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Accepted
Daniel Rivers
Asked: 2020-01-30 03:51:49 +0800 CST 2020-01-30 03:51:49 +0800 CST

Call a procedure of the calling class

  • 772

I'm new to delphi/free pascal and I think I have a misconception, since I'm used to java and maybe things can't be approached in the same way. I hope you can help me.

I am using Lazarus v1.6RC1.

I have a class TControlin a unit uControland a class TRedin a unit TRed. The unit uControlhas the unit uRedin uses. The case TControlhas an attribute called FRed, whose type is TRed(is an instance of TRed). The thing is that at a given moment, I need to call a procedure from . My intention is to have access to the instance of from , to be able to access its methods, that's something I did in java, but here it doesn't seem to work. What I am doing is the following:TControlTRedTControlTRed

class TControl:

TControl = class(TObject)
    procedure procesaEntradaRed(pComando : TComando)
public
    FRed : TRed;

class TRed:

TRed = class(TObject)
    constructor Create(pControl : TControl)
public
    FControl : TControl;

When I install TRedit TControlI do it like this:

FRed := TRed.Create(Self);

And the constructor of TRedis like this:

constructor TRed.Create(pControl : TControl)
begin
    FControl := pControl;
end;

The call in which I have the error is this:

FControl.procesaEntradaRed(lComando);

The error message is the following:

uRed.pas(662,12) Error: identifier idents no member "procesaEntradaRed"

I have tried to see which methods were visible TControlfrom TRed, and none of the ones I created in TControlare visible, although the own methods of TObject.

I have put only the parts of the code that are involved in the problem to be concise.

Thanks in advance and sorry for the inexperience.

EDIT:

The line where I make the call is found in uRed, at the end of a procedure whose declaration is:

procedure TRed.UDPRead(AThread: TIdUDPListenerThread; const AData: TidBytes; ABinding: TIdSocketHandle);

However, commenting out that line throws an error when instantiating TRed, passing Self as a parameter:

FRed := TRed.Create(Self);

The error is the following:

uControl.pas(146,69) Error: Incompatible type for arg no. 3: Got > >"UCONTROL.TControl", expected "CONTROLS.TControl"

It says that parameter 3 fails, it is because I also pass 2 strings, I do not show them so as not to confuse.

The fault seems to be in the Self, but I don't know how to fix it.

delphi

1 Answers

  1. Best Answer

    Your problem is that there are multiple classes with the same name inside the visibility and you don't properly qualify the name of the class you want to reference, and you're actually declaring another one.

    We are going to break a long story into several smaller ones.

    TControl

    TControl is a class already declared in the Controls unit. It is actually quite popular , as it is the source of all controls in FPC, just as it is in Delphi's VCL.

    That is, in FPC, just like in the VCL, any control (that is, anything that the user can see on his screen), directly or indirectly inherits from TControl. I follow the recommendation of not declaring other classes with the same names as the classes of the libraries of the tool itself: VCL/RTL/FMX, and less than the classes that are the basis of the class hierarchy.

    But Delphi/FPC does support different classes with the same name. For that, let's see.

    Identifier rules with the same name.

    In Delphi/FPC you can declare data types and variables with the same name, if they are in different visibility levels or in different units.

    In case of being in different units, the documentation tells us (free translation made by me):

    The order in which the units appear in the clause usesdetermines the order of their initialization and affects how identifiers are allocated by the compiler. If two units declare a variable, constant, type, procedure, or function with the same name, the compiler will use the name of the unit that appears last in the clause uses. (To access the identifier on the other drive, you must add the qualifier: NombreUnidad.Identificador.

    Going back to your question:

    In your case, there are two units that contain the TControl Class, to summarize, let's say they are:

    unit uControl;
interface

type
  TControl = class(TObject)
  //etcetera
  end;

    and we also have this other one, which is part of the FPC visual library.

    unit Controls
interface

type
  TControl = class(TComponent)
  //etcetera

    Since you use both units in the unit where you declare the class TRed, or else you wouldn't have this problem, the possible solutions are:

    • most recommended : change the name of your class, for example from TControlto TMiControl.

    • change the order in which the units are listed in the clause uses, so that the uControl unit appears after the Controls unit.

      uses UnaUnidad, OtraUnidad, Controls, uControl;

    • recommended : Qualify the name of the unit, where you declare the class member, so you don't depend on the order of the uses clause and there is no ambiguity in the declarations

    Your code would look like this:

    TRed = class(TObject)
    constructor Create(pControl : uControl.TControl)
public
    FControl : uControl.TControl;
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