When is array deallocating necessary?

Question

I have read that applying DEALLOCATE to an allocated array frees the space it was using. I deal with several allocatable arrays in my program, but never bother

Answer 1

Can deallocating variables no longer needed affect execution speed? Yes. Is it likely to in "normal" programs? No, if not preventing memory leaks.

There is no valuable heuristic of which I'm aware to help you determine usefulness of deallocating "for speed".

As previously mentioned, deallocating may be necessary for correctness or avoiding memory leaks.

However, if a program requires finalization of an allocatable variable for correctness then it will be necessary to have a deallocate statement for it: finalization does not occur when termination of execution comes about by a stop or end program statement.

---

Allocatable variables declared within a procedure (subroutine or function) without the save attribute (so-called unsaved local variables) are deallocated automatically when the procedure ends execution.

As a historical note, though, this wasn't true in Fortran 90. In Fortran 90 such variables were not deallocated and worse was that the allocation status of them became undefined (so that even the allocation status couldn't be queried). One really wanted a deallocate there. This deficiency was corrected in Fortran 95 but habits and code may live for a long time.