Mega Code Archive
Save memory with duplicate strings
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Title: save memory with duplicate strings?
{
Often there are lots of duplicate strings in a program,
for example take the following database:
Author Title
aaa xxx
aaa yyy
aaa zzz
Suppose you want to read it into your program and
create one record or class per line:
}
while not Table.EOF do
begin
author := Table['Author'];
title := Table['Title'];
MyClass := TMyClass(author, title);
StoreItSomewhere(MyClass);
Table.Next;
end;
{
You end up with three different strings containting the same author.
No problem with that as long as there are only three of them.
Now suppose you have got a database of 100,000 entries and lots of
duplicate author names.
If you do the above again you will end up with a huge memory overhead
due to fragmentation of the free memory on the heap.
In addition to that freeing the objects will result in a noticable delay
of the program while the memory manager is busy merging free memory blocks.
On the other hand, Delphi's AnsiStrings use reference counting so it
would be possible to assign the same string to many string variables
without copying the contents:
}
Author := 'aaa';
for i:=0 to 100000 do
begin
MyClass := TMyClass(Author);
StoreItSomewhere(MyClass);
end;
{
This will create 100000 strings containting 'aaa',
but store the actual contents only once,
because they are assigned the same string constant.
It would be nice to do the same with the above database example,
wouldn't it? But how do we get Delphi to realize that
the content is really the same?
Answer: A StringList
}
authors := TStringList.Create;
authors.Sorted := true;
authors.Duplicates := dupIgnore;
while not Table.EOF do
begin
author := Table['Author'];
title := Table['Title'];
authors.Add(author);
authors.Search(author, Idx);
author := authors[Idx];
MyClass := TMyClass(author, title);
StoreItSomewhere(MyClass);
Table.Next;
end;
authors.free;
{
This will only keep one string of each author's name and assign
it to all other occurences, even after the StringList has been freed again.
The wonders of reference counting!
I admit that the above example looks a bit odd. Nobody would load
such a database into memory. But I had a similar problem today and by using
the trick above I managed to reduce the memory usage of my program from 120 megabytes to 10
(basically nothing, given that it started out with 8 mb).
}
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