Question

For logging and reporting reasons I create objects that have the class name and message belonging to an Exception. I do this so I don’t have to manage the lifetime of the Exception object. What I would like to do is recover the advantages of RTTI identification that allow you to tell if an object is derived from a given class using the “is” operator in Delphi 6.

Is there a way to use a class name in string form to tell if the class the string contains is derived from another class?

Suppose I have the class of an object stored in strClassName and that class is “derivedClass”. Further, derivedClass is derived from baseClass. Is there a function I can write that can tell if the class in string form in strClassName is derived from baseClass? For example:

// Hypothetical function that returns TRUE if the class in strClassName is
//  derived from the class passed in theBaseClass
function isDerivedClass(strClassName: string; theBaseClass: TAnyClass): boolean;

What would the body of that method look like?

Answer 1

function isDerivedClass(strClassName: string; theBaseClass: TClass): boolean;
begin
  Result := FindClass(strClassName).InheritsFrom(theBaseClass);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  RegisterClass(TLabel); // must be registered to be found by FindClass/GetClass

  if isDerivedClass('TLabel', TWinControl) then
    ..

If you don’t want an exception to be raised when a class by the name ‘strClassName’ cannot be found, use GetClass instead of FindClass:

function isDerivedClass(strClassName: string; theBaseClass: TClass): boolean;
var
  aClass: TClass;
begin
  Result := False;
  aClass := GetClass(strClassName);
  if Assigned(aClass) then
    Result := aClass.InheritsFrom(theBaseClass);
end;