How should I allocate space for a list of items when I don't know in advance how many I'm going to have?

Question

In another question, David Heffernan posted a comment about his "all time least favourite Delphi construct":

SetLength(FItems, Length(FItems)+1);

The construct he dislikes was heavily used in the Pebongo project, such as in this excerpt:

procedure TBSONDocument.ReadStream( F: TStream ); 
var 
  len : integer; 
  elmtype : byte; 
  elmname : string; 
begin 
  Clear; 
  f.Read( len, sizeof( len ) ); 
  f.Read( elmtype, sizeof( byte ) ); 

  while elmtype <> BSON_EOF do // Loop
  begin 
    elmname := _ReadString( f ); 
    SetLength( FItems, length( FItems ) + 1 ); // This, spotted by TOndrej
    case elmtype of 
      BSON_ARRAY: FItems[high( FItems )] := TBSONArrayItem.Create; 
      BSON_BINARY: FItems[high( FItems )] := TBSONBinaryItem.Create; 
      ...
    end; 
    f.Read( elmtype, sizeof( byte ) ); 
  end; 
end; 

What are the alternatives?

Answer 1

It's not the SetLength() itself that is bad but incrementing the length in the loop. Example of bad code:

SetLength( Result.FItems, 0 ); 

for i := 0 to high( FItems ) do 
begin 
  SetLength(Result.FItems, Length(Result.Fitems)+1);
  Result.FItems[i] := FItems[i].Clone; 
end; 

In this case array is rearranged and reallocating memory on each iteration. Your posted example doesn't show bad usage of SetLength()