Getline() and cin manipulate dynamic array

Question

I'm totally lost and confused and could use some help. I'm currently working on a small command line-based game. For this I wrote a class Inventory, dynamically creating an array of invSpace-objects, each space representing a pair of a pointer to an Item (another class of mine) and a integer, depicting a quantity. Here's the code:

class invSpace {
public:
    Item *item;
    int quantity;
    invSpace() {
        item = NULL;
        quantity = 0;
    }
};

class Inventory {
private:
    invSpace* spaces = NULL;
    size_t size;

public:
int free_space() {
    int free = 0;
        for (int i = 0; i < size; i++) {
            if (spaces[i].item == NULL) {
                free++;
            }
        }
        return free;
    }

    Inventory() {}
    Inventory(size_t new_size) {                        
        size = new_size;
        spaces = new invSpace[size];
        for (int i = 0; i < size; i++) { //I know this is obsolete because
            spaces[i].item = NULL;       //of the invSpace constructor, I
            spaces[i].quantity = 0;      //just did this for testing
        }

    ~Inventory() {
        delete[] spaces;
    }

    invSpace& operator[](int index) {
        return spaces[index];
    }
};

There are some more methods in this class, like for adding, deleting and searching for items, but those don't matter now. So this is basically just a simple array within one object, dynamically allocating memory in the constructor and with some extra methods. After being created, the array contains zero elements, or Items, so the free_space() method should return the size of the array. But it doesn't. It returns about half of the size.

My first thought was that something went wrong with the allocation. But at a second glance I noticed that the Inventory is totally fine directly after being created; with exactly as many spaces as requested, all of them set to item=NULL/quantity=0. But after a call of getline() at the start of main() that scans user input and saves it to a string for further analyzing, some spaces get filled with random addresses and integers.

Even stranger, with each new call of getline() some spaces are freed, some others filled. As far as my debugging, experimenting and testing goes, none of these addresses belong to any variable in my program, they are just plain random. Also, at no point is there be any interference with the Inventory and the getline() function or the string it returns. In fact, after being created, no part of this object is used anywhere in the code beside the free_space() method. What's even stranger is that spaces in the Inventory class is marked private, so a method is required to meddle with this pointer/array (or so I would expect).

This problem occurs with getline() and cin but not with any of C's <stdio.h> input stream functions. Using malloc() instead of new[] makes no difference. Of course, I could use something like scanf() for the reading from the console. Still, I just want to know why all these things happen. I have absolutely no idea.

Thanks in advance for every answer!

EDIT: I narrowed the whole code so that it still produces the same error, also changed free_space() so that it prints adress and integer if present:

#include <iostream>
#include <string>
#include <map>
using namespace std;

class Item {
public:
    static map<string, Item*> itemlist;
    string name;
    string description;

    Item() {}
    Item(const string new_name, const string new_description) {
        name = new_name;
        description = new_description;
        itemlist.insert(pair<string, Item*> (name, this));      
    }
};

map<string, Item*> Item::itemlist;

/*The more Items are declared, the more random adresses appear in the
inventory*/
Item item01("sword", "A sharp and deadly weapon.");
Item item02("shield", "This will protect you. To a certain extent.");
Item item03("stick", "What is this for exactly?");
Item item04("bottle of water", "A bottle full of refreshing spring water.");

class invSpace {                    
public:
    Item *item;
    int quantity;

    invSpace() {
        item = NULL;
        quantity = 0;
    }
 };

class Inventory {
private:
    invSpace* spaces = NULL;
    size_t size;

public:
    int free_space() {
        int free = 0;
        for (int i = 0; i < size; i++) {
            if (spaces[i].item == NULL) {
                free++;
                cout << i << " = free" << endl;
            }
            else {
                cout << spaces[i].item << " / " << spaces[i].quantity << endl;
            }
        }
        return free;
    }

    Inventory() {}
    Inventory(size_t new_size) {    
        size = new_size;
        spaces = new invSpace[size];
        for (int i = 0; i < size; i++) {
            spaces[i].item = NULL;
            spaces[i].quantity = 0;

        }
    }

    ~Inventory() {
        delete[] spaces;
    }

};

class Player {
public:
    string name;
    Inventory inventory;

    Player(const string new_name) {
        inventory = Inventory(40);
        name = new_name;
    }
 };

Player player("Me");

int main() {
string input;
//Inventory inventory(40); //no error when declared outside the Player class

while (1) {
    cout << "\n>> ";
    getline(cin, input);
    if (input == "x") {
        break;      
    }
    else {
        player.inventory.free_space();
    }
}
}

Some things I noticed: No error occurs if the inventory isn't part of a Player-object. If it is but no Items are declared only the first inventory space receives a random adress (and int value) after the first call of getline(). The more Items there are, the more random adresses I get, it seems...