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Code Smells Catalog (luzkan.github.io)

submitted 2 weeks ago by [email protected] to c/[email protected]

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[–] [email protected] 11 points 2 weeks ago* (last edited 2 weeks ago) (2 children)

The items don't seem concise and always clear. But seems like a good, inspiring resource for things to consider.

If it is expected that a method might fail, then it should fail, either by throwing an Exception or, if not - it should return a special case None/Null type object of the desired class (following the Null Object Pattern), not null itself.

I've never heard of evading null with a Null object. Seems like a bad idea to me. Maybe it could work in some language, but generally I would say prefer result typing. Introducing a result type wrapping or extending the result value type is complexity I would be very evasive to introduce if the language doesn't already support result wrapper/state types.

[–] [email protected] 8 points 2 weeks ago

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

[–] [email protected] 2 points 2 weeks ago (2 children)

I’ve never heard of evading null with a Null object.

This is quite standard, and in fact it's even a safety feature. C++ introduced nullptr defined as an instance of std::nullptr_t explicitly with this in mind.

https://en.cppreference.com/w/cpp/language/nullptr

This approach is also quite basic in monadic types.

[–] [email protected] 4 points 2 weeks ago* (last edited 2 weeks ago) (2 children)

with this in mind

With what in mind? Evading NULL?

Languages that make use of references rather than pointers don't have this Dualism. C# has nullable references and nullability analysis, and null as a keyword.

What does your reasoning mean in that context?

[–] [email protected] 4 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

Languages that make use of references rather than pointers don’t have this Dualism.

It's not about references vs pointers. You could easily have a language that allowed "null references" (edit: too much C++; of course many languages allow null references, e.g. Javascript) or one that properly separated null pointers out in the type system.

I agree with your point though, using a special Null value is usually worse than using Option or similar. And nullptr_t doesn't help with this at all.

[–] [email protected] 0 points 2 weeks ago

With what in mind? Evading NULL?

Depends on your perspective. It's convenient to lean on type checking to avoid a whole class of bugs. You can see this either as avoiding NULL or use your type system to flag misuses.

Languages that make use of references rather than pointers don’t have this Dualism. C# has nullable references and nullability analysis, and null as a keyword.

C#'s null keyword matches the monadic approach I mentioned earlier. Nullable types work as a Maybe monad. It's the same concept shoehorned differently due to the different paths taken by these languages.

[–] [email protected] 0 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

"Monadic type" has something like three meanings depending on context, and it's not clear which one you mean. One of them is common in math, but not so common in programming, so probably not that. But neither "parametric types with a single argument" nor "types that encode a category-theoretic monad" have the property you say, as far as I know.

I imagine you're probably referring to the latter, since the optional monad exists. That's very different from returning null. The inhabitants of Integer in Java, for example, are the boxed machine ints and null. The inhabitants of Optional[Integer] (it won't let me use angle brackets here) are Optional.of(i) for each machine int i, Optional.empty(), and null.

Optional.empty() is not null and should not be called a "Null object." It's also not of type Integer, so you're not even allowed to return it unless the function type explicitly says so. Writing such function types is pretty uncommon to do in java programs but it's more normal in kotlin. In languages like Haskell, which don't have null at all, this is idiomatic.

[–] [email protected] 0 points 2 weeks ago (1 children)

I think you're trying too hard to confuse yourself.