How to Safely Downcast a Unique Pointer of Base to a Unique Pointer of Derived?

Downcasting from Unique Pointer of Base to Unique Pointer of Derived

When working with class hierarchies and inheritance, it may be necessary to downcast a unique pointer from a base class to a derived class to access specific functionality. This is commonly encountered when a factory method returns a unique pointer to the base class but the actual object is of a derived type.

To perform downcasting, it is often recommended to follow a two-step approach. First, release the object from the unique pointer using its release() method. This frees the underlying resource and returns a raw pointer to the derived class object. Secondly, use a function to cast the raw pointer to the desired derived class and reassign it to a new unique pointer.

Here's an example of a CastToDerived() function that performs a safe downcast:

template<typename Derived, typename Base>
std::unique_ptr<Derived> CastToDerived(Base* obj) {
    return std::unique_ptr<Derived>(static_cast<Derived*>(obj));
}

This function can be employed to downcast from a base class unique pointer to a derived class unique pointer:

unique_ptr<Derived> ptr = CastToDerived(DerivedAFactory());

Alternatively, if static casting is unreliable, you can use the following function templates, which perform static or dynamic casts based on the type safety of the downcast:

template<typename Derived, typename Base, typename Del>
std::unique_ptr<Derived, Del> 
static_unique_ptr_cast( std::unique_ptr<Base, Del>& p )
{
    auto d = static_cast<Derived *>(p.release());
    return std::unique_ptr<Derived, Del>(d, std::move(p.get_deleter()));
}

template<typename Derived, typename Base, typename Del>
std::unique_ptr<Derived, Del> 
dynamic_unique_ptr_cast( std::unique_ptr<Base, Del>& p )
{
    if(Derived *result = dynamic_cast<Derived *>(p.get())) {
        p.release();
        return std::unique_ptr<Derived, Del>(result, std::move(p.get_deleter()));
    }
    return std::unique_ptr<Derived, Del>(nullptr, p.get_deleter());
}

These functions ensure that the resource is disposed of properly and the unique pointer is constructed with the appropriate deleter function.

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