pointers are bi directional iterators?

Question

I have stumbled upon something that has me utterly confused.

std::reverse() takes bidirectional iterators as arguments into the function, but when I pass pointers to the function, it seems to work fine and actually reverse the string.

void reverseFour(char* str) {
    reverse(str, str + strlen(str));
}

int main {
    char *str = "hello";
    str += '/0';
    reverseFour(str);
}

So, my question is, how is this possible? Bidirectional iterators are not pointers, right?

no but pointers are bidirectional iterators, the iterator interface was chosen to be compatible with pointer syntax — john, Feb 27 '18 at 21:43
Pointers are even random access iterators, at least when they point to elements of an array. — nwp, Feb 27 '18 at 21:44
@nwp -- and any pointer that you're using as an iterator has to point into an array. If it doesn't, you can't increment it or decrement it. — Pete Becker, Feb 27 '18 at 21:44
If you want to show the code works fine, at least you need to post valid code. Your code has too many errors. — llllllllll, Feb 27 '18 at 21:47
The code shown has **undefined behavior**, because `main()` is passing a pointer to a string literal to `std::reverse()` (and a non-const pointer at that, which is not allowed in C++11, unless you tell the compiler to disable that restriction). `reverse()` will try to write to read-only memory, which will likely crash the app. To make the code valid, use `char str[] = "hello";` instead. — Remy Lebeau, Feb 27 '18 at 21:56

score 7 · Answer 1 · edited Feb 27 '18 at 21:52

7

A pointer is, in fact, a kind of RandomAccessIterator. This is even better than a BidirectionalIterator, in that any algorithm that requires a BidirectionalIterator will also accept a RandomAccessIterator, including pointers.

This is quite deliberate, and very useful.

There does, of course, exist other kinds of iterators, particularly some types that are dedicated to being just iterators and nothing more (you'll get these from std::set and std::map, for example). But the concept of an Iterator is something that C++ deliberately keeps rather general.

edited Feb 27 '18 at 21:52

Remy Lebeau

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answered Feb 27 '18 at 21:47

Lightness Races in Orbit

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How one category of iterators could be better than another? – Slava Feb 27 '18 at 21:48
@Slava: Because you can do more things with them, without any penalty. – Lightness Races in Orbit Feb 27 '18 at 21:48
Then integers are even better and we should stop using any iterators – Slava Feb 27 '18 at 21:49
2

@Slava: Integers are not a kind of iterator. Stop being silly. – Lightness Races in Orbit Feb 27 '18 at 21:49
That does not stop for them to be better. – Slava Feb 27 '18 at 21:50
1

@Slava: Better _iterators_. You're moving the goalposts. Your first comment talks about comparing categories of iterators. Now you're talking about who knows what. – Lightness Races in Orbit Feb 27 '18 at 21:50
@Slava `int` doesn't fulfill the requirements of any iterator category. – François Andrieux Feb 27 '18 at 21:50
Iterator category does not fullfill requirements to be better or worse. – Slava Feb 27 '18 at 21:51
@Slava: Well we'll just agree to disagree on that okay. – Lightness Races in Orbit Feb 27 '18 at 21:52
You are giving wrong impression, if `std::vector` provides better iterator then it is better container than `std::map` for example, which is simply wrong. – Slava Feb 27 '18 at 21:52
2

@Slava: Now you're posing a strawman argument, for that is not at all what I said. – Lightness Races in Orbit Feb 27 '18 at 21:53
Thanks @Remy, looks nice. "There do" was correct, though; "kinds" is plural. – Lightness Races in Orbit Feb 27 '18 at 21:53
1

@Slava `std::vector`'s iterators are objectively more convenient and powerful than `std::map`'s iterators, though that doesn't imply that `std::vector` is always superior to `std::map`. Only that it is so with regards to the type of iterators they provide. – François Andrieux Feb 27 '18 at 21:54
2

@Slava A random access iterator is better in the sense that you can do more operations with it (it is less restrictive). – NathanOliver Feb 27 '18 at 21:54
@NathanOliver is raw pointer better than iterator? – Slava Feb 27 '18 at 21:56
Since a pointer and an iterator can be the same thing I would say no. An iterator does allow you to have more stuff though but `iterator_traits` supplants the typedef's so it's kind of moot. – NathanOliver Feb 27 '18 at 21:57
2

I'm better than continuing this silly discussion, I know that much. – Lightness Races in Orbit Feb 27 '18 at 21:57
@NathanOliver But I do not follow, one side you say less restrictive is better, then you say it is restricted by `iterator_traits` and it is better than pointer. Can you make your mind? I do not follow your logic. Raw pointer is less restrictive than iterator so it is better right? – Slava Feb 27 '18 at 21:59
1

@Slava `iterator_triats` allows pointers to have the same typedef's that iterators have so an iterator doesn't really have an advantage about the typedefs is what said. A raw pointer and a random access iterator is basically the exact same thing. And with that I'm done. – NathanOliver Feb 27 '18 at 22:01
@NathanOliver right you cannot prove your point but you have to support, so you are done. Nice – Slava Feb 27 '18 at 22:02

pointers are bi directional iterators?

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