Unresolved external symbol (singleton class C++) [duplicate] - c++

Question

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 7 years ago.
I already looked up some answers in Stackoverflow for this type of problem, none of which are helping me out. This question describes how to resolve this error, and that I should provide a definition and not just a declaration. I've done that, but I'm still getting the following error:
Error 13 error LNK2019: unresolved external symbol "private: __thiscall NetworkManager::NetworkManager(void)" (??0NetworkManager##AAE#XZ) referenced in function "public: static class NetworkManager * __cdecl NetworkManager::Instance(void)" (?Instance#NetworkManager##SAPAV1#XZ) C:\Users\HIDDEN\Documents\AGK Projects\C++ Libraries\apps\template_windows_vs2013\NetworkManager.obj Template
Here's the code:
NetworkManager.h
#ifndef _H_NETWORKMANAGER_
#define _H_NETWORKMANAGER_
#include<iostream>
#include<vector>
class NetworkManager
{
private:
NetworkManager();
static NetworkManager * netManager;
public:
int networkID;
static NetworkManager * Instance();
int HostNetwork(std::string netName, std::string hostName, int port);
int JoinNetwork(std::string netName, std::string clientName);
bool IsNetworkActive(int netID);
};
#endif
NetworkManager.cpp
#include<iostream>
#include "NetworkManager.h"
#include "template.h"
NetworkManager * NetworkManager::netManager = NULL;
NetworkManager * NetworkManager::Instance()
{
if (!netManager)
netManager = new NetworkManager;
return netManager;
}
int NetworkManager::HostNetwork(std::string netName, std::string hostName, int port)
{
int networdID__;
const char * netName__ = netName.c_str();
const char * hostName__ = hostName.c_str();
networdID__ = agk::HostNetwork(netName__, hostName__, port);
return networdID__;
}
int NetworkManager::JoinNetwork(std::string netName, std::string clientName)
{
int networdID__;
const char * netName__ = netName.c_str();
const char * clientName__ = clientName.c_str();
networdID__ = agk::JoinNetwork(netName__, clientName__);
return networdID__;
}
bool NetworkManager::IsNetworkActive(int netID)
{
switch (agk::IsNetworkActive(netID))
{
case 0: return false; break;
case 1: return true; break;
}
}

Answer 1

You declared NetworkManager::NetworkManager() in the header file but there is no implementation of it in the source file.