使用带向量的自定义类：'std :: vector'默认构造函数错误

Question

EDIT: Adding in a default constructor changed nothing, but adding in a : itemlist(0) initialiser to the Inventory constructor removed that particular error. However, multiple instances of these two error still occur:

'Item': undeclared identifier

and

'std::vector': 'Item' is not a valid template type argument for parameter '_Ty'

I'm wondering if there's some sort of scope issue happening here as regards to my two separate classes?

---

I'm trying to create one class which defines an Item and another class which defines an Inventory, containing a vector list of Items. However, with the solution below, I'm getting multiple errors, most notably

'std::vector': no appropriate default constructor available

...and others which I can only assume lead on from that. Here's my definitions:

header.h

#include <iostream>
#include <string>
#include <vector>
#include "Item.h"
#include "Inventory.h"

Item.h

#include "header.h"
class Item
{
private:
    std::string name;
public:
    Item(std::string n, std::string d, int c);
    std::string getName();
};

Item.cpp

#include "header.h"
using namespace std;

Item::Item(string n)
{
    name = n;
}

string Item::getName()
{
    return name;
}

Inventory.h

#include "header.h"

class Inventory
{
private:
    std::vector<Item> itemlist;

public:
    Inventory();
    std::string getInventory();
    void addItem(Item x);
};

Inventory.cpp

#include "header.h"
using namespace std;

Inventory::Inventory()
{
}

string Inventory::getInventory()
{
    string output = "";

    for (int i = 0; i <= itemlist.size(); i++)
    {
        output = output.append(itemlist[i].getName());
    }

    return output;
}

void Inventory::addItem(Item x)
{
    itemlist.push_back(x);
}

I have a feeling it's something to do with my custom-defined object being somehow incompatible with vectors in the way I've attempted to use them. Is there something fundamentally wrong with all of this or have I just made a simple mistake somewhere?

Answer 1

You need to have a default constructor to use std::vector. The default constructor is the one that has no arguments, ie, Item::Item() { ... }

Answer 2

As mentioned in std::vector<> s reference documentation (emphasis mine):

T The type of the elements.
must meet the requirements of CopyAssignable and CopyConstructible .
The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements.

So you still need to provide a copy constructor and assignment operator as well. Also Item needs to be completely declared, when vector<Item> is instantiated.

---

You can store a smart pointer in your vector though, if it's not possible to provide the required functions for your class, eg:

std::vector<std::unique_ptr<Item>> itemlist;

or

std::vector<std::shared_ptr<Item>> itemlist;

This would have the advantage that you don't have your Item instances copied all the time.