Different behaviour between "int &&" and "auto &&"

Question

In C++11, if I try to do this:

int x = 5;

int && y = x;

It will fail to compile, with an error telling that an r-value reference cannot bind to an lvalue.

However if I do:

int x = 5;

auto && y = x;

It compiles with no errors. Why is it happening? I tried to get the type of y but typeid() takes away the reference attributes. Does auto && automatically collapses to a & or && depending on what is being assigned?

you mean `= x`, no? Don't forget you can always inspect the type with `typeid` — Bathsheba, Dec 11 '15 at 16:45
see [Universal References in C++11—Scott Meyers](https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers) — NathanOliver, Dec 11 '15 at 16:47
Now the official name is [forwarding references](http://stackoverflow.com/questions/33904462/whats-the-standard-official-name-for-universal-references/33904463#33904463) — 101010, Dec 11 '15 at 17:02

Nawaz · Accepted Answer · 2015-12-11T17:10:20.243

9

In the first case int && y, the variable y can bind to only rvalue which x is not.

In the second case auto && y however, the variable y can bind to anything, as the type of y would be deduced anyway — and reference-collapsing will be applied accordingly — which is why your code compiles:

auto && y = x;

Since x is an lvalue, auto is deduced to be int&, hence it becomes:

int& && y = x;

and after reference-collapsing, it becomes:

int & y = x;

which is fine.

To understand it in more detail, read about:

Universal Reference (or Forwarding Reference, as it has been proposed to improve the terminology)
Reference Collapsing

Hope that helps.

edited Dec 11 '15 at 17:10

answered Dec 11 '15 at 16:47

Nawaz

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The draft working paper for the C++ standard defines the term _forwarding reference_ in 14.8.2.1 [temp.deduct.call] paragraph 3 (i.e., a corresponding proposal was accepted at some point). – Dietmar Kühl Dec 11 '15 at 17:05
@DietmarKühl, thanks!!! I have been fighting with improper 'universal reference' for some time. – SergeyA Dec 11 '15 at 17:06

score 2 · Answer 2 · answered Dec 11 '15 at 16:49

2

int&& is an r-value reference to int. Whereas auto&& is a universal reference.

answered Dec 11 '15 at 16:49

Maxim Egorushkin

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I do not like the term universal reference. There is nothing universal about them. – SergeyA Dec 11 '15 at 16:50
@SergeyA The good thing about the term is that you immediately understood me without me having to wave my hands. I value that. – Maxim Egorushkin Dec 11 '15 at 16:51
Max, see above - now we have a proper term. – SergeyA Dec 11 '15 at 17:07
@SergeyA I am going to tattoo "forwarding reference" on my chest tonight. – Maxim Egorushkin Dec 11 '15 at 17:19
I'd certainly love to see the picture! – SergeyA Dec 11 '15 at 17:21

score 2 · Answer 3 · answered Dec 11 '15 at 16:49

2

In the second example, auto (in auto&&) would be anything which would make the code well-formed. In this case, int& && is well-formed, since int& && collapses to int& - thus all work well.

answered Dec 11 '15 at 16:49

SergeyA

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Different behaviour between "int &&" and "auto &&"

3 Answers3