Why I can not use abstract class in std::vector?

Question

I have came from those questions:

They all suggested that I should use pointer or smart pointers instead.

As far as I know, Data are dynamically allocated in std::vector which means that there are pointers internally in the std::vector. So why I can not use abstract classes directly? why I have to use pointers(The one I specified) for pointers(the internally) in order to use abstract classes with std::vector. I know some features like std::vector::resize won't work. However, std::vector::reserve and std::back_inserter will solve the problem.

Did you read the comment under the answer to the third question you linked - "The reason being that you are invoking the copy constructor, and thus trying to instantiate an abstract class." — Jonny Henly, Jun 19 '16 at 20:38

score 6 · Accepted Answer · answered Jun 19 '16 at 20:37

As far as I know, Data are dynamically allocated in std::vector which means that there is pointers internally in the std::vector

That's absolutely right. However, the pointer (actually, one of two pointers in the very common implementation) points to an array of identically-sized elements of type T, not to a single element of T or its subtype. Essentially, the pointer is used to represent an array, not to refer to a class or its subclass.

That is why you need an extra level of indirection - elements of an array T[] are not capable of storing subclasses of T without object slicing.

score 5 · Answer 2 · answered Jun 19 '16 at 20:35

5

Data are dynamically allocated in std::vector which means that there is pointers internally in the std::vector

No, that's a misconception. std::vector allocates an array of instances internally, and you can't create an instance of an abstract class.

answered Jun 19 '16 at 20:35

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score 2 · Answer 3 · answered Jun 19 '16 at 20:38

While it is true that memory for std::vector is allocated dynamically, the fact is that in std::vector<T> objects of type T are stored as copies. And you simply cannot copy abstract classes.

For example, if you have a base class A, a derived class B, and a std::vector<A>. Then, if you try to stored an object of type B in that vector, it will be copied and stored as an A. That is, it will be spliced.

class A {};
class B: public A {};

int main()
{
    std::vector<A> as;
    B b;
    as.push_back(b); //<-- splice!!!
}

That, assuming that class A is copiable (and non-abstract). If it is abstract the compiler will save you the trouble and fail the declaration of the vector.

score 0 · Answer 4 · answered Jun 19 '16 at 20:38

0

In order to insert something into a vector, the template has to first instantiate the class (since it uses the copy constructor). Since this is not possible with an abstract class, you can't do it!

Source

answered Jun 19 '16 at 20:38

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Why I can not use abstract class in std::vector?

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