The Wiert Corner – irregular stream of stuff

Though I try to refrain from using the singleton pattern when possible (I prefer dependency injection over accessing “globals” that usually tend up to be not so global after all), sometimes I need to.

In C# there has been a canonical singleton pattern implementation by John Skeet for a very long time at [WayBack] C# in Depth: Implementing the Singleton Pattern.

I just found out there has been similar implementations in Delphi both in this question (which like many useful questions is marked “closed”, oh the joy of StackOverflow): [WayBack] “Delphi Singleton Pattern – Stack Overflow:

• [WayBack] a implementation using locking by [WayBack] David Heffernan that creates at most one instance
• [WayBack] a lock free implementation by [WayBack] Ian Boyd that could create two instances but always returns the first and disposes the second when it detects the first was instantiated

I like both implmentation, but usually take the first as mostly locking has little overhead and sometimes two instances cannot co-exist (though that’s lessened when part of the singleton uses lazy initialisation, for instance from Spring4D).

–jeroen

via: [WayBack] May I please ask for your opinion on the following code:..singleton… – John Kouraklis – Google+

I think it was Stefan Glienke who pointed me at the Memoisation pattern: [WayBack] Implementing Memoize in Delphi 2009 – Community Blogs – Embarcadero Community

Or in laymans terms: caching with generic functions and dictionaries.

Note the Archive.is link is way better readable than the WayBack link, so below I’ve saved parts only in Archive.is.

It points to two other interesting reads:

• a series on Map/Reduce in Delphi using generics:
  • [Archive.is] Building a Generic Statistics Library, Part 1: Interface – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 2: Unit Tests – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 4: Map – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 4: Map – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 5: Implementation – Community Blogs – Embarcadero Community
• a benchmark class I already had forgotten about which is very useful in Unit Testing (or perhaps better phrased as Integration Testing) of algorithm speeds: [WayBack] Entropy Overload: Anonymous methods in testing / profiling situations written by Barry Kelly (ex Delphi compiler guru)

Later I found it was indeed Stefan who pointed me to the above links and also came up with this very small Memoisation example which is based on Spring4D:

and guess what? because we have interfaced based dictionary this is super easy, let me just hack an example, 5mins6:52 PM

function BuildCache: Tfunc<string,string>;
var
 cache: IDictionary<string,string>;
begin
 cache := TCollections.CreateDictionary<string,string>;
 Result :=
   function(s: string): string
   begin
     if not cache.TryGetValue(s, Result) then
     begin
       Sleep(1000); // hard work! ;)
       Result := ReverseString(s);
       cache.Add(s, Result);
     end;
   end;
end;

Since the returned anonymous method captures the dictionary its kept inside and once the anonymous method goes out of scope the dictionary is also cleaned, want to reset the cache? Just create a new one and throw the old one away.

Need thread safety? just add

System.TMonitoy.Enter/Leave(cache.AsObject);

–jeroen