String Sharing/Reference issue with objects in Delphi
- by jenakai123
My application builds many objects in memory based on filenames (among other string based information). I was hoping to optimise memory usage by storing the path and filename separately, and then sharing the path between objects in the same path. I wasn't trying to look at using a string pool or anything, basically my objects are sorted so if I have 10 objects with the same path I want objects 2-10 to have their path "pointed" at object 1's path (eg object[2].Path=object[1].Path);
I have a problem though, I don't believe that my objects are in fact sharing a reference to the same string after I think I am telling them to (by the object[2].Path=object[1].Path assignment).
When I do an experiment with a string list and set all the values to point to the first value in the list I can see the "memory conservation" in action, but when I use objects I see absolutely no change at all, admittedly I am only using task manager (private working set) to watch for memory use changes.
Here's a contrived example, I hope this makes sense.
I have an object:
TfileObject=class(Tobject)
FpathPart: string;
FfilePart: string;
end;
Now I create 1,000,000 instances of the object, using a new string for each one:
var x: integer;
MyFilePath: string;
fo: TfileObject;
begin
for x := 1 to 1000000 do
begin
// create a new string for every iteration of the loop
MyFilePath:=ExtractFilePath(Application.ExeName);
fo:=TfileObject.Create;
fo.FpathPart:=MyFilePath;
FobjectList.Add(fo);
end;
end;
Run this up and task manager says I am using 68MB of memory or something. (Note that if I allocated MyFilePath outside of the loop then I do save memory because of 1 instance of the string, but this is a contrived example and not actually how it would happen in the app).
Now I want to "optimise" my memory usage by making all objects share the same instance of the path string, since it's the same value:
var x: integer;
begin
for x:=1 to FobjectList.Count-1 do
begin
TfileObject(FobjectList[x]).FpathPart:=TfileObject(FobjectList[0]).FpathPart;
end;
end;
Task Manager shows absouletly no change.
However if I do something similar with a TstringList:
var x: integer;
begin
for x := 1 to 1000000 do
begin
FstringList.Add(ExtractFilePath(Application.ExeName));
end;
end;
Task Manager says 60MB memory use.
Now optimise with:
var x: integer;
begin
for x := 1 to FstringList.Count - 1 do
FstringList[x]:=FstringList[0];
end;
Task Manager shows the drop in memory usage that I would expect, now 10MB.
So I seem to be able to share strings in a string list, but not in objects. I am obviously missing something conceptually, in code or both!
I hope this makes sense, I can really see the ability to conserve memory using this technique as I have a lot of objects all with lots of string information, that data is sorted in many different ways and I would like to be able to iterate over this data once it is loaded into memory and free some of that memory back up again by sharing strings in this way.
Thanks in advance for any assistance you can offer.
