pass in the procedure as a var parameter TObject

Question

I read somewhere that reference types do not need to put var .

I have a procedure:

 procedure SetObjectData(var AObject: TObject; var AIndex: integer); begin if AObject is TFDQuery then with (Aobject as TFDQuery) do Params[0].Values[AIndex] := DT_START; TObjectQueue.Enqueu(AObject); AObject := TObjext.Create; AIndex := 0; end;

And do I understand correctly that when I create a new object inside this procedure, I get a new link and then I need to use var ? And can rewrite this code? And then the link to the link is not very.

The use of this word means passing a parameter by reference , i.e.
A reference is sent to the procedure to the object with which you are free to do anything.

Answer 1 · 2018-11-12T14:03:58

 function SetObjectData(AObject: TObject; var AIndex: integer): TObject; begin if AObject is TFDQuery then begin with (AObject as TFDQuery) do begin Params[0].Values[AIndex]:= DT_START; end; end; TObjectQueue.Enqueu(AObject); result := TObject.Create; AIndex:= 0; end; ... obj := SetObjectData(obj, i); ...

Igor

78k five 20 48

Forgive me for necroposing =) but IMHO is not the answer to the question, and if you know what happens with such an action, Pointer(AObject)=@SomeOtherPointer when transmitting as in your example, please explain. I really don’t know what will happen, but the logic suggests that it will not work without var to make such a trick, i.e change the link itself - Vladimir Klykov
@ Vladimir Klykov Yes, it will not work. Such code will change where the function parameter points inside the function, but it will not affect the variable used as a parameter when calling the function. - Igor
@ Vladimir Klykov About the answer / not the answer. The author was worried about the var parameter. I showed a code variant that does the same thing, but without a var before the TObject parameter. - Igor
Насчет ответ/не ответ. I did not quite understand the question correctly, read it diagonally and thought out my own, the question in my head sounded like "should var be used for reference data types", so yes, you are right and you have the exact answer ... =) - Vladimir Klykov
obj: = SetObjectData (obj, i); Is it faster than SetObjectData (var obj, i) ;? - Artem

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Answer 2 · 2018-11-14T04:39:20

If you use var then in fact you get that these same variables are passed to the method:

 SetObjectData(someObj, i); procedure SetObjectData(var AObject: TObject; var AIndex: integer); begin if someObj is TFDQuery then with (someObj as TFDQuery) do Params[0].Values[i] := DT_START; TObjectQueue.Enqueu(someObj); someObj := TObjext.Create; i := 0; end;

If you do NOT use var then copies of the variable values are transferred to the method and it looks like this:

 SetObjectData(someObj, i); procedure SetObjectData(AObject: TObject; AIndex: integer); begin someObj_copy := someObj; i_copy := i; if someObj_copy is TFDQuery then with (someObj_copy as TFDQuery) do Params[0].Values[i_copy] := DT_START; TObjectQueue.Enqueu(someObj_copy); someObj_copy := TObjext.Create; i_copy := 0; end;

and, as you can see, at the end of the method, only the variables inside the method changed, and those that were passed outside remained unchanged.

But!. here is a FreeAndNil(someObj_copy) , let's say you make FreeAndNil(someObj_copy) in the method - guess what will happen? The object pointed to by both someObj_copy and someObj will be destroyed, and the pointer someObj will remain and you will get AccessViolation when you try to work with it (or worse).

Because the expression "copy of a class instance" may be misunderstood.
I fully agree with @Igor, you have said копии значений переменных , Var A : TObject;
Now when I pass the "copy of the value of the variable " A , I will pass the copy of the class instance , since

pass in the procedure as a var parameter TObject

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