What does the cast AnsiString (myPAnsiChar) actually do?

Question

I have a C DLL that returns a pointer to a PAnsiChar string managed by the C DLL. I would like to make a copy of the string so that it can be managed on the Delphi side.

If I cast the returned PAnsiChar to an AnsiString, as in "str := AnsiString (myPAnsiChar)" what does the cast actually do? Does the cast allocate new memory for the string pointed to by PAnsiChar or should I make a copy of the string coming from the DLL first?

Answer 1

Yes. The compiler translates that cast into a RTL routine call that copies the string into a new AnsiString. If you build with Debug DCUs enabled you can trace into it in the debugger and see how it works. E.g:

var
    fromTheDll: PAnsiChar;
    localCopy: string;

localCopy := fromTheDll; //Delphi copies the string to fromTheDll variable

Answer 2

In fact the cast is superfluous. You can just as easily write str := myPAnsiChar.

Delphi string types use memory that is managed by the RTL. This means that they will never reuse the contents of a PChar. The only time you ever need to take steps to make sure an assignment creates a new copy is when the assignment is between two matching Delphi string types. That is AnsiString to AnsiString or UnicodeString to UnicodeString.