Apparently, the new Delphi 2009 TStringBuilder class has not got much attention yet, so I’ll give a short recap here.

Under .NET there is a StringBuilder class since the very beginning. This is because in .NET Strings are immutable, which means that once the content of a string instance is set, it is fixed. It cannot be changed anymore. “Quasi modifying” operations require new instances to be created and the old ones to be discarded:

s := s + ‘a’  means “create a new string instance and initialize it with the old content of s plus an ‘a’, then throw the old instance s pointed to into the garbage collector”

Lot of pressure on the memory management system that is.

Thus StringBuilder was invented. The core idea is to prepare some buffer and let all operations work on that buffer, so that you don’t need to recreate instances all the time. The buffer is still managed, and all are happy.

The question is now: “why do we need a TStringBuilder class in Delphi/Win32, and if it is a “real” class (i.e. not just a highly optimized type in System.pas), could it be really fast?”

Well, it appears to be fast, look at this code (test environment: Core 2 Duo, 2GHz):

// approx 2.2 sec ~45M operations/sec
LStart := GetTickCount;
for i := 0 to 100000000 do begin
SB.Append(' ');
end;
LStop := GetTickCount;
Writeln('StringBuilder: ', LStop - LStart);

// approx 3.8 sec ~ 26M operations/sec
s:='';
LStart := GetTickCount;
for i := 0 to 100000000 do begin
s := s + ' ';
end;
LStop := GetTickCount;
Writeln('String: ', LStop - LStart);

// approx 3.5 sec ~ 28M operations/sec
a:='';
LStart := GetTickCount;
for i := 0 to 100000000 do begin
a := a + ' ';
end;
LStop := GetTickCount;
Writeln('AnsiString: ', LStop - LStart);

So obviously TStringBuilder is useful for certain operations, as its significantly faster than String. As expected AnsiString is faster than String, but its still slower than TStringBuilder.

So this leaves to investigate when TStringBuilder has advantages, besides in this fairly artificial case. To be continued …