Deallocate array?

[Steven Schveighoffer]

On Wed, 08 May 2013 06:20:45 -0400, Matic Kukovec  
<matic.kukovec at pametnidom.si> wrote:

> Was playing around with the code and found that this WORKS:
> (note the "app_temp_array.reserve(50000000);" at the top, without this  
> line it doesn't work)

[snip]

> The memory usage shrinks from 400MB to 2MB, which is right!
> Why?

This is the conservative garbage collector at work.

What happens is there is a "false pointer", or a word-aligned 32-bit value  
on the stack, or in global registers, that points at one of the arrays  
that you allocate.

As you append, and the GC needs to allocate larger and larger arrays to  
hold your new data, the old ones are left behind for the GC to collect.   
But some random integer somewhere happens to "point" at one of those  
arrays.  It is then kept in memory.  It doesn't have to be a large array.   
GC.minimize can only shrink the OS-provided memory block, which is  
sequential.  If any data is still valid, even if there is a bunch of free  
pages below that data, it cannot be returned to the OS.

When you reserve, however, one, and only one, memory block is allocated.   
This block you are explictly freeing at the end, so it will be collected.   
Then minimize can do a good job.

Not saying you should do things this way, just explaining the behavior.   
arr.reserve is a good tool to use when you know how much data you will  
need.  But it can't fix every situation.

-Steve