I'm using Delphi7 (non-unicode VCL), I need to store lots of WideStrings inside a TFileStream. I can't use TStringStream as the (wide)strings are mixed with binary data, the format is projected to speed up loading and writing the data ... However I believe that current way I'm loading/writing the strings might be a bottleneck of my code ...
currently I'm writing length of a string, then writing it char by char ... while loading, first I'm loading the length, then loading char by char ...
So, what is the fastest way to save and load WideString to TFileStream?
Thanks in advance
Rather than read and write one character at a time, read and write them all at once:
procedure WriteWideString(const ws: WideString; stream: TStream);
var
nChars: LongInt;
begin
nChars := Length(ws);
stream.WriteBuffer(nChars, SizeOf(nChars);
if nChars > 0 then
stream.WriteBuffer(ws[1], nChars * SizeOf(ws[1]));
end;
function ReadWideString(stream: TStream): WideString;
var
nChars: LongInt;
begin
stream.ReadBuffer(nChars, SizeOf(nChars));
SetLength(Result, nChars);
if nChars > 0 then
stream.ReadBuffer(Result[1], nChars * SizeOf(Result[1]));
end;
Now, technically, since WideString
is a Windows BSTR
, it can contain an odd number of bytes. The Length
function reads the number of bytes and divides by two, so it's possible (although not likely) that the code above will cut off the last byte. You could use this code instead:
procedure WriteWideString(const ws: WideString; stream: TStream);
var
nBytes: LongInt;
begin
nBytes := SysStringByteLen(Pointer(ws));
stream.WriteBuffer(nBytes, SizeOf(nBytes));
if nBytes > 0 then
stream.WriteBuffer(Pointer(ws)^, nBytes);
end;
function ReadWideString(stream: TStream): WideString;
var
nBytes: LongInt;
buffer: PAnsiChar;
begin
stream.ReadBuffer(nBytes, SizeOf(nBytes));
if nBytes > 0 then begin
GetMem(buffer, nBytes);
try
stream.ReadBuffer(buffer^, nBytes);
Result := SysAllocStringByteLen(buffer, nBytes)
finally
FreeMem(buffer);
end;
end else
Result := '';
end;
Inspired by Mghie's answer, have replaced my Read
and Write
calls with ReadBuffer
and WriteBuffer
. The latter will raise exceptions if they are unable to read or write the requested number of bytes.
There is nothing special about wide strings, to read and write them as fast as possible you need to read and write as much as possible in one go:
procedure TForm1.Button1Click(Sender: TObject);
var
Str: TStream;
W, W2: WideString;
L: integer;
begin
W := 'foo bar baz';
Str := TFileStream.Create('test.bin', fmCreate);
try
// write WideString
L := Length(W);
Str.WriteBuffer(L, SizeOf(integer));
if L > 0 then
Str.WriteBuffer(W[1], L * SizeOf(WideChar));
Str.Seek(0, soFromBeginning);
// read back WideString
Str.ReadBuffer(L, SizeOf(integer));
if L > 0 then begin
SetLength(W2, L);
Str.ReadBuffer(W2[1], L * SizeOf(WideChar));
end else
W2 := '';
Assert(W = W2);
finally
Str.Free;
end;
end;
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