Reference to array

Question

This is probably easy for you but I am not really into references yet. I cannot see why this would not work. Please show me the correct style.

template <typename T>
void shuffle(T (&array)[], size_t len){};

And I try to call it like this:

uint_fast8_t dirBag[4]
Random::shuffle(dirBag, sizeof(dirBag)/sizeof(decltype(dirBag[0])));

The error is:

Error   1   error C2784: 'void Random::shuffle(T (&)[],size_t)' : could not deduce template argument for 'T (&)[]' from 'uint_fast8_t [4]'

The way I read 3.9/6, a pointer to an incomplete array is itself an incomplete type that can moreover never be completed. No mention is made of references to incomplete arrays, but I would assume the same holds. This is a bit vague, though... — Kerrek SB
– Kerrek SB, Commented Feb 24, 2013 at 2:27
This question actually has much more to do with deduction than about mechanics, as I first thought it did. I think I may drop my answer as a result. Though you may find it helpful, it isn't really an answer to the deeper question you seem to be asking, i.e. why doesn't this automagically deduce correctly. If you would rather I keep it up, I certainly will, but I think Kerrek's comment is very in-line with the question you asked, even if you didn't know you asked it =P — WhozCraig
– WhozCraig, Commented Feb 24, 2013 at 2:31

WhozCraig · Accepted Answer · 2013-02-24 02:26:45Z

5

Try:

template <typename T, size_t N>
void shuffle(T (&array)[N])
{
};

Further, within the body of the function, you have the size of the array: N. If you must pass a size as a parameter, I certainly suppose you can, but the parameter would be independent.

Site Note: Judging by the name of your function, you may also find this of interest:

uint_fast8_t dirBag[4];

// ...populate your array...

std::random_shuffle(std::begin(dirBag), std::end(dirBag));

edited Feb 24, 2013 at 2:26

answered Feb 24, 2013 at 2:20

WhozCraig

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4 Comments

Etherealone Over a year ago

This also takes away the ugly sizeofs. Also, I did not know about that shuffle function. Thanks a lot.

WhozCraig Over a year ago

@Tolga Certainly. I still think your question was deeper than you may first think (why didn't your call deduce correctly). But so long as this helped, I guess thats what we're ultimately here for. Don't forget to seed your RNG, btw, or use a generator, if you're using the random shuffler.

Etherealone Over a year ago

Yes I was actually wondering why it did not find it itself. It did compile (although did not test if it worked or not) without the reference. If it can copy the array it knows its size, but why not with reference.

WhozCraig Over a year ago

@Tolga Exactly, And I think Kerrek's comment actually addressed that pretty well. This answer is more of a "how to not have that problem in the first place by providing a more-complete type", but I think his assessment of the fact that without the size specification within the array decl, it is considered an incomplete type, and therefore cannot deduce completely is probably spot-on. (and +1 on the question which might be deeper than you first thought =P)

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