Instantiating an array of objects within a class

Question

I have a project that requires use of pre-defined classes to communicate with a remote web service. The primary class contains standard fields plus an array of objects defined within a different class. Instantiating the main class doesn't instantiate the lower level class thus producing an AV. The code below is an executable sample of the issue, where attempting to insert data in the 'ProductLines' array produces the error.

The question is how to instantiate the array objects? Tried constructor, setlength() without success. Any guidance MUCH appreciated.

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, 
  System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Soap.InvokeRegistry;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  ProductLine = class(TRemotable)
  private
    Description: string;
    Quantity: integer;
  end;

  ArrayOfProductLines = array of ProductLine;

  Customer = class(TRemotable)
  private
    Name: string;
    Comment: string;
    ProductLines: ArrayOfProductLines;
  end;

  // Customer Class
  //   Name
  //   Comment
  //   ProductLines (array)
  //             ProductLine
  //             ProductLine
  //             .....
var
  Form1: TForm1;
  TObj : Customer;     // Transfer object

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  TObj.Name := 'Test Name;';
  TObj.Comment := 'Test Comment';


  TObj.ProductLines[0].Description := 'Test Description 1';    // fails 
  here, how to instantiate?
  TObj.ProductLines[0].Quantity := 1;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  Close;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  TObj := Customer.Create;
  SetLength(Tobj.ProductLines,1);
end;

end.

You have to instantiate the ProductLine object

David Heffernan
– David Heffernan

2018-02-23 22:19:22 +00:00
Commented Feb 23, 2018 at 22:19 — David Heffernan
– David Heffernan, Commented Feb 23, 2018 at 22:19

LU RD · Accepted Answer · 2018-02-23 23:01:32Z

6

The question is how to instantiate the array objects?

SetLength(TObj.ProductLines,1);

This line creates the first element in an array and initializes it to nil.

In order to create the object, just do:

TObj.ProductLines[0] := ProductLine.Create;

Note that every instantiated element in the ProductLines array must be manually destroyed to avoid a memory leak.

A note on naming conventions:

For object types and types in general, use the capital letter T as the first letter. Example TCustomer
For variables, avoid the capital letter T as the first letter.

edited Feb 23, 2018 at 23:01

answered Feb 23, 2018 at 22:34

LU RD

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