Creating Array Of Objects

[nikel]

Hello, I'm trying to create array of objects. I have TMain, TEntireSongs and TSong classes. Here's my code

Code: Pascal  [Select][+][-]

1. unit EntireSongsHandler;
2.  
3. {$mode ObjFPC}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils,
9.   Config, Token;
10.  
11. type
12.   TEntireSongs = object
13.   private
14.     List      : TStringList;
15.     FArrSongs : array[0..200, 1..12] of string;
16.     Token     : PToken;
17.   public
18.     constructor Init;
19.     destructor Done;
20.  
21.     function GetSongByIndex(Num: Integer): TSong;
22.     ...

... and TSong class

Code: Pascal  [Select][+][-]

1. unit songhandler;
2.  
3. {$mode ObjFPC}{$H+}
4. {$TYPEDADDRESS ON}
5.  
6. interface
7.  
8. uses
9.   Classes, SysUtils,
10.   Token, Config, HistoryHandler;
11.  
12. type
13.   TSong = object(TEntireSongs)
14.   private
15.     FID        : Integer;
16.     FEnabled   : Boolean;
17.     FPlayCount : Integer;
18.     FAlbum     : string;
19.     FArtist    : string;
20.     FTitle     : string;
21.     FDuration  : Integer;
22.     FBpm       : Integer;
23.     FKey       : shortstring;
24.     FDate      : shortstring;
25.     FPriority  : Byte;
26.     FPath      : string;
27.   public
28.     property ID          : Integer read FID write FID;
29.     ...

I'm getting error

entiresongshandler.pas(21,50) Error: Identifier not found "TSong"

I tried TEntireSongs = array[0..200, 1..12] of string; but didn't work.

How can I fix errors?

[WooBean]

Hello, I'm trying to create array of objects. I have TMain, TEntireSongs and TSong classes. Here's my code

Code: Pascal  [Select][+][-]

1. unit EntireSongsHandler;
2.  
3. {$mode ObjFPC}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils,
9.   Config, Token, songhandler; //<- here add
10. type
11.   TEntireSongs = object
12.   private
13.     List      : TStringList;
14.     FArrSongs : array[0..200, 1..12] of string;
15.     Token     : PToken;
16.   public
17.     constructor Init;
18.     destructor Done;
19.  
20.     function GetSongByIndex(Num: Integer): TSong;
21.     ...

...
How can I fix errors?

But this will lead to circular unit reference so put both units in one.

[nikel]

Thanks for the quick reply. I tried using one file but got the same error. Here's my code so far

Code: Pascal  [Select][+][-]

1. unit EntireSongsHandler;
2.  
3. {$mode ObjFPC}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils,
9.   Config, Token;
10.  
11. type
12.   TEntireSongs = object
13.   private
14.     List      : TStringList;
15.     FArrSongs : array[0..200, 1..12] of string;
16.     Token     : PToken;
17.   public
18.     constructor Init;
19.     destructor Done;
20.  
21.     function GetSongByIndex(Num: Integer): TSong;
22.     function GetSongByPath(P: string): TSong;
23.  
24.     property ArrSongs : array[0..200, 1..12] of string read FArrSongs write FArrSongs;
25.   end;
26.  
27.   TSong = object(TEntireSongs)
28.   private
29.     FID        : Integer;
30.     FEnabled   : Boolean;
31.     FPlayCount : Integer;
32.     FAlbum     : string;
33.     FArtist    : string;
34.     FTitle     : string;
35.     FDuration  : Integer;
36.     FBpm       : Integer;
37.     FKey       : shortstring;
38.     FDate      : shortstring;
39.     FPriority  : Byte;
40.     FPath      : string;
41.   public
42.     property ID          : Integer read FID write FID;
43.     ...

Modify:

Code: Pascal  [Select][+][-]

1. unit EntireSongsHandler;
2.  
3. {$mode ObjFPC}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils,
9.   Config, Token, SongHandler;
10.  
11. type
12.   TEntireSongs = object
13.   private
14.     List      : TStringList;
15.     FArrSongs : array[0..200, 1..12] of string;
16.     Token     : PToken;
17.   public
18.     constructor Init;
19.     destructor Done;
20.  
21.     function GetSongByIndex(Num: Integer): TSong;
22.     function GetSongByPath(P: string): TSong;
23.  
24.     property ArrSongs : array[0..200, 1..12] of string read FArrSongs write FArrSongs;
25.   end;
26.  
27.   TSong = object(TEntireSongs)
28.   private
29.     FID        : Integer;
30.     ...

This gives me error:

entiresongshandler.pas(21,44) Error: Identifier not found "TSong"

[WooBean]

If you want to have access to unit EntireSongsHandler from (still existing) unit songhandler you need to add them (EntireSongsHandler) in uses clause of songhandler unit.

It may be used in implementation part (all depends on how all application is designed).

[Martin_fr]

You can't have circular references between "object". Unless you use pointers. Though maybe you meant to use classes, which can have such circles?

"object" in that aspect acts like "record"

You have

Code: Pascal  [Select][+][-]

1. type
2.   TEntireSongs = object
3.   private
4. ...
5.      function GetSongByIndex(Num: Integer): TSong; // FORWARD REFERENCE NOT POSSIBLE
6.   end;
7.  
8.  TSong = object(TEntireSongs)  // SINCE IT IS BASED ON TEntireSongs, IT CANNOT MOVE BEFORE TSong
9. ...
10.   end;
11.  

Is there a reason why you use "object" and not "class" ? (Mind "class" needs changes to the code, such as proper creation of instances).

Because with class you can do that. You can forward define "TSong = class" and then later define the details of TSong.


With object you can only forward define pointers to it

Code: Pascal  [Select][+][-]

1. type
2.   PSong = ^TSong;  // pointer to TSong, and in this case TSong can be defined later
3.  
4.   TEntireSongs = object
5.   private
6. ...
7.      function GetSongByIndex(Num: Integer): PSong; // returns pointer
8.   end;
9.  
10.  TSong = object(TEntireSongs)
11. ...
12.   end;
13.  

[Thaddy]

It looks like basically a song collection.
Here is how to handle that with the classes unit. I will add an example with the objects unit later, in the case that you prefer objects over classes.

Code: Pascal  [Select][+][-]

1. {$mode objfpc}{$apptype console}
2. uses
3.   classes;
4. type
5.  
6.   { TSong }
7.  
8.   TSong = class(TCollectionItem)
9.   private
10.     FName:string;
11.     FArtist:string;
12.     FAlbum:string;
13.     FLength: integer;
14.   public
15.     constructor Create(ACollection: TCollection); override;
16.   published
17.     //handled through TCollectionItem.ID
18.     //property ID:Integer read FID write FID;
19.     property Name:string read Fname write FName;
20.     property Artist:string read FArtist write FArtist;
21.     property Album:string  read FAlbum write FAlbum;
22.     property Length: integer read FLength write FLength;
23.   end;
24.  
25.   { TSongCollection }
26.  
27.   TSongCollection = class(TCollection)
28.   private
29.     function GetItems(Index: integer): TSong;
30.     procedure SetItems(Index: integer; AValue: TSong);
31.   public
32.     constructor Create;
33.   public
34.     function Add: TSong;
35.     property Items[Index: integer]: TSong read GetItems write SetItems; default;
36.   end;
37.  
38.  
39. constructor TSong.Create(ACollection: TCollection);
40. begin
41.   if Assigned(ACollection) then
42.     inherited Create(ACollection);
43. end;
44.  
45. { TSongCollection }
46.  
47. function TSongCollection.GetItems(Index: integer): TSong;
48. begin
49.   Result := TSong(inherited Items[Index]);
50. end;
51.  
52. procedure TSongCollection.SetItems(Index: integer; AValue: TSong);
53. begin
54.   Items[Index].Assign(AValue);
55. end;
56.  
57. constructor TSongCollection.Create;
58. begin
59.   inherited Create(TSong);
60. end;
61.  
62. function TSongCollection.Add: TSong;
63. begin
64.   Result := inherited Add as TSong;
65. end;
66.  
67. var
68.   Songs: TSongCollection;
69.   NewSong:TSong;
70.   i:integer;
71. begin
72.   Songs := TSongCollection.Create;
73.   NewSong := Songs.Add;
74.   NewSong.Name := 'Jumping Jack Flash';
75.   NewSong.Artist:= 'The Rolling Stones';
76.   NewSong.Album := 'The Rolling Stones Rock and Roll Circus';
77.   NewSong.Length := 100;
78.   for TCollectionItem(NewSong) in Songs do
79.     begin
80.      writeln(Newsong.name);
81.      writeln(NewSong.ID);
82.     end;
83.   Songs.Free;
84.   readln;
85. end.

[Thaddy]

And here is one that uses the objects unit instead:

Code: Pascal  [Select][+][-]

1. {$mode objfpc}{$apptype console}
2. uses
3.   objects;
4. type
5.  
6.   { TSong }
7.   PSong = ^TSong;
8.   TSong = object
9.   private
10.     FName,
11.     FArtist,
12.     FAlbum:string;
13.   public
14.     constructor Init(aName,aArtist,aAlbum:string);
15.     destructor done; virtual;
16.     property Name  :string read Fname write FName;
17.     property Artist:string read FArtist write FArtist;
18.     property Album :string read FAlbum write FAlbum;
19.   end;
20.  
21.  
22.   { TSongCollection }
23.   PSongCollection = ^TSongCollection;
24.   TSongCollection = object(TCollection)
25.   private
26.     function GetItems(Index: integer): TSong;virtual;
27.     procedure SetItems(Index: integer; AValue: TSong);virtual;
28.   public
29.     destructor Done;virtual;
30.     property Items[Index: integer]: TSong read GetItems write SetItems; default;
31.   end;
32.  
33. constructor TSong.Init(aName, aArtist, aAlbum: string);
34. begin
35.   Name := aName;
36.   Artist:= aArtist;
37.   Album := aAlbum;
38. end;
39.  
40. destructor Tsong.Done;
41. begin
42.   //inherited done;
43. end;
44.  
45. { TSongCollection }
46.  
47. function TSongCollection.GetItems(Index: integer): TSong;
48. begin
49.   Result := TSong(At(Index)^);
50. end;
51.  
52. procedure TSongCollection.SetItems(Index: integer; AValue: TSong);
53. begin
54.    inherited Insert(PSong(@AValue));
55. end;
56.  
57. destructor TSongCollection.Done;
58. begin
59.    DeleteAll;
60.    Inherited Done;
61. end;
62.  
63.  
64. var
65.   Songs: PSongCollection;
66.   Song : PSong;
67.   i:integer;
68. begin
69.   { Songs collection with initial capacity of 50 and grow factor 10 }
70.   New(Songs, init(50,10));
71.   New(Song,Init('Jumping Jack Flash',
72.                    'The Rolling Stones',
73.                    'The Rolling Stones Rock and Roll Circus'));
74.   Songs^.Insert(Song);
75.   New(Song,Init('It''s Only Rock ''n Roll (But I Like It)',
76.                    'The Rolling Stones',
77.                    'It''s Only Rock ''n Roll'));
78.   Songs^.Insert(Song);
79.   writeln(songs^.count);
80.   writeln(Songs^.Items[0].name);
81.   writeln(Songs^.Items[0].Artist);
82.   writeln(Songs^.Items[0].Album);
83.   writeln;
84.   writeln(Songs^.Items[1].name);
85.   writeln(Songs^.Items[1].Artist);
86.   writeln(Songs^.Items[1].Album);
87.   Dispose(Song, done);
88.   Dispose(Songs,done);
89.   readln;
90. end.

Example inspired by the TCollection example in the Turbo Vision 2.0 programming guide, 1992.
This is available from the internet archive, but I also have it in print.

Better not mix the objects.TCollection with classes.TCollection etc: They are very very different.

[nikel]

Thanks for the replies. I'm working on Thaddy's first example.

[Thaddy]

@nikel
I will show you how to load/save the collection for both examples tomorrow.