[fpc-pascal] Clone a instance of object

[Michael Van Canneyt]

On Mon, 23 Sep 2013, patspiper wrote:

> On 23/09/13 10:02, Michael Van Canneyt wrote:
>> 
>> 
>> This is wrong. You are typecasting a class reference to an object instance.
>> 
>> The following works:
>> 
>> {$mode objfpc}
>> 
>> Type
>>   TBaseObject = class(TObject)
>>   public
>>   constructor Create; virtual;
>>   function Clone:TBaseObject;
>>   end;
>>   TBaseObjectClass = Class of TBaseObject;
>> 
>> constructor TBaseObject.Create;
>> 
>> begin
>> end;
>> 
>> Function TBaseObject.Clone:TBaseObject;
>> 
>> Var
>>   C : TBaseObjectClass;
>> begin
>>   C:=TBaseObjectClass(ClassType);
>>   Result:=C.Create;
>> end;
> What would happen if Result := Self.ClassType.Create were used instead?

The wrong constructor would be called. 
Classtype is equivalend to 'Class of TObject'.

The typecast is needed to ensure that the TBaseObject.Create constructor is called.
Your basic polymorphism problem...

Michael.