[fpc-devel] Newbie question: how does the compiler know the class type of an object ( the Is operator)

[Andrew Haines]

On 08/22/14 13:53, Dennis Poon wrote:
>
>
> Howard Page-Clark wrote:
>> program Project1;
>>
>> uses classes;
>> begin
>>   WriteLn('InstanceSize of TObject=',TObject.InstanceSize);
>>   ReadLn;
>> end.
>
> I tried and it says the instancesize is 4 (on win 32 bit).
> My question actually was how does the "as" and "is" get the class type
> of an object.
> So, does that imply the 4 byte is used to store a pointer to the class
> type and Virtual Method table?
>

http://www.freepascal.org/docs-html/prog/progsu168.html#x212-2250008.2.13

Table 8.6 is the class instance memory layout and 8.7 is the class vmt
which has info on it's name and ancestor classes.

Regards,

Andrew Haines