On Fri, 29 Oct 2010 09:06:24 -0400, Bruno Medeiros
<brunodomedeiros+s...@com.gmail> wrote:
On 06/10/2010 17:34, Andrei Alexandrescu wrote:
or similar. However, a sealed range does not offer references, so trying
e.g.
swap(r1.front, r2.front);
will not work. This is a problem.
Why doesn't a sealed range offer references? Is it to prevent modifying
the elements being iterated?
(I searched google and TDPL but couldn't find any info on sealed ranges)
Because a sealed range then has complete power over memory allocation of
its elements (really, I think sealed ranges is a misnomer, it's really
ranges on sealed containers).
Example: dcollections has a default allocator that allocates nodes in the
collection as chunks (large numbers of nodes that fit into a page) to
avoid over-using the GC. It also uses free-lists to further avoid GC
allocations.
If a range on such a container returned a reference, then someone could
keep that reference indefinitely. When the container removed that element
and re-used it for another, the reference now would refer to the same
element.
Although the range essentially is still a pointer, you can use mechanisms
to prevent abuse, such as using a mutation counter on the container/range
to track changes (dcollections doesn't do this, but it is possible).
These kinds of things are impossible if the range offers references. Once
a reference gets out, you have lost control over the memory.
-Steve
