Vector of unique pointers of an abstract class type creates problems

Question

I am designing a c++ program aiming to show some object oriented practices. My goal is to have:

1. An "abstract" class that can't be initialized that has two subclasses that can.
2. Use unique pointer to maintain that there is only one copy of these subclasses at once.

I have therefore created a class with a property that is a vector to unique pointers of the abstract class. The idea being that those unique pointers/the vector might hold a mix of the two subclasses.

I have written the following code and I am not sure why it doesn't compile. Somewhere it complains about v tables suddenly when adding the lines in main and I am not sure why that fixes it.

#include <iostream>
#include <vector>
#include <memory>


class Abstract {
protected:
    int capacity;
  public:

    int getCapacity() {
      return capacity;
    }
public:
  virtual~Abstract() = 0;
  virtual bool stub();
};

class Specific : public Abstract {
public:
  Specific() {
    capacity = 9;
  }
  bool stub() {
    return false;
  }
};

class Test {
public:
  std::vector<std::unique_ptr<Abstract>> vec;
  void addIt(std::unique_ptr<Abstract> thing) {
    vec.push_back(std::move(thing));
  }
};


int main() {
  Test t;
  std::unique_ptr<Specific> testPtr(new Specific());
  t.addIt(std::move(testPtr));
}

Perhaps I am going about abstract classes wrong? Perhaps a vector of unique pointers to abstract class objects is a bad idea. Really not sure.