Re: Exceeding memory while using STL containers
Martin Bonner wrote:
blwy10 wrote:
What happens when in the process of using STL containers, we
insert so many elements that our computer runs out of memory
to store them. What happens then? I assume that this
behaviour is implementation-defined and dependent on what
STL am I using and what OS I am running but I'm just curious
to know what would/should typically happen. To be more
precise, I am using Windows XP SP 2 and VC Toolkit 2003,
including its STL implementation. The container specifically
is std::set.
In principal (as everybody has said), the implementation
should throw std:bad_alloc. In practice, when a Windows
program runs out of memory, there is a good chance that it
will take the whole system down with it.
I've never experienced this. I know that Linux used to (and
probably still does in some configurations) use lazy allocation,
and would start terminating random processes when it ran out of
memory -- AIX also had this problem in the distant past.
Although I've not experienced this, at least one person has told
me that in one case, it killed the login process, so it became
impossible to log into the system -- as a result of a simple
user process using too much memory. Under Windows, I've had a
pop-up window appear, asking me to kill some other processes
manually, in order to obtain more memory -- the system request
didn't return until it could fulfill the request for memory.
Of course, on any system, if you have a lot more virtual memory
than real memory, you'll start thrashing long before the system
runs out of memory. While the system isn't down, there's not
necessarily much you can do with it.
This is not a failure of the STL, but of all the drivers and other
programs that can't cope with out of memory. There is thus little
point in trying to handle out-of-memory failures (and yes, I do realize
that is part of the reason we got here!)
Note that failing to throw std:bad_alloc doesn't mean the STL is
failing to adhere to the standard - there is a general get-out clause
for "resource limit exceeded" in the standard.
It's debatable whether this clause applies when the standard
provides an official means of signaling the error. In practice,
however, when you're out of memory, you also stand a definite
risk of stack overflow when calling operator new. Which is
definitly covered by this clause.
On the other hand, in the older versions of AIX (older, in this
case, being those from more than 8 or 10 years ago), and some
configurations of Linux, the system returns a valid pointer even
when no memory is available; your program (or someone elses!)
then crashes when it attempts to use the pointer. I find it
hard to justify this under the "resource limits exceeded"
clause, because the system has told me that the resource was
there; I'm not trying to use additional resources when it
crashes, but rather resources that I have already successfully
acquired.
--
James Kanze GABI Software
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