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delphi – Why variables are declared as TStrings and created as TStringList? – Stack Overflow

Posted by jpluimers on 2019/07/24

Blast from the past: [WayBack] delphi – Why variables are declared as TStrings and created as TStringList? – Stack Overflow

Q

Why variables are declared as TStrings and created as TStringList?

A

Because that way you could put another TStrings descendant in the SL variable (I’d probably call that Strings, not SL).

In your case, that is moot, since the logic around SL contains the creation of a TStringList and no external assignment or parameter parsing.

But if you ever split the logic away from the assignment, then you could benefit from using any TStrings descendant.

For instance, a TMemoy.Lines, TListBox.Items, TComboBox.Items, etc.
From the outside it looks like they are TStrings, but internally they do not use a TStringList but their own descendant.

A few examples of classes that descend from TStrings:

source\DUnit\Contrib\DUnitWizard\Source\DelphiExperts\Common\XP_OTAEditorUtils.pas:
     TXPEditorStrings = class(TStrings)
source\fmx\FMX.ListBox.pas:
       TListBoxStrings = class(TStrings)
source\fmx\FMX.Memo.pas:
     TMemoLines = class(TStrings)
source\rtl\common\System.Classes.pas:
     TStringList = class(TStrings)
source\vcl\Vcl.ComCtrls.pas:
     TTabStrings = class(TStrings)
     TTreeStrings = class(TStrings)
     TRichEditStrings = class(TStrings)
source\vcl\Vcl.ExtCtrls.pas:
     TPageAccess = class(TStrings)
     THeaderStrings = class(TStrings)
source\vcl\Vcl.Grids.pas:
     TStringGridStrings = class(TStrings)
     TStringSparseList = class(TStrings)
source\vcl\Vcl.Outline.pas:
     TOutlineStrings = class(TStrings)
source\vcl\Vcl.StdCtrls.pas:
     TCustomComboBoxStrings = class(TStrings)
     TMemoStrings = class(TStrings)
     TListBoxStrings = class(TStrings)
source\vcl\Vcl.TabNotBk.pas:
     TTabPageAccess = class(TStrings)

–jeroen

This entry was posted on 2019/07/24 at 12:00 and is filed under Conference Topics, Conferences, Delphi, Development, Event, Software Development. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

One Response to “delphi – Why variables are declared as TStrings and created as TStringList? – Stack Overflow”

  1. Arnaud Bouchez said

    2019/07/24 at 13:16

    Note that this is a nice example of the Liskov substition principle. See https://en.wikipedia.org/wiki/Liskov_substitution_principle

    Reply

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