delphi demo of class pointer

unit ptest;

//0.0.0.1       20200408        DX      demo for the pointer of class and record

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

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

var
  Form1: TForm1;

implementation

{$R *.dfm}
type
  Myrecord = record       {define the record}
     name_record: string;
end;

type
 Myclass = class          {define the class}
   public
     name_class: string;
end;

TPRec = ^Myrecord;        {define the pointer for the record}
TPClass= ^Myclass;        {define the pointer for the class}

var
  ARec:        Myrecord;  {define record}
  PRec:        TPRec;     {define record pointer}

  AClass:      Myclass;   {define class}
  PClass:      TPClass;   {define class pointer}
  
  P:           Pointer;   {define pointer}

procedure ShowMessRecord( AP: TPRec);
begin
  ShowMessage(AP^.name_record); {??}
  ShowMessage(AP.name_record);  {??}
end;  

procedure ShowMessClassA( AP: TPClass);
begin
  ShowMessage(AP^.name_class); {??}
  ShowMessage(AP.name_class);  {??}
end;

procedure ShowMessClassB( AP: Pointer);
begin
  //ShowMessage(TPClass(AP^).name_class); {not work}
  ShowMessage(TPClass(AP).name_class);    {??}
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
//https://www.bbsmax.com/A/gAJGnK8nzZ/
  ARec.name_record := 'day';
  P := @ARec;
  ShowMessage(TPRec(P).name_record); {??}
  ShowMessRecord(P);
  PRec:= @ARec;
  ShowMessRecord(PRec);

//https://blog.youkuaiyun.com/wbcp2000/article/details/1385922
  AClass:= Myclass.Create;
  AClass.name_class:='month';
  P := @ AClass;
  ShowMessage(TPClass(P).name_class); {??}
  ShowMessClassA(P);
  ShowMessClassB(P);
  PClass:=@ AClass;
  ShowMessClassA(PClass);
  ShowMessage(PClass.ClassName);      {??}

//http://blog.sina.com.cn/s/blog_77b05b190100x48o.html
  if PClass = Pointer(Myclass) then
    ShowMessage('The pointer is to the class')
  else if  PClass.ClassType = Myclass then
  begin
    ShowMessage('PClass is Myclass')
  end;
  
//https://blog.youkuaiyun.com/Musicwind/article/details/7107  
  try
    PRec:=nil;

    AClass.Free;                       {Only release all the memory releases it requested at the time of creation}
    AClass:=nil;
    PClass:=nil;
    P:=nil;
    ShowMessage('Release all');
  except
    ShowMessage('error')
  end;

end;

end.