For what reason do functions like std::sort() directly change the vector instead of return a value?

Question

I'm a novice with C++ and am wondering why std::sort() directly changes the array. I would naturally think that the sort() function should be like this:

anyVector = sort(anyVector.begin(), anyVector.end());

My question is: how std::sort() understands that it must change the order of the array content if you don't mention the array itself to be re-assigned?

I know that it could appear as a trivial question, but I didn't find any explanation (maybe because it's obvious for many).

[`std::sort`](https://en.cppreference.com/w/cpp/algorithm/sort) does a sort *in place*. — Eljay, Jun 09 '21 at 16:11
`std::sort()` sorts a range, provided by pair of iterators - the first for begin of the sequence, the second - one behind the last. Passing whole `vector` is just a variant of use. — Slava, Jun 09 '21 at 16:12
I just needed the "in place" keyword thank you, now I read what means "pass by reference", I refreshed my old C notions with pointers, now it's clear the reason why sort use the reference and modify directly the value. Thank you very much @Eljay! — SYS64738, Jun 09 '21 at 17:10
You're welcome. :-) "*I would naturally think...*" I also naturally think that way, as there are less surprises (referential transparency, immutability... which are more in the *functional programming* camp). With *in place* mutability comes smaller memory footprint, which can be critical in many rather common situations, and provides some guarantees. I just wish the name was `std::sort_in_place`. Alas, that ship has sailed. — Eljay, Jun 09 '21 at 17:38

score 8 · Answer 1 · edited Jun 09 '21 at 16:23

8

Algorithms like std::sort don't know about the container. You pass iterators and the algorithm works on iterators. Hence std::sort has no way to return a vector.

The more profane reason is that you always have to choose between doing something in-place or creating a copy. When the algorithms does its job in-place, you are still free to make a copy:

std::vector<int> my_sort(const std::vector<int>& v) {
      auto result = v; // passing v by value and returning it defeats NRVO
      std::sort(result.begin(), result.end());
      return result;
}

But the other way around is not possible. If the algorithm would make a copy, you cannot easily turn it into something that does not make a copy.

edited Jun 09 '21 at 16:23

Bathsheba

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answered Jun 09 '21 at 16:13

463035818_is_not_an_ai

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Your example could do with a tweak as NRVO cannot be exploited since `v` is a function parameter. It takes an unnecessary value copy. – Bathsheba Jun 09 '21 at 16:14
@Bathsheba move into some local vector, then return that? – 463035818_is_not_an_ai Jun 09 '21 at 16:16
Or pass by const reference and create a copy inside – Slava Jun 09 '21 at 16:16
@Slava like that? – 463035818_is_not_an_ai Jun 09 '21 at 16:17
It's a really evil part of C++, see https://stackoverflow.com/questions/14856344/when-should-stdmove-be-used-on-a-function-return-value – Bathsheba Jun 09 '21 at 16:17
Yes the edit is how I would do it. Churlish of me not to have upvoted this before now, but here you go! – Bathsheba Jun 09 '21 at 16:18
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@Bathsheba I find the whole copy elision thing a bit annoying, because lots has changed, and I don't understand it fully. – 463035818_is_not_an_ai Jun 09 '21 at 16:20
That makes two of us. – Bathsheba Jun 09 '21 at 16:20
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I am afraid quite soon if not already nobody would fully understand what is going on there. – Slava Jun 09 '21 at 16:21
@Slava: I've taken the liberty of adding a useful comment. I'd allow a comment like that in my codebase. – Bathsheba Jun 09 '21 at 16:23
@Bathsheba it wasnt immediately obvious to me, so thats the kind of comments I do use in code. Actually now I am curious and motivated to give it another try to understand, because naively I looks like a step back that I have to take a reference and cannot make the function signal to the caller that it makes a copy of the parameter, lets see how long that motivation will last – 463035818_is_not_an_ai Jun 09 '21 at 16:29

For what reason do functions like std::sort() directly change the vector instead of return a value?

1 Answers1