undefined reference to `State::Inventory[abi:cxx11]'

Question

I know this question has been asked before but I can't figure out what's wrong with my program.

I have a class and a header file:

State.cpp

    #include "State.h"
#include "GameObject.h"
#include <iostream>
#include <iomanip>
#include <memory>
#include <iterator>
#include <vector>
#include "wordwrap.h"




using std::string;
using std::unique_ptr;
using namespace std;
using std::list;

/**
 * Current state of the game.
 */

/**
 * Display the description of the room the player is in. */

void State::announceLoc() const {
    this->currentRoom->describe();
}

/**
 * Constructor.
 * @param startRoom Pointer to the room to start in.
 */
State::State(const Room *startRoom) : currentRoom(startRoom) {};

/**
 * Move to a specified room and print its description.
 * @param target Pointer to the room to move to.
 */
void State::goTo(const Room *target) {
    this->currentRoom = target;
    this->announceLoc();
}

GameObject State::addToInv(GameObject* gameobject) {
    Inventory.push_back(gameobject);
}

void State::dispInv(){

    for (auto test : Inventory){
        wrapOut(test->getName());
        wrapEndPara();
    }
}


/**
 * Return a pointer to the current room.
 * @return Pointer to the current room.
 */
const Room* State::getCurrentRoom() const {
    return this->currentRoom;
}

and State.h

#ifndef TEXTADV_STATE_H
#define TEXTADV_STATE_H


#include "Room.h"
#include "GameObject.h"
#include <list>
using std::string;

using namespace std;
using std::list;


class State {
    const Room *currentRoom;
    static list<GameObject*> Inventory;

public:
    GameObject addToInv(GameObject* gameobject);
    void dispInv();
    explicit State(const Room *startRoom);
    void goTo(const Room *target);
    void announceLoc() const;
    const Room* getCurrentRoom() const;

};

However when I try running it I get the issue:

CMakeFiles\ott.dir/objects.a(State.cpp.obj): In function ZN5State8addToInvEP10GameObject': H:/.0 Year 3/C and C++/CW/ott/State.cpp:49: undefined reference to State::Inventory[abi:cxx11]' CMakeFiles\ott.dir/objects.a(State.cpp.obj): In function ZN5State7dispInvEv': H:/.0 Year 3/C and C++/CW/ott/State.cpp:54: undefined reference to State::Inventory[abi:cxx11]' collect2.exe: error: ld returned 1 exit status mingw32-make.exe[3]: * [ott.exe] Error 1 mingw32-make.exe[2]: [CMakeFiles/ott.dir/all] Error 2 CMakeFiles\ott.dir\build.make:225: recipe for target 'ott.exe' failed CMakeFiles\Makefile2:66: recipe for target 'CMakeFiles/ott.dir/all' failed CMakeFiles\Makefile2:78: recipe for target 'CMakeFiles/ott.dir/rule' failed mingw32-make.exe[1]: [CMakeFiles/ott.dir/rule] Error 2 Makefile:117: recipe for target 'ott' failed mingw32-make.exe: * [ott] Error 2

What am I doing wrong and how do I fix this??