meaw | Utility for Unicode East Asian Width cat2 | User Interface library

There are a few problems with your code:

1. Animal lacks any virtual methods. At the very least, it needs a virtual destructor, so that the destructors of Cats and Dog are called correctly when shared_ptr calls delete on its held Animal* pointer.

2. add() is creating an instance of Animal specifically, regardless of T. So your vector contains only real Animal objects. add() needs to create an instance of T instead. A std::shared_ptr can be assigned to a std::shared_ptr when T derives from Animal. Of course, add() is redundant, main() can just create and add the new objects directly to its vector without using add() at all.

3. When main() calls operator<< on an Animal, it will not call the operator<< defined by Cats or Dog, only the operator<< defined by Animal. This can be fixed by having operator<< in Animal call a virtual method that Cats and Dog override. There is no need to define operator<< in derived classes when the base class also has an operator<<.

Try this instead: