Hi,

Maybe it is not a correct list, but I am going to ask my question anyway :)

Is there a penalty for calling SetLength if array already has the length we need?

And if there is a penalty - is there a way to write "generic" function SmartSetLength(array,length) that will only call SetLength if necessary?

I hope other people find these questions interesting enough :)

Thanks,
Gena

Yes, there's always penalty for operations that you don't need. FPC will generate normal call, pass arguments, etc. I checked if SetLength with ansistring detects that requested size is same: It doesn't, it performs whole allocation and moving.

And if there is a penalty - is there a way to write "generic" function SmartSetLength(array,length) that will only call SetLength if necessary?

No. Compiler internally emits information about element size, how are they refcounted etc. as a pointer. Your best bet would be "hacking" FPC to emit such information to your procedure and then pass it to internal fpc setlength.
I would personally like it a lot if there was way to access "secret" information stored along with dynamic arrays/ansistrings, however it's not easy task. I think best bet would be storing secret information pointers along with refcount/length and then making functions that allow editing those data. Then we could have generic "dynamic array" type (I assume?).

Not always.
Keep in mind that dynamic arrays have references and the array memory itself is a reference counted structure.

SetLength creates a unique copy of the array, that means an array with reference count 1. If it is already unique nothing is changed. For example:

var
  A,B: array of string;
begin
  SetLength(A,10); // this allocates, A points to unique array
  A[0]:='bla'; // the A array is still unique
  SetLength(A,10); // no reallocation
  B:=A; // reference count increased to 2, not unique anymore
  SetLength(A,10); // reallocation to create a second array, now A
                   // and B point to two unique arrays
  SetLength(A,10); // no reallocation
  SetLength(B,10); // no reallocation
end;

Mattias