Archive for the ‘Coding’ Category

A TDictionary explanation

On the recent Delphi 2009 roadshow in Australia, I had a few people ask me about the new TDictionary<TKey, TValue> container in Generics.Collections. If you haven’t played with it yet,  Roland Beenhakker has a nice write up on using it, but most of the questions I was getting were not about how to use it, but about what it’s for.

The way I try and explain it is by stepping back and looking at collections that people already are used to. Leaving implementation details aside, a lot of the Delphi collections store values and let you retrieve those values using a key.

For example, a TStringList lets you store strings indexed by an integer value. So in this case, the value would be a string and the key would be an integer. A TList lets you store pointers and retrieve them using an integer. Again, the key is an Integer, the value is a pointer.

The next step is to think about TList<T>. It’s not that different, the key is still an integer, just the value can be parameterised using generics.

If you’re ok with that, then a TDictionary<TKey, TValue> is pretty easy to understand. It’s a generic container that lets you parameterise not only the value, but the key also. So, to get somewhat similar behaviour to a TStringList, you could use a TDictionary<integer, string>. Now, of course the implementation details of how TDictionary would store those strings is wildly different to a TStringList, but in terms of understanding it, I think it helps.

Equally, a TList<T> might be replaced in broad terms with a TDictionary<Integer, T>, with T being whatever type you like.

The really cool thing though, is that the key part does not have to be an Integer. You could instead of TDictionary<Integer, String> do something like TDictionary<string, string>, so a string could be retrieved by using another string as a key. If you’re trying to think of an example of that, what about the TDictionary’s real world namesake, a dictionary. The key string would be the word, the definition would be the Value.

Further, it could be TDictionary<TGuid, TState>, where you might be using a GUID as a sessionID to retrieve an object that stores your session state in some sort of distributed system.

An interesting take on this is to use a TDictionary to store code in the form of anonymous methods. Anonymous methods are defined as types, so no reason they can’t be stored in a collection just like data. I find this really interesting. If you’re a fan of using table-driven development to replace convoluted if..then…else or case statements, this might spark some ideas. In fact in my next blog post I’ll show an example of a generic Factory implementation that uses a TDictionary internally to store anonymous factory methods.

Anyway, there is a lot more to know about TDictionary’s, such as hashing functions, comparers, etc, but hopefully this has at least given you a high-level understanding of where you might look at using them.

Delphi 2009 : TQueue<T>.Dequeue vs TObjectQueue<T>.Dequeue

I was poking around inside Generics.Collections the other day while writing some code for a future post on pooling, and I came up against something puzzling.

Generics.Collections has a class called TQueue<T>, which is a generic version of the classic first-in-first-out datastructure. The nice thing of course is that it can now be made type-safe by specifying a type parameter. If you’ve ever looked at a queue, such as the non-generic one that ships with Delphi already, or even thought about a queue datastructure, looking at TQueue<T> will not hold many surprises.

Generics.Collections also has a class called TObjectQueue<T>, which descends from TQueue<T> and adds object ownership (ie. the queue will destroy any objects it holds when you destroy it). This is implemented using an interesting looking notification process that probably deserves investigation in a future post, but again, the object ownership concept should be one many Delphi developers would be familiar with from TObjectList, etc, in previous versions of Delphi.

TQueue<T> has a function called Dequeue which returns the front item off the queue. The signature looks like this:

However, when I came to use TObjectQueue<T> its Dequeue method looks like this:

 

That’s right, a procedure. Internally it still takes the front item off the queue, in fact all it does is call the inherited TQueue<T>.Dequeue, but it doesn’t give it back to you.

I have to admit, my initial reaction to this was to assume I’d navigated to the wrong class so I went back to the starting point and walked my way through it again. Nope, still a procedure. No matter how I navigated to it, it wouldn’t turn into a function.

At this point I took the dogs for a walk and thought about it some more. When I got back, unfortunately, it was still a procedure. Further, after working through the notification code a little, it turns out that not only does TObjectQueue<T>.Dequeue take the front item off the queue and not give it back to you, it also Frees the item.

Now, I’d love to tell you that after careful examination of the rest of the code I discovered what was going on but that would be complete bollocks. Instead, I dashed off an email to Barry Kelly.

The answer comes down to the fact that because TObjectQueue<T> owns the objects, you need the ability to do two different things:

  • Remove the front item from the queue and have it automatically destroyed by its owner, the queue. Maybe this is after you’ve looked at it using Peek and decided you don’t want it. This is what TObjectQueue<T>.Dequeue is for.
  • Remove the front item from the queue, and have it returned to you. Part of this is that it will be removed from the queue’s ownership and it becomes your responsibility to free. This is what the method Extract is for, which to be honest I didn’t notice was there, and is in fact defined up on the TQueue<T> as well.

If you think about the names Dequeue vs Extract, this kind of makes sense. Some small part of me still finds it a little disconcerting that a function turns into a procedure between TQueue<T> and TObjectQueue<T>, but I’m mostly over it by now. Chances are if I’d stumbled upon Extract before Dequeue, none of this would have occurred. Anyway, I changed my pool code to use Extract instead of Dequeue, all my unit tests passed and I was free to go watch the Brisbane Broncos get robbed of a spot in the Rugby League finals.

Anyway, I hope this will help some future poor soul googling away to try and find out why Dequeue changes from a function to a procedure.

Anonymous Methods, Generics and Enumerators

I’ve been playing around with Anonymous Methods in Delphi 2009 a little bit lately, and I thought one of my experiments might be worth sharing.

I decided I would try to extend TList<T> so that when you enumerate over it in a for..in loop, not every item would be returned. Specifically, only items that passed a filter criteria would be returned, and I would use an anonymous method to specify the filter criteria.

This actually turned out to be kind of fun, as I got to play with Enumerators, Generics and Anonymous Methods all in one fairly short piece of code.

So, first I looked at TList<T>. I was hoping there was some way I could specify that it should use a different Enumerator than the default, but unfortunately I could not find a way. So I descended from TList<T> to create a TFilteredList<T> to:

  • add a couple of methods, ClearFilter and SetFilter, which I’ll come back to later
  • define TFilteredEnumerator<T>
  • reintroduce the GetEnumerator method to create an instance of TFilteredEnumerator<T> instead of TList<T>’s standard TEnumerator<T>

I also declared an anonymous method type called TFilterFunction<T> which takes a parameter of type T and returns true or false depending on whether the parameter passed or failed the filter criteria respectively.

Here’s the definition:

Most of the methods on TFilteredEnumerator<T> are not terribly interesting. The constructor takes a reference to an instance of our TFilterFunction<T> anonymous method, which it stores in the FFilterFunction field. This constructor gets called from the aforementioned GetEnumerator method of TFilteredList<T>.

Most of the hard work is done by the MoveNext method, which looks like this:

It is invoked when the Enumerator wants to move to the next item in the List, returning True if it does this successfully, otherwise returning False.  In this case, it:

  • first checks to see if we’re already at the end of the List, in which case it bails out returning False, we’re at the end of the list.
  • otherwise, it keeps incrementing the position until either we’re passed the last item or we hit an item that passes our filter criteria.

ShouldIncludeItem invokes our FIlterMethod if one is defined, passing in the current item in the list. It looks like:

The following code creates a TList<TPerson> and loads it up, then enumerates over each TPerson in the list that is older than 18. You can see the call to SetFilter where we specify the anonymous method that will do the actual filtering. It then clears the filter and enumerates over all the TPerson objects.

 

This produces the following output:

————— Filtered Person List —————

Name = Fred Adult, Age = 37

Name = Mary Adult, Age = 18

————— Unfiltered Person List ————-

Name = Fred Adult, Age = 37

Name = Julie Child, Age = 15

Name = Mary Adult, Age = 18

Name = James Child, Age = 12

The generics come into play so that I can use the same list for something other than TPerson objects, in the example below, Integers:

Now, I’m not sure that I really want to write my code like this. I like the fact that there’s a nice separation between the code that decides which items to act on, and the code that actually acts on them, rather than having them all jumbled together inside the for..in loop.

Ironically, that’s also the bit that I don’t like, as I can see that the filter code could possibly be overlooked by someone not familiar with anonymous methods, and think that we were acting on all the items in the list.

Yet again, I want to have my cake and eat it to.

However, as I said at the start this was an experiment to teach me a bit more about anonymous methods, so from that point of view it worked, and hopefully you got something out of it as well.

You can download the code from my delphi-experiments repository on github.

Nullable Types in Delphi

I was traveling a bunch over the last two weeks, USA, Japan and Korea. In Osaka, I did a session on Generics for the Japan Developer Camp.

Afterwards, I was having a further play with Highlander and Generics and stumbled across something rather nice. Maybe everyone else is aware of this already, but the new Generics support in Delphi for .NET also gives us access to Nullable types. So the following code:

produces the following output in the Listbox:

false

true

18

I’m currently reading through Professional .NET 2.0 Generics to get a better understanding of all the possibilities, but already I’m getting excited by what this allows. I’ll post more as I explore further.


Now, all we need do is to deliver Tiburon to give me Generics in Win32, and I’ll be a happy boy.

Update : Hallvard has written up a good overview of generics in Highlander here