CODEWITHSUNDEEP
delphicharansistring
Gonzalez
Gonzalez

Reputation: 711

How to find out char code for a character of an Ansistring

In older versions of Delphi, like D7, you could do like ord(s[i]) where s was a string, but trying this with an AnsiString results in an exception (access violation).

P.S. I was w/delphi 7 for a long time.

Here are the steps to reproduce the error: Create a new project and through a memo on the form (let it be memo1) than add the following code to the form create event handler:

procedure TForm1.FormCreate(Sender: TObject);
var u: ansistring;
begin
    u := 'stringtest';
    memo1.Lines.Add(inttostr(ord(u[2])));
end;

For me this code produces an AV.

Upvotes: -2

Views: 3145

Answers (1)

Johan
Johan

Reputation: 76724

It does work with an ansistring, but you cannot read past the end of it and you must make sure the string is initialized. 

function CharCode(const S: ansistring; pos: integer): byte;
begin
  if pos <= 0 then result:= 0
  //else if s='' then Result:= 0 //unassigned string;
  else if Length(s) < Pos then Result:= 0  //cannot read past the end.
  else Result:= Ord(s[pos]);
end;

Note that if s='' is the same as asking if pointer(s) = nil. An empty string is really a nil pointer.
This is probably why you where getting an access violation.

If you want to force the ansistring to be a certain length you can use SetLength(MyAnsistring, NewLength);

The length of the (ansi)string is variable. That means it grows and shrinks as needed. If you read past the end of the string you may get an access violation.
Note that you don't have to get an AV, the RTL leaves a bit of slack in its allocation; it usually allocates a slightly bigger buffer than requested, this is due to performance and architectural reasons.

The other reason why you may not get an AV if reading past the end of a string is that your program may own both the string buffer and whatever happens to be right next to it.

For this reason it is a good idea to enable range checking in debug mode {$R+} it adds extra checks to protect against reading past the end of structures.

The difference between shortstring and (ansi)string
A short string has a fixed length and it lives on the stack.
A long string (ansi or wide) is a pointer to a record that gets allocated on the heap; it looks like this: 

type
  TStringRecord = record
    CodePage: word;     
    ElementSize: word; //(1, 2 or 4)    
    ReferenceCount: integer;    
    Length: Integer;    
    StringData: array[1..length(s)] of char;
    NullChar: char;
  end;

The compiler hides all these details from you.
see: http://docwiki.embarcadero.com/RADStudio/Seattle/en/Internal_Data_Formats

Upvotes: 3

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