problem with accessing value

[question]

Dear All,
I have two units, Unit1 and Unit2 , i would like to access a boolean from Unit1 to Unit2 , i have tried it in the following way, but i have got the error "Zugriffsverletzung bei Adresse 023C4B58 in Modul 'Test.exe'. Lesen von Adresse 0000"

Code:

//Unit1
Unit Unit1
type TCalculation = class(TForm)
  private
  protected
   FNewvalue: Boolean;
  public
   property Newvalue: Boolean read FNewvalue;
   
  end;

//Unit2
Unit Unit2
type Taccounts = class(TForm)

 interface

uses
  Unit1
   
  private
  protected
  public
    procedure Checkvalue(Sender: TObject);
  end;

implementation

procedure Taccounts .Checkvalue(Sender: TObject);
var
  f : TCalculation ; // i have created an instance of TCalculation    
begin
if f.Newvalue = True then  // i have called the boolean  
//check the condition
End;

could you please tell me ,how can i make it works?

[Furtbichler]

Zitat:

Zitat von question (Beitrag 1226069)

Code:

...procedure Taccounts .Checkvalue(Sender: TObject);
var
  f : TCalculation ; // i have created an instance of TCalculation    
begin
if f.Newvalue = True then  // i have called the boolean  
End;

... but you are not accessing the instance your have created. Instead, you access 'f', which is undefined at this point.

Where is your created instance of 'TCalculation'?

[question]

Could you please tell me ,how can i create the instance of 'TCalculation'? i am reading some tutorial,but i am not so clear, could you please give me an example ?

[Uwe Raabe]

Zitat:

Zitat von question (Beitrag 1226072)

Could you please tell me ,how can i create the instance of 'TCalculation'? i am reading some tutorial,but i am not so clear, could you please give me an example ?

Delphi-Quellcode:

var
  f: TCalculation; // declaration
begin
  f := TCalculation.Create; // create instance
  ...

[question]

Hi , i got the error " Nicht genünged wirklich parameter"

Code:

var
  f: TCalculation; 
begin
  f := TCalculation.Create; // Here pointing the error("Nicht genünged wirklich parameter")
  ...

[DeddyH]

TCalculation is of kind TForm, whose constructor expects an Owner-parameter. If you don' t want to set an owner, simply pass nil.

Delphi-Quellcode:

f := TCalculation.Create(nil);

I think you should read some basics-tutorial, e.g. here.

[question]

Thanks for the link, i am following this tutorial,but i have another question, i have done the progem, now there is no error,but i am not getting the expected value of Modified(its a Boolean var)

Code:

var
  f: TCalculation; 
begin
  f := TCalculation.Create(nil);
  if f.FNewvalue = True then  // the FNewvalue is always zero, which is unexpected
  //this will hapen
  else
    //this
end;

[DeddyH]

This is not unexpected, you create a new instance, therefore its properties are set to default unless something happens which changes them.

[edit] BTW: You should never compare to true or false, this can lead to unexpected results.

Delphi-Quellcode:

if var = true then

-->

Delphi-Quellcode:

if var then

[/edit]

[question]

Is it also possible to create a class exactly as it is(not as a new instance), i mean i want to create a class of TCalculation so that i can access any of its variable with the exact value from class of TAccounts

i even dont know whether its possible or not, i was looking in tutorial but dont get any idea yet, can you please help me ?

[DeddyH]

You have 2 different classes derived from TPersistent with similar properties? In that case you can override the "Assign" or even better the "AssignTo"-Method. This should do exactly what you want. There should be examples for both methods in this forum: Assign, AssignTo.

[question]

Thanks, i am looking on it.

By the way, Can you please suggest me, any Book/online learning material to learn Delphi with MYSQL(from beginner to expert), even it need to buy ,i will buy it,but i am looking for it, i have a book but it is in German language, a learning material in English would be easily understable in my situation.