基于范围的for循环，其中抽象类为声明C ++

Question

I want to iterate through a list of an abstract class list<abstract> using a for loop.

Here's my code:

class Component {
    protected:
        string name;
        int price;

    public:
        Component(string name, int price): name(name), price(price){};

        virtual int getPrice() =0;
};

class Composite : public Component {
    list<Component> components;

    public:
        int getPrice() {
            for(Component comp : components) {
                return comp.getPrice();   
            } 
        }   
};

class Leaf : public Component {
    public:
        int getPrice() {
            return price;
        }
};

But I get an error; cannot allocate an object of abstract type 'Component'. This error makes sense, as abstract types can't be instantiated.

Answer 1

Your code has some problems:

You declare virtual int getPrice() = 0; and this makes your Component pure virtual -> you CANNOT instantiate an object of it -> list<Component> components; does NOT make sense to appear. Please consider to use list<Component*> (or smart-pointer) instead!

Range-based for loop:

for(Component comp : components) {
    return comp.getPrice();   
}

Suppose you have updated your list to list<Component*> then the for loop could be:

for(auto& comp : components) {
    return comp->getPrice();   
}

Answer 2

list<Component> means the list can only contain objects whose type is exactly Component . It cannot contain objects of some other type (even if derived from Component).

Since it is not possible to create (complete) objects of type Component , this list is not usable.

To create a list which can store any object having Component as a base class, the list items must be stored by reference. But then you have to think about lifetime management of the objects in the list.

---

One way to do this would be:

std::list< std::shared_ptr<Component> > components;

and the loop might be:

for(auto& comp : components) {
     return comp->getPrice();   
} 

(although why you're using a loop when you always exit on the first item is another question).

There are other options available too which represent different ownership semantics. I would recommend reading a guide on smart pointers in C++ for further understanding of this topic.

Answer 3

You have to indicate that you want a reference:

for(Component & comp : components)

Or simply

for(auto & comp : components)

If you want (or need), you can also add const modifier.

for(const Component & comp : components)

Note

While my anser might be incomplete, it does explain the error reported in the question. As other have not reported, it make no sense to create a list of abstract object by value.

However the question was specific about the problem related to the loop too. As such, I think that my answer was adequate given the question.