Re: How to write previous element in STL list
In message
<ea2bef8b-b9d9-4455-a463-27787283e7c7@o32g2000yqm.googlegroups.com>,
cornelis van der bent <kees.van.der.bent@gmail.com> writes
On 27 jul, 20:16, Rolf Magnus <ramag...@t-online.de> wrote:
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?
The list is doubly-linked, so giving back an iterator one postion
earlier/further does not seem to be a big deal.
One position not, that's why there are increment/decrement operators, but
you cannot define addition/subtraction operators that only work with the
value 1. They would have to be usable with any value, and adding e.g. a
million to such an iterator (provied that the list has that many elements
elements) would be possible, but very slow.- Tekst uit oorspronkelijk
bericht niet weergeven -
- Tekst uit oorspronkelijk bericht weergeven -
I agree that in language design you could leave out useless (e.g. due
to low performance) constructions. On the other hand having uniform
interfaces as much as possible has its virtues too. If it would be
possible to implement list<T>::iterator + N, I think I would choose to
do so and leave the performance issues to the user.
Which is no doubt why the uniform interface provided by std::advance is
available. It can be applied to input, forward, bidirectional and
random-access iterators, N can be negative for bidirectional and
random-access, and performance depends on the iterator type.
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