unique_ptr and polymorphism

Question

I have some code that currently uses raw pointers, and I want to change to smart pointers. This helps cleanup the code in various ways. Anyway, I have factory methods that retur

Answer 1

Initially I decided to simply change the definition of Push to use unique_ptrs too, but this generates compile errors when trying to use derived types.

You likely did not correctly deal with uniqueness.

void push(std::unique_ptr);
int main() {
    std::unique_ptr i;
    push(i); // Illegal: tries to copy i.
}

If this compiled, it would trivially break the invariant of unique_ptr, that only one unique_ptr owns an object, because both i and the local argument in push would own that int, so it is illegal. unique_ptr is move only, it's not copyable. It has nothing to do with derived to base conversion, which unique_ptr handles completely correctly.

If push owns the object, then use std::move to move it there. If it doesn't, then use a raw pointer or reference, because that's what you use for a non-owning alias.