Solved

Storing a record in a TList with different struct

Posted on 2010-09-18
6
692 Views
Last Modified: 2012-05-10
Hello,

What is the best way to identify wich kind of record is in the list?

Example

type
  TSomethingA = record //A = ANSI
    chBla: array [0..9] of AnsiChar;
  end;
  PSomethingA = ^TSomethingA;

  TSomethingW = record //W = UNICODE
    chBla: array [0..9] of WideChar;
  end;
  PSomethingW = ^TSomethingW;

//global var
var
  lsList: TList;

//the lsList is created and inside has a lot of PSomethingW and PSomethingA stored
procedure Something;
var
  SomethingA: PSomethingA;
  SomethingW: PSomethingW;
  i: Integer;
begin
  //now, walking in lsList, an item can be of type SomethingA or SomethingW
  for i := 0 to Pred(lsList.Count) do
  //what is the best method to identify, if the current item "lsList[ i ]" is a SomethingA or SomethingW?
  end;
end;
0
Comment
Question by:cebasso
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
6 Comments
 
LVL 25

Expert Comment

by:epasquier
ID: 33707814
you can't with records, or it would require a complex set of records declarations that would follow a simple rule : having a Type field before (integer)

Easiest way : use class instead
type
  TSomethingA = Class //A = ANSI
    chBla: array [0..9] of AnsiChar;
  end;
//  PSomethingA = ^TSomethingA;

  TSomethingW = Class //W = UNICODE
    chBla: array [0..9] of WideChar;
  end;
//  PSomethingW = ^TSomethingW;

//global var
var
  lsList: TObjectList; // Change TList to TObjectList

//the lsList is created and inside has a lot of PSomethingW and PSomethingA stored
procedure Something;
var
  SomethingA: TSomethingA;
  SomethingW: TSomethingW;
  i: Integer;
begin
  //now, walking in lsList, an item can be of type SomethingA or SomethingW
  for i := 0 to Pred(lsList.Count) do
   if lsList[ i ] is TSomethingA Then // Use Is operator on TObject
    begin
...
    end;
end;

Open in new window

0
 
LVL 4

Author Comment

by:cebasso
ID: 33707836
hey epasquier!

cool!
but, the struct has more members like

chBla: array [0.9] of WideChar;
dwFlags: DWORD;
iSomething: Integer;
//and more

no problem?

and actually i'm using New() to store in TList, like

var
  SomethingA: PSomethingA;
begin
  New(SomethingA);
  FillChar(SomethingA^, SizeOf(TSomethingA), 0);
  //fill members
  lsList.Add(SomethingA);
  Dispose(SomethingA);
end;

now, with your code, i need just to change PSomethingA to TSomethingA as a class, do the same and store in a TObjectList instead TList like your example?
0
 
LVL 25

Expert Comment

by:epasquier
ID: 33707845
no, you can't create objects with New, and do not use FillChar on them.
It's possible that your needs are not compatible with Objects... So you might have to do it the hard way, with a Type field in records.
I wont have time to explain you that alternate today, I'll think of the best thing for you and explain monday
0
Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
LVL 25

Accepted Solution

by:
epasquier earned 500 total points
ID: 33709026
Alternate method with a field identifying the record. It could be an integer with some constants defined, but that is as well done with an enumerated type.

With that method, you won't have much to change from your older code. I recommend that you use these kind of techniques for API purpose only where you can't use Delphi objects. You can always create objects wrapper around these structures if you use a lot of those in your application.

type
  RecordType=(rtSomethingA,rtSomethingW);
  pRecordType=^RecordType;

  TSomethingA = record //A = ANSI
    RecType:RecordType; // = rtSomethingA
    chBla: array [0..9] of AnsiChar;
  end;
  PSomethingA = ^TSomethingA;

  TSomethingW = record //W = UNICODE
    RecType:RecordType; // = rtSomethingW
    chBla: array [0..9] of WideChar;
  end;
  PSomethingW = ^TSomethingW;

//global var
var
  lsList: TList;

//the lsList is created and inside has a lot of PSomethingW and PSomethingA stored
procedure Something;
var
  i: Integer;
begin
 for i := 0 to Pred(lsList.Count) do
  Case pRecordType(lsList[i])^ of
   rtSomethingA: With PSomethingA(lsList[i])^ do ... 
   rtSomethingW: With PSomethingW(lsList[i])^ do ... 
  end;
end;

Open in new window

0
 
LVL 8

Expert Comment

by:GeneralTackett
ID: 33712662
How about using records with variant parts  example:
TPerson = record
  FirstName : string[40];
  LastName : string[40];
{ Fixed portion of record begins here }
  BirthDate: TDate;
  case
     Citizen: Boolean of
{ variant portion of record begins here }
      True: (BirthPlace: string[40]);
      False:
        (
        Country: string[20];
        EntryPort: string[20];
        EntryDate: TDate;
        ExitDate: TDate
        );
end;
0
 
LVL 4

Author Closing Comment

by:cebasso
ID: 33746792
Thanks!
0

Featured Post

Free Tool: IP Lookup

Get more info about an IP address or domain name, such as organization, abuse contacts and geolocation.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This article explains how to create forms/units independent of other forms/units object names in a delphi project. Have you ever created a form for user input in a Delphi project and then had the need to have that same form in a other Delphi proj…
Hello everybody This Article will show you how to validate number with TEdit control, What's the TEdit control? TEdit is a standard Windows edit control on a form, it allows to user to write, read and copy/paste single line of text. Usua…
In an interesting question (https://www.experts-exchange.com/questions/29008360/) here at Experts Exchange, a member asked how to split a single image into multiple images. The primary usage for this is to place many photographs on a flatbed scanner…

738 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question