std::unordered_map and linker errors - c++

Question

I'm have some problems with compiling my new project.
Compiler outputs:
1>------ Build started: Project: IRPCore, Configuration: Debug Win32 ------
1> core.cpp
1> Creating library D:\Repo\Inter Role Play Gamemode\sa-mp-0.2-plugin-sdk\IRP\Debug\IRPCore.lib and object D:\Repo\Inter Role Play Gamemode\sa-mp-0.2-plugin-sdk\IRP\Debug\IRPCore.exp
1>core.obj : error LNK2001: unresolved external symbol "public: static class std::tr1::unordered_map<int,struct item,class std::hash<int>,struct std::equal_to<int>,class std::allocator<struct std::pair<int const ,struct item> > > core::itemCache" (?itemCache#core##2V?$unordered_map#HUitem##V?$hash#H#std##U?$equal_to#H#3#V?$allocator#U?$pair#$$CBHUitem###std###3##tr1#std##A)
1>main.obj : error LNK2001: unresolved external symbol "public: static class std::tr1::unordered_map<int,struct item,class std::hash<int>,struct std::equal_to<int>,class std::allocator<struct std::pair<int const ,struct item> > > core::itemCache" (?itemCache#core##2V?$unordered_map#HUitem##V?$hash#H#std##U?$equal_to#H#3#V?$allocator#U?$pair#$$CBHUitem###std###3##tr1#std##A)
1>D:\Repo\Inter Role Play Gamemode\sa-mp-0.2-plugin-sdk\IRP\Debug\IRPCore.dll : fatal error LNK1120: 1 unresolved externals
Here comes core.cpp
#include <iostream>
#include <unordered_map>
#include "core.h"
core::core(void)
{
}
core::~core(void)
{
}
void core::Item_Insert(int uid, item Item)
{
core::itemCache.insert(std::make_pair<int,item>(uid, Item));
return;
}
std::string core::convertNativeStringToString(AMX *amx, cell input)
{
char *string = NULL;
amx_StrParam(amx, input, string);
return string ? string : "";
}
Should I link some more libs or so?
Thanks in advance.
Edit: Here comes core definition from core.h
class core
{
public:
static std::unordered_map<int, item> itemCache;
core(void);
~core(void);
static void Item_Insert(int uid, item Item);
static std::string convertNativeStringToString(AMX *amx, cell input);
private:
};

Answer 1

Add a definition of core::itemCache to core.cpp. In general, static data members of classes are declared in the class definition and defined in a source file.

Answer 2

As #Pete Becker is saying, put this at the top of the cpp:
std::unordered_map<int, item> core::itemCache;
in addition to the
static std::unordered_map<int, item> itemCache;
already in the .h.