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Article #17225: Using RTTI to determine property of ancestor type

 Question and Answer Database

     FAQ: FAQ2225C - Using RTTI to determine property of ancestor type
Category: VCL
Platform: All-32Bit
 Product: All-CBuilder,   


Say that I have a pointer to a ClassB that I know decends from
ClassA and I want the value of Property1 from ClassB, however this
property is protected at the ClassA level and therefore not accessable.

How can I use the RTTI ( Run Time Type Information ) to get at the
information in Property1 of ClassB when I don't know _exactly_
what ClassB is?


Good question, luckily there are some Delphi runtime library functions
that will let you do this.

The following example responds to a button click and sets the caption
property of a label with the lable of the button (or any other
control decended from TControl) using RTTI to figure out what 
sort of pointer to use (sort of).

NOTE: EmptyStr is a string type constant that contains NULL.

__fastcall TForm1::TForm1(TComponent* Owner)
  : TForm(Owner)
AnsiString TForm1::GetCaption(TControl *AComp)
  PPropInfo   propInfo;
  AnsiString  result;
  AnsiString  theCaption;

  result = EmptyStr;

  if (AComp == NULL)
    return result;

  propInfo = GetPropInfo((TTypeInfo*)(AComp->ClassInfo()), "Caption");

  if (propInfo != NULL)
    result = GetStrProp(AComp, propInfo);

  return result;
void __fastcall TForm1::Button1Click(TObject *Sender)
  Label1->Caption = GetCaption(dynamic_cast(Sender));

This information was gleaned from the Borland Newsgroups as a submission
by Ralph Friedman of the amazing Team B.

6/24/99 9:30:42 AM

Last Modified: 01-SEP-99