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 return objects and its the caller's responsibility to manager them. Ownership isn't shared and so I figure unique_ptr would be suitable. The objects I return generally all derive from a single base class, Object.

For example,

class Object { ... };
class Number : public Object { ... };
class String : public Object { ... };

std::unique_ptr<Number> State::NewNumber(double value)
    {
        return std::unique_ptr<Number>(new Number(this, value));
    }

std::unique_ptr<String> State::NewString(const char* value)
    {
        return std::unique_ptr<String>(new String(this, value));
    }

The objects returned quite often need to be passed to another function, which operates on objects of type Object (the base class). Without any smart pointers the code is like this.

void Push(const Object* object) { ... } // push simply pushes the value contained by object onto a stack, which makes a copy of the value
Number* number = NewNumber(5);
Push(number);

When converting this code to use unique_ptrs I've run into issues with polymorphism. 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. I could allocate objects as the base type, like

std::unique_ptr<Object> number = NewNumber(5);

and pass those to Push - which of course works. However I often need to call methods on the derived type. In the end I decided to make Push operate on a pointer to the object stored by the unique_ptr.

void Push(const Object* object) { ... }
std::unique_ptr<Object> number = NewNumber(5);
Push(number.get());

Now, to the reason for posting. I'm wanting to know if this is the normal way to solve the problem I had? Is it better to have Push operate on the unique_ptr vs the object itself? If so how does one solve the polymorphism issues? I would assume that simply casting the ptrs wouldn't work. Is it common to need to get the underlying pointer from a smart pointer?

Thanks, sorry if the question isn't clear (just let me know).

edit: I think my Push function was a bit ambiguous. It makes a copy of the underlying value and doesn't actually modify, nor store, the input object.

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>);
int main() {
    std::unique_ptr<int> 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.