Why is forwarding variadic parameters invalid?

Question

Consider the variadic function parameter:

func foo(bar:Int...) -> () { }

Here foo can accept multiple arguments, eg foo(5,4). I am curious about the type of Int... and its supported operations. For example, why is this invalid?

func foo2(bar2:Int...) -> () {
    foo(bar2);
}

Gives a error:

Could not find an overload for '_conversion' that accepts the supplied arguments

Why is forwarding variadic parameters invalid?

What is the "conversion" the compiler is complaining about?

You'd think that, with the focus the language authors clearly had on righting [the wrongs of C](http://stackoverflow.com/questions/150543/forward-an-invocation-of-a-variadic-function-in-c), that they would have made this possible. — jscs, Jun 04 '14 at 21:27
Reference: http://stackoverflow.com/questions/24024376/passing-an-array-to-a-function-with-variable-number-of-args-in-swift/24024428#24024428 — GoZoner, Jun 04 '14 at 21:42

score 7 · Accepted Answer · edited May 23 '17 at 11:51

7

When you call foo, the compiler expects a series of arguments, each of which must be an Int.

In the body of foo2, bar2 summarises all the passed arguments and actually has the type Int[] for all practical purposes. Thus, you cannot pass it directly to foo — as foo wants Int arguments, and not an Int[].

As for a solution to this: see my answer to this question.

edited May 23 '17 at 11:51

Community

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answered Jun 04 '14 at 21:20

Jean-Philippe Pellet

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Is there anyway to turn `Int[]` into a series of arguments so that what the user is attempting to accomplish can be accomplished? – nhgrif Jun 04 '14 at 21:21
So within the body of `foo` you can essentially treat `bar` as a `Int[]` – yizzlez Jun 04 '14 at 21:22
See http://stackoverflow.com/questions/24046142/how-to-forward-functions-with-variadic-parameters. Basically, you overload `foo` with an `Int[]` argument. – Jean-Philippe Pellet Jun 04 '14 at 21:22
@awesomeyi Yes, that's it. – Jean-Philippe Pellet Jun 04 '14 at 21:22
Kind of defeats the convenience of variadic parameters in the first place though, doesn't it! – devios1 Jul 24 '14 at 02:27
1

Yes, partially at any rate. Hopefully the language can provide a better solution in a future revision. – Jean-Philippe Pellet Jul 24 '14 at 06:06
what if my `foo` function is actually an apple function, like for example `localizedStringWithFormat`? – pvinis Feb 19 '18 at 09:40

score 0 · Answer 2 · answered Aug 12 '14 at 03:25

0

You can forward a variadic argument but the function you forward it to has to be defined with a parameter that is an array, not a variadic. So write your foo function as

func foo(bar:[Int]) -> () { }

and it works.

answered Aug 12 '14 at 03:25

RobertL

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Why is forwarding variadic parameters invalid?

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