How to Safely Downcast a `unique_ptr` to a Derived Type?

How to Perform "Downcasting" with unique_ptr

In certain programming scenarios, you may encounter a need to convert a unique_ptr of a base class to a unique_ptr of a derived class. This is known as "downcasting." Understanding how to perform this operation effectively is crucial for maintaining type safety and avoiding potential errors.

Consider a situation where you have factories that return unique_ptr of a base class. However, these factories internally work with pointers to various derived types, such as DerivedA, DerivedB, and so on. You may require a way to convert the returned base class unique_ptr to a specific derived class level, as illustrated in the following pseudocode:

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

While this approach might seem intuitive, it can lead to runtime errors or unexpected behavior. A more robust and recommended solution involves releasing the object from the base class unique_ptr and then casting the raw pointer to the desired derived type, as shown below:

unique_ptr<Derived> CastToDerived(Base* obj) {
    return unique_ptr<Derived>(static_cast<Derived*>(obj));
}

By employing this technique, you can safely perform downcasting without compromising type safety. However, keep in mind that the release operation should be explicitly performed by the caller before invoking the CastToDerived function.

To cater to scenarios where the factories reside in dynamically loaded DLLs, you may need to consider using function templates like static_unique_ptr_cast and dynamic_unique_ptr_cast. These templates ensure that the produced objects are destroyed in the same context where they were created. Moreover, they offer two variants of casting:

• static_unique_ptr_cast: Use this template when you have certainty that the pointer is a valid representation of the Derived type.
• dynamic_unique_ptr_cast: Employ this template if you need to dynamically verify that the pointer is a valid representation of the Derived type through dynamic_cast.

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