Static unordered_map pointer initialitation in C++

Question

I got this C++ code:

Client.h:

#include <unordered_map>

using namespace std;

class Client{
    static unordered_map<int, Client*>* clients;
public:
    static void initializeClients();
}

Client.cpp

#include "Client.h"
#include <unordered_map>

using namespace std;

 void Client::initializeClients(){
     clients = new unordered_map<int, Client*>();
}

But the linker gives me a LNK2001 unresolved external symbol for the unordered_map. I have no idea of what I am doing wrong, but it seems that I am missing something. I am using Visual Studio 2013.

Any idea? Thank you in advice!

Answer 1

you need to declare clients in your cpp file :

#include "Client.h"
#include <unordered_map>

using namespace std;
unordered_map* Client::clients;

 void Client::initializeClients(){
     clients = new unordered_map<int, Client*>();
}

explenation: header file only tells the linker what kinds of symbols some cpp file(s) has. declaring a variable in the header file does not makes this variable appear by it self. here you need to instantiate the static map pointer in the cpp file by declaring it , and make other files know about him by specifing it in the header file

summery : static member variables and global variables need to be declared also in some cpp file in order to get instantiated