How to convert strings to array of byte and back

Question

4I must write strings to a binary MIDI file. The standard requires one to know the length of the string in bytes. As I want to write for mobile as well I cannot use AnsiString, which was a good way to ensure that the string was a one-byte string. That simplified things. I tested the following code:

TByte = array of Byte;

function TForm3.convertSB (arg: string): TByte;
var
   i: Int32;
begin
   Label1.Text := (SizeOf (Char));
   for i := Low (arg) to High (arg) do
   begin
      label1.Text := label1.Text + ' ' + IntToStr (Ord (arg [i]));
   end;
end; // convert SB //

convertSB ('MThd');

It returns 2 77 84 104 100 (as label text) in Windows as well as Android. Does this mean that Delphi treats strings by default as UTF-8? This would greatly simplify things but I couldn't find it in the help. And what is the best way to convert this to an array of bytes? Read each character and test whether it is 1, 2 or 4 bytes and allocate this space in the array? For converting back to a character: just read the array of bytes until a byte is encountered < 128?

Answer 1

Delphi strings are encoded internally as UTF-16. There was a big clue in the fact that SizeOf(Char) is 2.

The reason that all your characters had ordinal in the ASCII range is that UTF-16 extends ASCII in the sense that characters 0 to 127, in the ASCII range, have the same ordinal value in UTF-16. And all your characters are ASCII characters.

That said, you do not need to worry about the internal storage. You simply convert between string and byte array using the TEncoding class. For instance, to convert to UTF-8 you write:

bytes := TEncoding.UTF8.GetBytes(str);

And in the opposite direction:

str := TEncoding.UTF8.GetString(bytes);

The class supports many other encodings, as described in the documentation. It's not clear from the question which encoding you are need to use. Hopefully you can work the rest out from here.