C++ auto function return?

Question

lately I've seen some code with functions labeled like this:

auto Function(...) -> BOOL

Is there some difference between using just

BOOL Function(...)

The first one looks nicer to me though, maybe I'm just weird, so is it just visual, or it has some other benefits?

It's called *trailing return type* an can be very useful if the returned type depends on the arguments to your function. Otherwise they are just the same the way you show it. — Some programmer dude, Jun 11 '18 at 10:56
I'm with you stylistically. It makes all the functions align nicely to the start of the line, without messing with spaceing. — StoryTeller - Unslander Monica, Jun 11 '18 at 10:59

KostasRim · Answer 1 · 2018-06-11T11:06:56.227

7

The statement auto Function(...) -> some_type is used when you need type deduction of the arguments before the return type

template<class T>
   decltype(a*b) add(T a, T b){
   return a + b;
}

But this wont work so you need:

 template<class T>
   auto add(T a, T b) -> decltype(a + b) {
   return a + b;
 }

edited Jun 11 '18 at 11:06

answered Jun 11 '18 at 10:56

KostasRim

`bool` is clearly not dependent upon the parameters. – LogicStuff Jun 11 '18 at 10:58
1

Because you answer is false. It is used for type deduction of the return type based on the arguments. And usually that's only relevant in meta programming. The bool versions as shown in the question make no difference at all. – Simon Kraemer Jun 11 '18 at 10:58
And your code in invalid as well, as it doesn't really show the trailing return type. – Some programmer dude Jun 11 '18 at 11:01
@SimonKraemer I was implying that type deduction (i.e. return type) is based on the arguments. I hope my example is now clearer – KostasRim Jun 11 '18 at 11:04
Thank you all guys, didn't really expect that much heat there lol – Martin Prince Jun 11 '18 at 11:06
It is "heat" only for sake of clarity, accuracy, and posterity. It is not intended as an insult, although sometimes it may appear to be a dogpile. – Eljay Jun 11 '18 at 14:11
@Eljay Couldn't agree more! – KostasRim Jun 11 '18 at 17:13

score 7 · Accepted Answer · answered Jun 11 '18 at 11:02

7

Is there some difference between using just [...]

No - in your particular example, they are equivalent.

it just visual, or it has some other benefits?

Trailing return types have a few benefits:

Easier to switch to automatic return type deduction in the future (just delete everything after ->)
Can use parameters as part of the return type
Can access class C type aliases without having to say C::

In your particular example, these do not apply.

answered Jun 11 '18 at 11:02

Vittorio Romeo

2

Read also this [answer](https://stackoverflow.com/questions/11215227/should-the-trailing-return-type-syntax-style-become-the-default-for-new-c11-pr) and this [article](http://www.cprogramming.com/c++11/c++11-auto-decltype-return-value-after-function.html). It can be very useful with lambda and decltype, sizeof – Clonk Jun 11 '18 at 11:03
1

Not sure if deleting everything after the `->` is really easier than changing the return type from `BOOL` to `auto`. – Baum mit Augen Jun 11 '18 at 11:05

2 Answers2