Default value

[karl Jonson 0]

Hi,

How can I give Index variable in TExample a default value of -1 ?

TExample = class  
  Index : integer;
end

TIA

[Lajos Juhász 238]

type
  Texample = class
  private
    findex: integer;
  public
    constructor Create; reintroduce;
  published
   property Index: integer read fIndex write fIndex default -1;
  end;

constructor TExample.create;
begin
  inherited;
  findex:=-1;
end;

If it's a published property you should use the default directive (when the value of the published property equals to it's default value it's not streamed to the dfm). Even if you set the default directive it's the job of the constructor to initialize the field value.

[haentschman 92]

TExample = class  
private
  FIndex : Integer;
public
  constructor Create;
  property Index: Integer read FIndex write FIndex;
end;
...

constructor TExample.Create;
begin
  FIndex := 999;
end;

 

...the same idea.

Edited November 19, 2020 by haentschman

[David Heffernan 2288]

Fields of a class are default initialised upon instantiation. That means, for an integer, a value of 0. If you want to assign a different value, you need to do that yourself, in the constructor.

[Stefan Glienke 1899]

uses
  Spring;

type
  TExample = class(TManagedObject)
    [Default(-1)]
    Index: integer;
  end;

Just because I can 😎

Edited November 20, 2020 by Stefan Glienke

[Mahdi Safsafi 225]

33 minutes ago, Stefan Glienke said:

uses
  Spring;

type
  TExample = class(TManagedObject)
    [Default(-1)]
    Index: integer;
  end;

Just because I can 😎

IIRC Extended-Pascal had similar thing but it also applies to type(declared type can have default value as well).

 

[Stefan Glienke 1899]

6 minutes ago, Mahdi Safsafi said:

IIRC Extended-Pascal had similar thing but it also applies to type(declared type can have default value as well).

Interesting - I think I am going to add that to the TManagedObject functionality 

[Attila Kovacs 617]

Too bad there is no multiple inheritance.

 

Is it possible to hook a constructor on startup?

Edited November 20, 2020 by Attila Kovacs

[Mahdi Safsafi 225]

18 minutes ago, Attila Kovacs said:

Too bad there is no multiple inheritance.

multiple inheritance = multiple trouble 

Quote

Is it possible to hook a constructor on startup?

Yep if you install it before it gets created.

[Stefan Glienke 1899]

No need for multiple inheritance - you can also apply the mechanism from Spring.TManagedObject to any other class - simply override NewInstance and FreeInstance 

[balabuev 97]

Slow...