Re: [fpc-pascal] dynamic array reference count

[Michael Schnell]

On 03/15/2014 03:20 PM, Jürgen Hestermann wrote:



I added the last sentence which may help you to avoid false assumptions:


Well done !

OTHO, IMHO this perfectly documents that the decision (supposedly by 
Borland) ) to do it this way is a very nasty and the conceptional 
difference between Strings and dynamic arrays, that both use reference 
counting and "SetLength" is not really easy to be communicated to the 
users code programmers. (Even though it might be possible to do nice 
tricks with them when you exactly know what you are doing.)


-Michael

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Re: [fpc-pascal] dynamic array reference count

[Michael Van Canneyt]

On Sat, 15 Mar 2014, Xiangrong Fang wrote:


Thanks, this is important to know!


The latest version of the docs mentions this explicitly:

http://www.freepascal.org/docs-html/ref/refsu18.html#x42-460003.3.1

Michael.
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Re: [fpc-pascal] dynamic array reference count

[Xiangrong Fang]

​Thanks, this is important to know!​
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Re: [fpc-pascal] dynamic array reference count

[Jürgen Hestermann]

Am 2014-03-15 15:06, schrieb Xiangrong Fang:
> I just want to confirm one thing:
> var
>   a, b: array of Integer;
> begin
>   SetLength(a, 100);
>   SetLength(b, 100);
>   a := b;
> end.
> Now "a" will point to the same memory block as "b". Since dynamic arrays are 
ref counted,
> I don't need to worry about the memory originally allocated for "a", right?

Yes, you don't need to worry about the old array.

But there other peculiarities of dynamic arrays which are very inportant to 
know IMO.
There was a larger discussion about them recently.
I added some more information to http://wiki.freepascal.org/Dynamic_array 
because
some very important details about dynamic arrays were not mentioned in any 
documentation.

I added the last sentence which may help you to avoid false assumptions:

---
Although writing to elements of dynamic arrays does not create a new instance
of the array (no copy-on-write as it exists for Ansistrings) using SetLength
on such arrays does create a copy! So if 2 dynamic array variables point to
the same array (one has been assigned to the other) they do not do so after
having used SetLength on one of them (or on both).
Afterwards both variables are separated arrays each
with elements independend from the other array."
---

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[fpc-pascal] dynamic array reference count

[Xiangrong Fang]

Hi there,

I just want to confirm one thing:

var
  a, b: array of Integer;
begin
  SetLength(a, 100);
  SetLength(b, 100);
  a := b;
end.

Now "a" will point to the same memory block as "b". Since dynamic arrays
are ref counted, I don't need to worry about the memory originally
allocated for "a", right?

Thanks,
Xiangrong
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