Re: Memory issue

"Alf P. Steinbach" <>
Mon, 10 Aug 2009 21:09:18 +0200
* kathy:

I want to use std::vector::pushback function to keep data in RAM. What
will happened if no more memory available?

In practice, what happens on a modern system as free memory starts to become
exhausted and/or very fragmented, is that the system slows to a crawl, so,
you're unlikely to actually reach that limit for a set of small allocations.

However, a very large allocation might fail.

In that case, the C++ standard guarantees a std::bad_alloc exception as the
default response, provided the allocation at the bottom was via 'new'.

This default response can be overridden in three ways:

   * By replacing the class' operator new (this is just an unfortunate name
     for the allocation function; it's *called* by a 'new' expression, which
     after that call proceeds to call the specified constructor, i.e. you're
     not overriding what a 'new' expression does by defining operator new).

   * By replacing the global namespace operator new (ditto comment).

   * By installing a new-handler (see set_new_handler, I think the name was).

In general it's difficult to handle memory exhaustion in a good way. What to do
depends on whether it was a small allocation (uh oh, skating on thin ice,
further execution will likely fail, best to terminate) or a very large
allocation (hm, perhaps inform the user of failure to do whatever it was that
caused the allocation), or something in between (what to do?). And so many, if
not most, programs simply assume that there will be no allocation failure, ever,
although that's of course just the ostrich policy adopted by default.

Cheers & hth.,

- Alf

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