Generic `count` function

Asked May 23 '20 at 17:17

Active May 23 '20 at 17:23

Viewed 114 times

In the C++ book, I have met next function:

template<class Container>
int count(const Container& container) {
    int sum = 0;
    for(auto&& elt : container) {
        sum += 1;
    }
    return sum;
}

I am just wondering why we use here reference to reference auto&& instead of just reference auto& ?

asked May 23 '20 at 17:17

ceth

44,198
62
180
289

1

It is known as **rvalue reference**. – Ardent Coder May 23 '20 at 17:18
There are no references to references – StoryTeller - Unslander Monica May 23 '20 at 17:21
@ArdentCoder, ok. But what I lose if I will use **reference** in this example, instead of **rvalue reference** ? – ceth May 23 '20 at 17:27
@ArdentCoder Actually, it's a forwarding reference. – Asteroids With Wings May 23 '20 at 17:29
@demas Yes, that's your original question. My comment was just intended to let you know of the exact term for "reference to reference", which is not actually a reference to reference. – Ardent Coder May 23 '20 at 17:29
@AsteroidsWithWings There are many more terms like "universal reference" as well ;) – Ardent Coder May 23 '20 at 17:30
Indeed. But it may not be an rvalue reference. – Asteroids With Wings May 23 '20 at 17:30
@AsteroidsWithWings I was just not being too specific: "forwarding reference is an rvalue reference to a cv-unqualified template parameter" :P – Ardent Coder May 23 '20 at 17:31
@ArdentCoder That's not a good description of a forwarding reference; it seems to be describing the symbols rather than the semantics, and doing so incorrectly. The `auto` part makes it different from any old `T&&` for some concrete `T`. – Asteroids With Wings May 23 '20 at 17:32
@AsteroidsWithWings You shall ask a separate question on terminology if you're not getting what I mean – Ardent Coder May 23 '20 at 17:35
@ArdentCoder I don't need to ask a question because I know what a forwarding reference is, and how it differs from an rvalue reference. :) – Asteroids With Wings May 23 '20 at 17:40
@AsteroidsWithWings Of course there's a difference, and [I didn't deny that](https://stackoverflow.com/questions/61975831/generic-count-function#comment109614724_61975831). – Ardent Coder May 23 '20 at 17:41
@Ardent Presumably then you will be removing your initial comment wherein you claim there is an rvalue reference in the question. :) – Asteroids With Wings May 23 '20 at 17:42
@AsteroidsWithWings As you can see in [this comment](https://stackoverflow.com/questions/61975831/generic-count-function#comment109614675_61975831), my initial comment was intended to show "reference to reference" is not yet a feature of C++. – Ardent Coder May 23 '20 at 17:44
@ArdentCoder May as well say "it is known as **strawberry milkshake**" to make the same point. – Asteroids With Wings May 23 '20 at 17:46
@AsteroidsWithWings I just felt that "rvalue reference" would give more of a starting point than "strawberry milkshake" to the OP. Moreover, I don't want the OP to go outside and buy milkshakes in this season. – Ardent Coder May 23 '20 at 17:49
@ArdentCoder It may be possible to get them on Amazon Prime :) – Asteroids With Wings May 23 '20 at 17:49
@AsteroidsWithWings Oh no, more problems in that case: poor delivery boys would suffer as well, unless you've made virtual reality a reality. – Ardent Coder May 23 '20 at 17:50
2

Optimus Prime could kick Amazon Prime's butt. – user4581301 May 23 '20 at 17:51
@user4581301 Can't argue with that tbh – Big Temp May 23 '20 at 21:27

Generic `count` function

0 Answers0