C++ – Which is better to use? (Enum or Constant)

I rather avoid things like

ViewStatus viewStatus;

since it introduces a new class without giving you anything in return when compared to

boolean isViewed;

But that's a matter of personal taste.

By setting the reference to null you can have a third value, which is not really good, since you can get an unexpected NPEsomewhere; null is just no "first class value".

EnumSet pretty much exists specifically for this purpose, but... it seems like just two much overhead. Especially when reading the values in the part of the code that builds the HQL.

The overhead of an EnumSet is reasonable, it caches the class and an array of all values, and uses a long for storing the data (as long as the enum is small enought). Its equals method is quite fast, but obviously slower than reference comparison. You mentioned using HQL, so most probably the runtime will be dominated by the database access, so IMHO you're needless and premature optimization.

The mutability of the EnumSet may be a problem sometimes, but there's ImmutableEnumSet in guava.

I'll go for an EnumSet because I want to introduce methods on that enum that wouldn't make sense to be called on ALL. For example ViewStatus.isViewed().

I don't think you stated all your alternatives right. You can have both

enum ViewStatus {VIEWED, UNVIEWED}

and

enum ViewStatuses {VIEWED, UNVIEWED, ALL}

where the second is an replacement for the EnumSet. That said, I agree with your decision.

I agree with others that this seems overengineered. Usually, you want either a simple enum or a complex hierarchy of classes, it's not a good idea to combine the two.

But if you really want to do this (in C#), I think it's useful to recap what exactly do you want:

• Separate types for the hierarchy Kingdom, Phylum, etc., which do not form inheritance hierarchy (otherwise, Phylum could be assigned to Kingdom). Though they could inherit from a common base class.
• Each expression like Animalia.Chordata.Aves has to be assignable to a variable, which means we have to work with instances, not nested static types. This is especially problematic for the root type, because there are no global variables in C#. You could solve that by using a singleton. Also, I think there should be only one root, so the code above would become something like Organisms.Instance.Animalia.Chordata.Aves.
• Each member has to be a different type, so that Animalia.Chordata compiled, but Plantae.Chordata didn't.
• Each member needs to somehow know all its children, for the IsMember() method to work.

The way I would implement these requirements is to start with a class like EnumSet<TChild> (though the name could be better), where TChild is the type of the children of this level in hierarchy. This class would also contain a collection of all its children (see later about filling it). We also need another type to represent leaf level of the hierarchy: non-generic EnumSet:

abstract class EnumSet
{}

abstract class EnumSet<TChild> : EnumSet where TChild : EnumSet
{
    protected IEnumerable<TChild> Children { get; private set; }

    public bool Contains(TChild child)
    {
        return Children.Contains(child);
    }
}

Now we need to create a class for each level in the hierarchy:

abstract class Root : EnumSet<Kingdom>
{}

abstract class Kingdom : EnumSet<Phylum>
{}

abstract class Phylum : EnumSet
{}

And finally some concrete classes:

class Organisms : Root
{
    public static readonly Organisms Instance = new Organisms();

    private Organisms()
    {}

    public readonly Animalia Animalia = new Animalia();
    public readonly Plantae Plantae = new Plantae();
}

class Plantae : Kingdom
{
    public readonly Anthophyta Anthophyta = new Anthophyta();
}

class Anthophyta : Phylum
{}

class Animalia : Kingdom
{
    public readonly Chordata Chordata = new Chordata();
}

class Chordata : Phylum
{}

Notice that children are always fields of the parent class. What this means is that to fill the Children collection, we can use reflection:

public EnumSet()
{
    Children = GetType().GetFields(BindingFlags.Instance | BindingFlags.Public)
                        .Select(f => f.GetValue(this))
                        .Cast<TChild>()
                        .ToArray();
}

One problem with this approach is that Contains() always works only one level down. So, you can do Organisms.Instance.Contains(animalia), but not .Contains(chordata). You can do that by adding overloads of Contains() to the specific hierarchy classes, e.g.:

abstract class Root : EnumSet<Kingdom>
{
    public bool Contains(Phylum phylum)
    {
        return Children.Any(c => c.Contains(phylum));
    }
}

But this would be a lot of work for deep hierarchies.

After all of this, you end up with quite a lot of repetitive code. One way to fix that would be to have a text file that describes the hierarchy and use a T4 template to generate all the classes based on that.