Re: About std::list

"Alan Carre" <>
Sun, 29 Jun 2008 03:06:19 +0700
"Doug Harrison [MVP]" <> wrote in message

On Sat, 28 Jun 2008 14:03:32 +0200, "Bo Persson" <> wrote:

It also depends on the type of your data. Is it very expensive to
copy? How often do you add and delete elements?

For std::list, complexity of removing an element is actually O(1),
which ignores the constant factor k in k * 1, where k is much larger
than for a std::vector.

Yet in absolute terms, it's still incredibly tiny. It's just a couple of
pointer updates over what std::vector does.

This is something I've always wondered about... which is, yes: "For
std::list, complexity of removing an element is actually O(1)", that is if:

line 1: std::list<T>::iterator itr;
line 2: std::list<T>::erase(itr);

Then the complexity of "line 2" is *certainly* O(1). But what about "itr"?
In order to erase an element (or elements) from a list, I assume we mean to
erase something specific. That is, normally we're going to wind up doing
something like...

MyType SpecificElement(specifics);
std::list<MyType>::iterator itr = MyList.find(SpecificElement);
if(itr != MyList.end()) MyList.erase(itr);

And the complexity becomes ~= O(N) again due to "find".

Of course one might never need to do this. That is say, if we *only* removed
elements during iterations, but I have found in practice that more often
than not I'm searching for a specific element to remove *outside of an
iteration*. Typical example: you might have a list of "All 'MyType' objects"
(as pointers say); added-to on construct, and deleted-from on destroy. This
can be very useful if we want to apply something to "every MyType". But now
the complexity of "MyType::~MyType()" becomes ~= O(NumObjects). So with lots
of MyType objects around, deleting a specific one (or having one fall out of
scope) becomes extremely costly. I mean, if there were say, a million
MyType's then this simple code:

   MyType t;
   t.OutputConsole("Hello World");

Would have an average complexity of 500,000!


So my question is this... is there a *safe* way one could store off the
iterator corresponding with a particular object? That way, on destroy one
could write:


Reducing the complexity back to O(1).

Many thanks in advance,
- Alan Carre

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