CODEWITHSUNDEEP
c++c++11vectorpolymorphismabstract-class
Humam Helfawi
Humam Helfawi

Reputation: 20294

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

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.

Upvotes: 4

Views: 556

Answers (4)

gsamaras
gsamaras

Reputation: 73394

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

Upvotes: 0

rodrigo
rodrigo

Reputation: 98476

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.

Upvotes: 2

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726849

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.

Upvotes: 6

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

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.

Upvotes: 5

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