Appending UnicodeString to WideString in Delphi

Question

I'm curious about what happens with this piece of code in Delphi 2010:

function foo: WideString;
var 
   myUnicodeString: UnicodeString; 
begin
  for i:=1 to 1000 do
  begin
    myUnicodeString := ... something ...;

    result := result + myUnicodeString;  // This is where I'm interested
  end;
end;

How many string conversions are involved, and are any particularly bad performance-wise?

I know the function should just return a UnicodeString instead, but I've seen this anti-pattern in the VCL streaming code, and want to understand the process.

Answer 1

To answer your question about what the code is actually doing, this statement:

result := result + myUnicodeString;

Does the following:

1. calls System._UStrFromWStr() to convert Result to a temp UnicodeString

2. calls System._UStrCat() to concatenate myUnicodeString onto the temp

3. calls System._WStrFromUStr() to convert the temp to a WideString and assign it back to Result.

There is a System._WStrCat() function for concatenating a WideString onto a WideString (and System._UStrCat() for UnicodeString). If CodeGear/Embarcadero had been smarter about it, they could have implemented a System._WStrCat() overload that takes a UnicodeString as input and a WideString as output (and vice versa for concatenating a WideString onto a UnicodeString). That way, no temp UnicodeString conversions would be needed anymore. Both WideString and UnicodeString are encoded as UTF-16 (well mostly, but I won't get into that here), so concatenating them together is just a matter of a single allocation and move, just like when concatenating two UnicodeStrings or two WideStrings together.