C ++如何将对象向量声明为类的成员

Question

I'm trying to declare a vector<Item> as a private member of another class Inventory , but it is giving me an error saying that Item is not in scope. Both classes are declared in the same file. I don't know how to change the scope that it looks at or whatever you are supposed to do to make it work.

Here is the code to make absolutely clear what I'm trying to do.

class Inventory {
public:

private:
    vector<Item> inventory;
};

class Item {
public:
    void SetName(string nm)
        { name = nm; };
    void SetQuantity(int qnty)
        { quantity = qnty; };
    void SetPrice(int pric)
        { price = pric; };
    virtual void Print()
        { cout << name << " " << quantity << " for $" << price 
          << endl; };
    virtual ~Item()
        { return; };
protected:
    string name;
    int quantity;
    int price;
};

Answer 1

Item must be defined before its usage as a template argument.

Technically, you may be able to get away with a forward declaration in specific contexts, but to save you time and frustration with learning the exact rules, it is easier to just make sure you have defined it first.

In general, the order of declarations are important. If you use a type in the declaration of another type, the used type must already be defined. Exceptions to this rule involve usage by pointer and reference which only require forward declaration.

Answer 2

As std::vector<Item> is a type in its own right, it must be declared after the declaration of the Item class.

(It's similar to the rule that for class Child : public Base , the declaration of Base needs to appear above that line).

A forward declaration is not insufficient.

One way round this is to use a std::vector<std::shared<Item>> (a vector of smart pointers) but that, of course, changes the structure of the vector. In that case a forward declaration is sufficient.

Answer 3

Define Item first, then Inventory.

class Item {

public:
    void SetName(string nm)
        { name = nm; };
    void SetQuantity(int qnty)
        { quantity = qnty; };
    void SetPrice(int pric)
        { price = pric; };
    virtual void Print()
        { cout << name << " " << quantity << " for $" << price 
          << endl; };
    virtual ~Item()
        { return; };
protected:
    string name;
    int quantity;
    int price;
};

class Inventory {
public:

private:
    vector<Item> inventory;
};