Alternatives of static_pointer_cast for unique_ptr

Question

I understand that using static_pointer_cast with unique_ptr would lead to a shared ownership of the contained data.
In other terms, what I'd like to do is:

unique_ptr<Base> foo = fooFactory();
// do something for a while
unique_ptr<Derived> bar = static_unique_pointer_cast<Derived>(foo);

Anyway doing that results with two unique_ptr that should never exist at the same time, so it is simply forbidden.
Right, it makes sense, absolutely, that's why there doesn't exist anything like static_unique_pointer_cast indeed.

So far, in cases where I want to store pointers to those base classes, but I also need to cast them to some derived classes (as an example, imagine a scenario involving type erasure), I've used shared_ptrs because of what I've above mentioned.

Anyway, I was guessing if there are alternatives to shared_ptrs for such a problem or if they are really the best solution in that case.

Answer 1

Raw pointers

The solution for your problem is to get the raw (non-owning) pointer and cast it - then just let the raw pointer go out of scope and let the remaining unique_ptr<Base> constrol the lifetime of the owned object.

Like this:

unique_ptr<Base> foo = fooFactory();

{
    Base* tempBase = foo.get();
    Derived* tempDerived = static_cast<Derived*>(tempBase);
} //tempBase and tempDerived go out of scope here, but foo remains -> no need to delete

Unique_pointer_cast

The other option is to use the release() function of unique_ptr to wrap it into another unique_ptr.

Like this

template<typename TO, typename FROM>
unique_ptr<TO> static_unique_pointer_cast (unique_ptr<FROM>&& old){
    return unique_ptr<TO>{static_cast<TO*>(old.release())};
    //conversion: unique_ptr<FROM>->FROM*->TO*->unique_ptr<TO>
}

unique_ptr<Base> foo = fooFactory();

unique_ptr<Derived> foo2 = static_unique_pointer_cast<Derived>(std::move(foo));

Remember that this invalidates the old pointer foo

Reference from raw pointers

Just for completeness of the answer, this solution was actually proposed as a small modification of the raw pointers by the OP in the comments.

Similar to using raw pointers one can cast the raw pointers and then create a reference out of them by derefering. In this case it is important to guarantee that the lifetime of the created reference does not exceed the lifetime of the unique_ptr.

Sample:

unique_ptr<Base> foo = fooFactory();
Derived& bar = *(static_cast<Derived*>(foo.get()));
//do not use bar after foo goes out of scope