Programming

We use null objects at work, and as another person said they are a safety feature. Here’s how they work: they are wrappers around another type. They provide the same interface as the wrapped type. They store one global instance of the wrapped type, default initialized, in a memory page marked read-only.

Here’s why they are considered a safety feature (note: most of this is specific to c++).

Suppose you have a collection, and you want to write a function that finds an item in the collection. Find can fail, of course. What do you return in that case? Reasonable options would be a null pointer, or std::nullopt. Having find return a std::optional would be perfect, because that’s the exact use case for it. You either found the item or you did not.

Now, the problem is that in most cases you don’t want to copy the item that was found in the collection when you return it, so you want to return a pointer or a reference. Well, std::optional<T&> is illegal. After all, an optional reference has the same semantics as a pointer, no? This means your find function cannot return an optional, it has to return a pointer with the special sentinel value of nullptr meaning “not found”.

But returning nullptr is dangerous, because if you forget to check the return value and you accidentally dereference it you invoke undefined behavior which in this case will usually crash the program.

Here’s where the null object comes in. You make find just return a reference. If the item is not found, you return a reference to the null object of the relevant type. Because the null object always exists, it’s not UB to access it. And because it is default initialized, trying to get a value from it will just give you the default value for that data member.

Basically it’s a pattern to avoid crashing if tou forget to check for nullptr