does SetLength for an array actually release memory?

[dummzeuch 1505]

Given code like this:

var
  arr: array of TSomeLargeRecord;
begin
  SetLength(arr, 5000);
  // fill the array
  SetLength(arr, 100);
  // work with the smaller array
end;

Does the last call to SetLength actually reduce the memory required for the array?

 

I'm suspecting that it only reduces the internally stored capacity but the memory will remain allocated until the array is finalized, that is it goes out of scope, is set to NIL or SetLength(arr,0) is called.

[Guest]

According to the documentation, it should:
 

Quote

For a dynamic array variable, SetLength reallocates the array referenced by S to the given length.

 

[Lajos Juhász 293]

I peeked at the source code and it calls ReallocMem thus should work as expected.

[Stefan Glienke 2006]

That's up to the implementation of ReallocMem - the one that comes with Delphi (FastMM4) (I just looked into getmem.inc) has this comment:

Quote

It's a downsize. Do we need to allocate a smaller block? Only if the new size is less than a quarter of the available size less SmallBlockDownsizeCheckAdder bytes