"
#include <vector>
#include <string>
#include <sstream>

namespace cge
{
    class NetMessageParser
    {
        static std::stringstream ss;
        static void clearStream();
    public:
        NetMessageParser(void);
        static int parseInt(const std::string &str);
        static float parseFloat(const std::string &str);
        static double parseDouble(const std::string &str);
        static std::vector<int> parseIntVectorString(
            std::string str, char startKey, char endKey, char separator);
        static std::string numberToString(int n);
        static std::string numberToString(float n);
        static std::string numberToString(double n);
        virtual ~NetMessageParser(void);
    };

}
#endif

Which produces the following linker error:

Error 3 error LNK2001: unresolved external symbol "private: static class std::basic_stringstream,class std::allocator > cge::NetMessageParser::ss" (?ss@NetMessageParser@cge@@0V?$basic_stringstream@DU?$char_traits@开发者_运维问答D@std@@V?$allocator@D@2@@std@@A) NetMessageParser.obj

What could be wrong?

You have to define static members outside of the class as well or they will be considered external. add this:

static std::stringstream NetMessageParser::ss;

outside of your class and the linker error should go away.

You have to declare storage for static variables in the CPP file. You probably want something like this

std::stringstream NetMessageParser::ss;

Oh and do not put this in header file otherwise you will get errors about multiple definitions