stl vector.push_back() abstract class doesn't compile

Question

Hi,

Let's say I have an stl vector containing class type "xx". xx is abstract. I have run into the issue where the compiler won't let me "instantiate" when i do something like the following:

std::vector<xx> victor;
void pusher(xx& thing)
{
    victor.push_back(thing);
}

void main()
{
    ;
}

I assume this is because the copy constructor must be called. I have gotten around this issue by storing xx*'s in the vector rather than xx's. Is there a better solution? What is it?

Answer 1

When you use push_back, you are making a copy of the object and storing it in the vector. As you surmised, this doesn't work since you can't instantiate an abstract class, which is basically what the copy-construction is doing.

Using a pointer is recommended, or one of the many smart-pointer types available in libraries like boost and loki.

Answer 2

std::vector (and the whole STL in general) is designed to store values. If you have an abstract class, you don't intent to manipulate value of this type, but to manipulate pointer or references to it. So have a std::vector of abstract class doesn't make sense. And even if it isn't abstract, it doesn't usually make sense to manipulate class designed to be base classes as value, they usually should be noncopiable (copy constructor and assignment operator declared private and not implemented is the standard trick, inheriting from a suitable class whose lone purpose is to make its descendant non copiable -- boost::noncopyable for instance -- has become fashionanle).

Answer 3

To be more Catholic than the Pope (or in this case Steve Guidi) the requirements for objects stored in STL containers are that they are copy-constructable and assignable and an abstract class is neither. So in this case a container of pointers is the way to go. Another thing to consider in case you decide to fix things by not making the class abstract would be slicing.