[fpc-pascal] Array clearing

Sven Barth pascaldragon at googlemail.com
Tue Apr 4 17:01:01 CEST 2017

On 04.04.2017 16:54, Ryan Joseph wrote:
>> On Apr 4, 2017, at 9:46 PM, Sven Barth via fpc-pascal <fpc-pascal at lists.freepascal.org> wrote:
>> SetLength() allocates a single block of memory, cause array access is
>> ordinary pointer arithmetic. However if you have an array of array then
>> the first array stores a pointer to each sub array.
>> E.g. the following would be valid, too:
>> === code begin ===
>> var
>>  arr: array of array of Integer;
>> begin
>>  SetLength(arr, 10, 10);
> “then the first array stores a pointer to each sub array.”
> Could you illustrate this is code? I don’t think I’m understanding this exactly like it’s represented in memory. There’s only one “sub array” in this 2x2 array so how does that look in memory?

Let's look at a smaller array, let's say 3 x 4, then it would look like

arr = @arr[0], @arr[1], @arr[2]

arr[0] = 0, 0, 0, 0
arr[1] = 0, 0, 0, 0
arr[2] = 0, 0, 0, 0

Essentially you'd have four arrays, namely the outer array arr which
basically contains pointers to the other arrays, and the inner arrays
which are each three separate four element arrays. So all four of these
arrays could reside at completely different locations of the heap.

Is this clearer?


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