[fpc-pascal] Generic a "Clone" method

Martin Friebe fpc at mfriebe.de
Tue Jun 16 14:20:01 CEST 2009

Torsten Bonde Christiansen wrote:
> Hi.
> I was wondering if it is posible to make this type of construct for 
> creating a generic Clone method:
> (I put it in pastebin):  http://pastebin.com/fb495ed7
> Explained in words - I'm trying to make a clone method that should be 
> able to copy the content of private variables to a destination object.
> Naturally this destination would have to be of the same class as the 
> original, but is it posible to determine the size of used space in 
> each derived class, simply by using Sizeof(...). Or do I need to 
> create a seperate clone method for each individual derived class.
> If it is posible, what memory copy function should I use?

There is Object.InstanceSize, which sounds as if it may be what you are 
looking for.
But you have a bigger problem than that.

If any member (object variable) is of a ref-counted type (that is 
strings and dynamic arrays), then any "memory copy" will mess up.

Because in the objects memory, there only are the pointers to the data 
of strings, and dyn-arrays. If you copy those pointer by hand, then the 
reference-counts in the actual string or dyn-array are not updated.
this leads to at least 2 problems.

If the original object was the only holder (or last remaining) of a 
reference to this string, and your original or new object is destroyed, 
the ref-count goes down to zero => the string is freed, and the other 
object points to unallocated memory, or meory allocated to some 
new/other data) => crash

Again if, your old object was the only holder, then the ref-count is 
still 1. Strings are "copy on modify", => 2 copies of a string share 
memory until one is changed (in which case a copy is made). The need for 
making the copy is determined by ref-count > 1.
Your ref-count is 1 => no copy is made, the new object is updating the 
strings of the old object too

Best Regards

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