Re: Will std::bad_alloc be derived from std::system_error?

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 26 Apr 2011 14:22:12 CST

Message-ID:

<ip5sao$hoc$1@dont-email.me>

On 2011-04-26 09:41, DeMarcus wrote:

On 2011-04-25 21:50, Daniel Kr?gler wrote:

Am 25.04.2011 14:55, schrieb DeMarcus:

Hi,

In C++0x, when trying to create a new thread or mutex one may get
std::system_error if there was not enough memory.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2678.htm

Will std::bad_alloc be derived from std::system_error?

AFAIK, there is no such proposal and the FDIS does not show this
derivation. It would probably require a good reason, because an
out-of-memory situation usually is a very drastic situation that you
might not expect to be handled by a generic std::system_error exception
type as well.

So why should std::bad_alloc be derived from std::system_error?

As I understand, std::system_error is a way to report failure from the
operating system.

http://www.stdthread.co.uk/doc/headers/system_error/system_error.html

I agree with that point of view.

I was just curious whether an out-of-memory failure would be categorized
as an operating system failure. I'm working on sorting out a failure
handling strategy for myself.

Maybe, but I see at least one severe problem with that new base class of std::bad_alloc: It would potentially break C++03 programs by a silent change in the runtime behaviour, because the effect would be that now an exception handler for std::runtime_error could catch a std::bad_alloc. This looks very dangerous to me and I'm not sure that this risk is worth the uniformity realized by that approach.

There was one such change in the library, where std::ios_base::failure has been changed to use std::system_error as a base-class, though, but it is at least arguably whether this these changes would be comparable. Also, the reason would be different: For std::ios_base::failure the reason was to allow for more specific error descriptions, but AFAIK for std::bad_alloc there would not exist such an advantage.

HTH & Greetings from Bremen,

Daniel Kr?gler

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