Printing a map of object, which has another object as Key

I am trying to print the contents of my map <Object, int>. Here is the code in question:

void Inventory::print_initial_inventory()
{
    for(auto p : inventory) {
        std::cout << p.first << " : " << p.second << std::endl;
    }
}

std::ostream& operator<<(std::ostream& outstream, Inventory& inv) 
{
    outstream << inv.first << "\nNumber of parts: " << inv.second << std::endl;
    return outstream;
}

I know the problem is at p.first? because std::cout doesn't know how to print an object, so I tried to overload the operator<< , but I'm not sure how to do it. Can anyone point me in the right direction?

EDIT Here's how I've tried the problem again. I was suggested to pass the key type to the operator<< overload. Here is my code now:

void Inventory::print_initial_inventory()
{
    for(auto x : inventory) {
        std::cout << x.first << " : " << x.second << std::endl;
    }
}

std::ostream& operator<<(std::ostream& outstream, Auto_Part& inv)
{
    outstream << "Type: " << inv.type << "\nName: " << inv.name << "\nPart Number: " << inv.part_number << "\nPrice: $" << inv.price << std::endl;
    return outstream;
}

I'm still getting an invalid binary expression error pointing to x.first.