Singleton causing linker errors: "already defined" - c++

Question

I wrote a simple class which manages the current execution session. Therefore, I decided to use a singleton pattern but the compilation crashes at linked step. This is the error:
error LNK2005: "class Session * session" (?session##3PAVSession##A) already defined in ClientTsFrm.obj
(...)client\FrmSaveChat.obj TeamTranslate
error LNK2005: "class Session * session" (?session##3PAVSession##A) already defined in ClientTsFrm.obj
(...)client\Login.obj TeamTranslate
error LNK2019: unresolved external symbol "protected: __thiscall Session::Session(void)" (??0Session##IAE#XZ)
referenced in function "public: static class Session * __cdecl Session::Instance(void)" (?Instance#Session##SAPAV1#XZ)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_nick" (?_nick#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_service" (?_service#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_serverAddress" (?_serverAddress#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_googleAPI" (?_googleAPI#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_language" (?_language#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static int Session::_numbLanguageSelected" (?_numbLanguageSelected#Session##0HA)
(...)client\Session.obj TeamTranslate
error LNK2001: unresolved external symbol "private: static char const * const Session::_translationEngine" (?_translationEngine#Session##0PBDB)
(...)client\Session.obj TeamTranslate
error LNK1120: 8 unresolved externals
(...)client\bin\TeamTranslate.exe TeamTranslate
IntelliSense: identifier "wxZipStreamLink" is undefined
(..)\include\wx\zipstrm.h 417 5
session.h (singleton)
#ifndef __SESSION_H__
#define __SESSION_H__
#include <cstring>
#include <stdio.h>
class Session {
public:
static Session* Instance();
void setNick(char* nick);
const char* getNick();
void setService(char* serv);
const char* getService();
void setLanguage(char* lang);
const char* getLanguage();
const char* getServerAddress();
void setServerAddress(char *sv);
void setGoogleAPIKey(char* code);
const char* getGoogleAPIKey();
void setNumbLanguageSelected(int v);
int getNumbLanguageSelected();
const char* Session::getTranslationEngine();
void Session::setTranslationEngine(char *sv);
char* Session::getGoogleURLTranslation();
void update();
bool read();
protected:
Session();
private:
static Session* _instance;
static const char* _nick; // client nickname
static const char* _service; // service used to translation (Google, Bing,....)
static const char* _serverAddress;
static const char* _googleAPI;
static const char* _language;
static int _numbLanguageSelected;
static const char* _translationEngine;
};
#endif
Session.cpp
#include "Session.h"
Session* Session::_instance = 0;
Session* Session::Instance() {
if (_instance == 0) {
_instance = new Session;
_instance->read();
}
return _instance;
}
void Session::setGoogleAPIKey(char* code){
_googleAPI = strdup(code);
}
const char* Session::getGoogleAPIKey(){
return _googleAPI;
}
void Session::setLanguage(char* lang){
_language = strdup(lang);
}
const char* Session::getLanguage(){
return _language;
}
void Session::setNick(char* nick){
_nick = strdup(nick);
}
const char* Session::getNick(){
return _nick;
}
void Session::setService(char* serv){
_service = strdup(serv);
}
const char* Session::getService(){
return _service;
}
const char* Session::getServerAddress(){
return _serverAddress;
}
void Session::setServerAddress(char *sv){
_serverAddress = strdup(sv);
}
void Session::setNumbLanguageSelected(int v){
this->_numbLanguageSelected = v;
}
int Session::getNumbLanguageSelected(){
return _numbLanguageSelected;
}
const char* Session::getTranslationEngine(){
return _translationEngine;
}
void Session::setTranslationEngine(char* sv){
_translationEngine = strdup(sv);
}
void Session::update(){
FILE* config = fopen("conf\\config.txt", "w");
fprintf(config, "%s\n", _serverAddress);
fprintf(config, "%s\n", _nick);
fprintf(config, "%d\n", _numbLanguageSelected);
fprintf(config, "%s\n", _language);
fprintf(config, "%s", _translationEngine);
fflush(config);
fclose(config);
}
bool Session::read(){
FILE * config;
if (config = fopen("conf\\config.txt", "r"))
{
fscanf(config, "%s", _serverAddress);
fscanf(config, "%s", _nick);
fscanf(config, "%d", _numbLanguageSelected);
fscanf(config, "%s", _language);
fscanf(config, "%s", _translationEngine);
fflush(config);
fclose(config);
return true;
} else
return false;
}
char* Session::getGoogleURLTranslation(){
return "https://www.googleapis.com/language/translate/v2?key=";
}
Example about how I call the singleton:
#include "../data/Session.h"
static Session* session = Session::Instance();
Can you help me to fix the errors?
thanks in advance.

Answer 1

One of your headers (which you didn't show, but it is included in both "ClientTsFrm.cpp" and "FrmSaveChat.cpp") defines a variable called "session" of type Session*.
The other errors are caused by your forgetting to define most static members of Session.