[fpc-pascal] Dynamic arrays, yet another pitfall

[Michael Van Canneyt]

On Mon, 17 Feb 2014, Lukasz Sokol wrote:

> On 17/02/14 06:02, Jürgen Hestermann wrote:
>>
>> Am 2014-02-16 17:16, schrieb Michael Van Canneyt:
>>> It does exactly that, it says:
>>> 1) No COW
>>
>> As said already: SetLength *is* a write!
>>
>>
>
> I'm not of compiler background - but I think you're confusing things here.
>
> To 'write' usually means : changing DATA element(s). That's it. You have a sheet of paper of
> known size and you write on it/erase/change the text. But the size of the array remains the same.
>
> SetLength is /not/ a write in this sense : because you're resizing the paper you're
> going to write on (e.g. adding another page or cutting it to size)
>
> As far I understood, SetLength is changing the /description/ of the array (meta-data?) but not the data itself :
> in this sense, it is not a 'write data' operation.
>
> It does 'write' as in: change parameters - of the array DESCRIPTION area, but that area is not ARRAY DATA;
> and enlarging/shrinking the memory allocation - but that does not change the content of the already
> written-to array elements: it may add some blank ones or remove some written-to ones, but the rest
> remains UNCHANGED, i.e. they (the just-added or remaining-after-removal ones) are not being written-to
> by doing SetLength.)

That is the long version of what I was trying to say. Thank you :)

Michael.