Do I need to setLength a dynamic array on initialization?

Question

type Tmyclass = class(TObject)
  somearray: array of TSometype
  FBool: Boolean;
  Fint: Integer;
  Fstr: string;
  constructor Create;
  destructor Destroy; overrid         


        

Answer 1

All the memory belonging to objects are initialized to 0 on object construction (heap allocation), so usually you don't need to initialize anything belonging to a class, default values (zero memory) are:

• strings will be ''
• integers will be 0
• floating point variables will be 0
• boolean will be false
• pointers and object references will be nil
• dynamic arrays will contain zero elements.

and so on.

For local variables and anything related to stack, it will contain garbage, so you're responsible to provide a meaningful value before using the variables.

Something like:

procedure Something;
var
  x: Integer;
begin
  ShowMessage(IntToStr(X));
end;

will show a random value.

Answer 2

Dynamic arrays are managed types and so are always initialized to nil, equivalent to SetLength(..., 0). You never need to do this.

The only time you can get caught out is when you are returning a dynamic array from a procedure as a function return value. In fact a function return value is in fact just an implicit var parameter.

Consider the following code:

function Foo: string;
begin
  Result := Result + 'X';
end;

var
  i: Integer;

begin
  for i := 1 to 5 do
    Writeln(Foo);
  Writeln(Foo);
  Writeln(Foo);
end;

Output

X
XX
XXX
XXXX
XXXXX
X
X

What's going on here is that the compiler, as an optimization is electing not to re-initialize the implicit local variable inside the loop.

This can catch you out from time to time so I recommend setting to nil return values that are dynamic arrays, strings, interfaces etc. You don't need to do so for class members and it's more idiomatic to let the constructor zero-initialise them automatically.

Answer 3

Both SetLength calls are superfluous since dynamic arrays are initialized and finalized.

All fields in a class instance are initialized to their respective zero value, i.e. 0, nil, False, '' etc.

Local variables are initialized (and finalized) only if they are of a lifetime-Managed type like string, dynamic array or interface.

Answer 4

As the other answerers have said, you do not need to initialize class member fields.

You also asked about local variables. Yes, you do need to initialize those. All of them, except variables of one of the managed types:

• AnsiString
• UnicodeString
• WideString
• an interface type
• dynamic array type
• Variant

List taken from Barry Kelly's answer that Sertac pointed out in the comments: Which variables are initialized when in Delphi?

Answer 5

If you know in advance how large the array will be it could be a good practice to allocate it as a whole, it will avoids too many reallocations later, using memory better. Also clearing the array when you no longer use it will realease it memory (but Delphi knows how to clear them when they go out of scope).