Delphi inherit Create from TObject

Question

SITUATION

This question may be very easy but I am new to Delphi and I'm studying it right now. To better understand classes, I've made one that calculates the solutions of second degree equations. This is the code:

type
 TArrayOfStrings = array of string;

type
 TEqSecGrado = class(TObject) sealed
  private
   a, b, c: double;
   delta: double;
   solutions: TArrayOfStrings;
   function getDelta(vala, valb, valc: double): double; overload;
  public
   constructor Create(a, b, c: double);
   function getDelta: double; overload;
   function getSolutions: TArrayOfStrings; virtual;
 end;

This is pretty easy indeed but I'd like to focus on the constructor.

QUESTION

From the book I'm reading I know that (even if the (TObject) is not needed) my class is actually a sub-class of TObject. For this reason I can call the constructor Create with no parameters by default. My question is:

constructor TEqSecGrado.Create(a, b, c: double);
begin
 //inherited; -> is it needed?
 Self.a := a;
 Self.b := b;
 Self.c := c;
 delta := 0;
end;

Do I need to call the inherited? I have studied that, using the keyword I've just mentioned, I am going to "copy" the behavior of the Create constructor in TObject inside my class. I need to create the object for sure, but then I also need to set default values for my parameters.

Since it's not very well explained, I haven't understood when I have to use inherited. Should I do it in this case?

Possible duplication: Have a look at stackoverflow.com/questions/772336/… — Ralf Bönning
I have an error if I put the inherited in my Create. I'd like to see what others have to suggest — Alberto Rossi
inherited; in a constructor with parameters tries to call the same constructor with the same parameters of the superclass. That doesn't exist, so you get an error. Try inherited Create; instead. Now TObject.Create doesn't do anything, but if, one day, you decide to inherit from another class, it will still be valid. — Rudy Velthuis
The word inherit always inherits constructor or other method from parent class. If such constructor or method is not found in parent class Delphi tries to search it in its parent class etc. If such constructor or method can't be found in any of the parent classes an error is raised. Now — SilverWarior
@AlbertoRossi BTW since in last two questions of your it seems you are using classes to only contain some code which returns you certain result you might want to check on class methods (docwiki.embarcadero.com/RADStudio/Seattle/en/Methods) and class fields (docwiki.embarcadero.com/RADStudio/Seattle/en/Fields). Creating a class just to do some simple calculation and then destroying it is not the best practice because class creation and class destruction can impact the performance. By using class methods and class fields you don't need to create the class object every time. — SilverWarior

Dsm Dsm · Accepted Answer · 2016-08-19T21:34:36

Inherited is not strictly needed if you know the parent is TObject. (If you look at the constructor of TObject it is empty). However it is bad practice, in my opinion not to call the inherited constructor, for several reasons that I will outline in a moment. But first, how to call the inherited constructor is like this

constructor TEqSecGrado.Create(a, b, c: double);
begin
 inherited Create; // Note that we need to explicitly write "Create" here because it doesn't have the same parameters as our "Create"
 Self.a := a;
 Self.b := b;
 Self.c := c;
 delta := 0;
end;

But if the inherited constructor is empty, why would we need to call it?

It is all to do with maintenance, and what happens six months down the line when we have forgotten what we did and why.

Firstly it may be that we decide to refactor and inherit from something other than TObject. If we have included the inherited constructor now, either it will still be valid later, or the compiler will tell us we need to do something.

Secondly, we do not control TObject, the Delphi compiler writers do. It may be that in the future TObject.Create is not empty. Imagine having to go through all our constructors to add the inherited one in! Of course, there would be an outcry from all those programmers who thought it was a waste of time, so it will never happen. Probably.

Delphi inherit Create from TObject

1 Answers