Re: How to write previous element in STL list

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Mon, 27 Jul 2009 12:28:34 -0400

Message-ID:

<h4kkjj$4po$1@news.datemas.de>

cornelis van der bent wrote:

On 27 jul, 17:42, Victor Bazarov <v.Abaza...@comAcast.net> wrote:

cornelis van der bent wrote:

In my code I want to go through all combinations of two items in a
list. Here is my code:
    list<Instance*>::iterator i;
    for (i = instances.begin(); i != --instances.end(); i++)
    {
        list<Instance*>::iterator j;
        for (j = i + 1; j < instances.end(); j++)
        {
            // Do something!
        }
    }
I get a big error message at i + 1.

Such an operation is only defined for random-access iterators, and the
list iterator isn't one.

Do you know reason(s) why this has nog been implemented for a list?

My guess would be because addition is slow and implementing it would
just tempt folks to use it with unfortunate performance impacts.

The list is doubly-linked, so giving back an iterator one postion
earlier/further does not seem to be a big deal.

Yes, but the compiler does not need to recognize that the value you're
adding or subtracting from the iterator is '1'. It sees the addition or
the subtraction and needs to resolve it. It can't. No compiler I know
would convert (a+1) into anything except operator+(a,1) for user-defined
types.

Thanks for the insight! I used a vector instead.

It's probably easier to use indices with the vector instead of iterators.

[..]

V
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