When and Why Should I Use TStringBuilder?

Basically, I use these idioms for building strings. The most important differences are:

• TStringBuilder.Create and Append pattern which adds new characters to the TStringBuilder instance.
• TStringList.Create and Add pattern which adds new lines the to the Text of the TStringList instance.
• The Format function to assemble strings based on format patterns.
• Simple concatenation of string types for expressions with 3 or fewer values.

For complex build patterns, the first make my code a lot cleaner, the second only if I add lines and often includes many of Format calls.

The third makes my code cleaner when format patterns are important.

I use the last one only when the expression is very simple.

A few more differences between the first two idioms:

• TStringBuilder has many overloads for Append, and also has AppendLine (with only two overloads) if you want to add lines like TStringList.Add can
• TStringBuilder reallocates the underlying buffer with an over capacity scheme, which means that with large buffers and frequent appends, it can be a lot faster than TStringList
• To get the TStringBuilder content, you have to call the ToString method which can slow things down.

So: speed is not the most important matter to choose your string appending idiom. Readable code is.

To the best of my knowledge TStringBuilder was introduced just for some parity with .NET and Java, it seems to be more of a tick the box type feature than any major advance.

Consensus seems to be that TStringBuilder is faster in some operations but slower in others.

Your program sounds like an interesting one to do a before/after TStringBuilder comparison with but I wouldn't do it other than as an academic exercise.

I tried to improve a old routine that was parsing a text file (1.5GB). The routine was pretty dumb and it was building a string like this: Result:= Result+ buff[i];

So, I though that TStringBuilder will add significant speed improvements. It turned out that the 'dumb' code was actually 114% faster than the 'improved' version with TStringBuilder.

So, building a string from characters is NOT a place where you can obtain speed improvement with TStringBuilder.

---

My StringBuilder (below) is 184.82 times (yes 184!!!!!!) faster than the classic s:= s+ chr. (Experiment on a 4MB string)

Classic s:= s + c
Time: 8502 ms

procedure TfrmTester.btnClassicClick(Sender: TObject);
VAR
   s: string;
   FileBody: string;
   c: Cardinal;
   i: Integer;
begin
 FileBody:= ReadFile(File4MB);
 c:= GetTickCount;
 for i:= 1 to Length(FileBody) DO
  s:= s+ FileBody[i];
 Log.Lines.Add('Time: '+ IntToStr(GetTickCount-c) + 'ms');     // 8502 ms
end;

---

Prebuffered

Time:  
     BuffSize= 10000;       // 10k  buffer = 406ms
     BuffSize= 100000;      // 100k buffer = 140ms
     BuffSize= 1000000;     // 1M   buffer = 46ms

Code:

procedure TfrmTester.btnBufferedClick(Sender: TObject);
VAR
   s: string;
   FileBody: string;
   c: Cardinal;
   CurBuffLen, marker, i: Integer;
begin
 FileBody:= ReadFile(File4MB);
 c:= GetTickCount;

 marker:= 1;
 CurBuffLen:= 0;
 for i:= 1 to Length(FileBody) DO
  begin
   if i > CurBuffLen then
    begin
     SetLength(s, CurBuffLen+ BuffSize);
     CurBuffLen:= Length(s)
    end;
   s[marker]:= FileBody[i];
   Inc(marker);
  end;

 SetLength(s, marker-1); { Cut down the prealocated buffer that we haven't used }  
 Log.Lines.Add('Time: '+ IntToStr(GetTickCount-c) + 'ms');
 if s <> FileBody
 then Log.Lines.Add('FAILED!');
end;

---

Prebuffered, as class

Time:    
 BuffSize= 10000;       // 10k  buffer = 437ms       
 BuffSize= 100000;      // 100k buffer = 187ms        
 BuffSize= 1000000;     // 1M buffer = 78ms     

Code:

procedure TfrmTester.btnBuffClassClick(Sender: TObject);
VAR
   StringBuff: TCStringBuff;
   s: string;
   FileBody: string;
   c: Cardinal;
   i: Integer;
begin
 FileBody:= ReadFile(File4MB);
 c:= GetTickCount;

 StringBuff:= TCStringBuff.Create(BuffSize);
 TRY
   for i:= 1 to Length(FileBody) DO
    StringBuff.AddChar(filebody[i]);
   s:= StringBuff.GetResult;
 FINALLY
  FreeAndNil(StringBuff);
 END;

 Log.Lines.Add('Time: '+ IntToStr(GetTickCount-c) + 'ms');
 if s <> FileBody
 then Log.Lines.Add('FAILED!');
end;

---

And this is the class:

{ TCStringBuff }

constructor TCStringBuff.Create(aBuffSize: Integer= 10000);
begin
 BuffSize:= aBuffSize;
 marker:= 1;
 CurBuffLen:= 0;
 inp:= 1;
end;

function TCStringBuff.GetResult: string;
begin
 SetLength(s, marker-1);                    { Cut down the prealocated buffer that we haven't used }
 Result:= s;
 s:= '';         { Free memory }
end;

procedure TCStringBuff.AddChar(Ch: Char);
begin
 if inp > CurBuffLen then
  begin
   SetLength(s, CurBuffLen+ BuffSize);
   CurBuffLen:= Length(s)
  end;

 s[marker]:= Ch;
 Inc(marker);
 Inc(inp);
end;

Conclusion: Stop using s:= s + c if you have large (over 10K) strings. It might be true even if you have small strings but you do it often (for example, you have a function that is doing some string processing on a small string, but you call it often).

_

PS: You may also want to see this: https://www.delphitools.info/2013/10/30/efficient-string-building-in-delphi/2/

TStringBuilder was introduced solely to provide a source code compatible mechanism for applications to perform string handling in Delphi and Delphi.NET. You sacrifice some speed in Delphi for some potentially significant benefits in Delphi.NET

The StringBuilder concept in .NET addresses performance issues with the string implementation on that platform, issues that the Delphi (native code) platform simply does not have.

If you are not writing code that needs to be compiled for both native code and Delphi.NET then there is simply no reason to use TStringBuilder.