What use is this code?

Question

I can't figure out the use for this code. Of what use is this pattern?

[code repeated here for posterity]

public class Turtle<T> where T : Turtle<T>
{
}

Its turtles all the way down... – Juliet Sep 05 '09 at 02:50 — Juliet, Sep 05 '09 at 02:50
Wha!? I've gotta be missing something... – Justin Niessner Sep 05 '09 at 02:51 — Justin Niessner, Sep 05 '09 at 02:51
Needs more turtles, if you ask me. – Corey Sep 05 '09 at 02:51 — Corey, Sep 05 '09 at 02:51

score 9 · Accepted Answer · answered Sep 05 '09 at 05:03

9

This pattern essentially allows you to refer to a concrete subclass within the parent class. For example:

public abstract class Turtle<T> where T : Turtle<T>
{
    public abstract T Procreate();
}

public class SeaTurtle : Turtle<SeaTurtle>
{
    public override SeaTurtle Procreate()
    {
        // ...
    }
}

Versus:

public abstract class Turtle
{
    public abstract Turtle Procreate();
}

public class SnappingTurtle : Turtle
{
    public override Turtle Procreate()
    {
        // ...
    }
}

In the former, it's specified that a SeaTurtle's baby will be a SeaTurtle.

answered Sep 05 '09 at 05:03

dahlbyk

75,175
8
100
122

Do you think, this kind of thing would not be required, if there is support for contra variance? – shahkalpesh Sep 05 '09 at 05:20
1

It's got more uses. It may implement interfaces for the subclass. Like, in Java, `java.lang.Enum` uses the pattern to implement `java.lang.Comparable` for the subclass. I'm sure C# has something similar. – Johannes Schaub - litb Sep 05 '09 at 07:07
There are certainly other uses, but they all involve the parent class needing to use the type of its subclass. Regarding implementation of interfaces, you have two choices: implement the interface for T or Turtle. IComparable would only let you compare items of the same subclass, where IComparable> would let you compare any Turtle. And since .NET 4.0's IComparable will be contravariant in T, you will be able to use an IComparable> as an IComparable because T : Turtle. – dahlbyk Sep 05 '09 at 19:40

score -1 · Answer 2 · answered Sep 05 '09 at 02:50

-1

There is no use that I can see. Basically, it's the same as

public class Turtle
{
}

answered Sep 05 '09 at 02:50

AngryHacker

59,598
102
325
594

1

It's not the same because in the code given, `Turtle` cannot be instantiated. – strager Sep 05 '09 at 02:53
1

Yes it can... see Marc Gravell's comment here http://stackoverflow.com/questions/194484/whats-the-strangest-corner-case-youve-seen-in-c-or-net/1332344#1332344 – Thomas Levesque Sep 05 '09 at 02:58
@Levesque, And see RCIX's comment following. – strager Sep 05 '09 at 03:01
My question is where does it have any use? – RCIX Sep 05 '09 at 03:15
Something just occured to me. You can subclass `Turtle` and instantiate that but you can't instantiate a copy of `Turtle` itself. – RCIX Sep 05 '09 at 03:32
@RCIX, But can you subclass Turtle? Doesn't it require a generic parameter? With generics, you are creating a type, not a "copy" or instance. – strager Sep 05 '09 at 03:33
@strager : Yes, you can. Look at Marc's comment, he gives an example. – Thomas Levesque Sep 05 '09 at 03:45
Ah, I misunderstood the comments to that answer. Sorry! – strager Sep 05 '09 at 05:42

What use is this code?

2 Answers2