Unresolved external symbol (singleton class C++)

Question

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.