--begin(container) - is defined behavier?

Question

I am implementing decrement operator for my custom bidirectional iterator. Is decrementing iterator pointing at 1st element of Range is defined behavior? Does it have some special value after decrementing, like Range.end()?

Just remember: en.wikipedia.org/wiki/Principle_of_least_astonishment — Loki Astari
– Loki Astari, Commented Sep 26, 2012 at 20:19

Benjamin Lindley · Accepted Answer · 2012-09-26 17:40:54Z

8

In the iterators of all of the standard library containers, that is undefined behavior. But if you are making your own iterator class, it doesn't have to be.

answered Sep 26, 2012 at 17:40

Benjamin Lindley

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2 Comments

Cubbi Over a year ago

The standard library almost has an example of this: std::forward_list has a before-begin iterator, but it's not reachable by decrementing the begin()

Hindol Over a year ago

Ah, that is why you said almost, :). I did not know it is unreachable. Thanks for the info.

Dietmar Kühl · Accepted Answer · 2012-09-26 20:24:01Z

3

The iterator requirements are rather clear: In 24.2.6 [bidirectional.iterators], Table 110:

--r (Expression) X& (Return type) pre: there exists s such that r == ++s.

Since there is no such s for c.begin(), it can't be decremented without violating the precondition.

answered Sep 26, 2012 at 20:24

Dietmar Kühl

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