What happens when AnsiString is cast to PAnsiString?

Question

I have the method (Delphi 2009):

procedure TAnsiStringType.SetData(const Value: TBuffer; IsNull: boolean = False);
begin
  if not IsNull then
    FValue:= PAnsiS         


        

Answer 1

An AnsiString is implemented as a pointer. An AnsiString variable holds nothing but an address. The address is that of the first character in the string, or nil if the string is empty.

A PAnsiString is a pointer to an AnsiString variable. It's a pointer to a pointer to the first character of a string. When you say PAnsiString(Buffer), you're telling the compiler to treat the pointer in Buffer as though it were a pointer to an AnsiString instead of a pointer to character data. The address 5006500 is the location of the first character of the string, C.

You have a record in memory that represents the string:

                +-----------+
                | $ffffffff | -1 reference count (4 bytes)
                +-----------+
Buffer:         | $00000001 | length (4 bytes)
+---------+     +-----------+
| 5006500 | --> |       'C' | first character (1 byte)
+---------+     +-----------+
                |        #0 | null terminator (1 byte)
                +-----------+

Buffer holds the address of the byte with C in it. You type-cast that to have type PAnsiString instead of AnsiString. You told the compiler that you had this layout:

                                  +-----------+
                                  |       ... |
                                  +-----------+
Buffer:                           |       ... |
+---------+     +-----------+     +-----------+
| 5006500 | --> | $00000043 | --> |   garbage | first character
+---------+     +-----------+     +-----------+
                                  |       ... |
                                  +-----------+

When I'm reasoning about pointers, I draw diagrams just like this. If you don't keep some paper next to you on your desk, you're doing yourself a disservice.