Can you cast a pointer, into an array?

Question

I have a pointer "a", it is of type A*. I now that there is n objects of type A at that address and I want to iterate over them.

I would like to cast it to A[n], so that I can use the c++11 range-for and write for (auto temp : a){...}. Of course, I can use a classic for(int i=0; i<n; i++) {temp=a[i]; ...} but the range-for is cleaner.

@user2370139 C++ does not allow you to cast a raw pointer to an array type . So no. You need to implement this solution: http://stackoverflow.com/a/12444983/126769 — nos, Apr 06 '17 at 13:10
Possible duplicate of [Casting pointer to Array (int\* to int\[2\])](http://stackoverflow.com/questions/20046429/casting-pointer-to-array-int-to-int2) — apmccartney, Apr 06 '17 at 13:11
You could use a [`gsl::span`](http://codexpert.ro/blog/2016/03/07/guidelines-support-library-review-spant/) — NathanOliver, Apr 06 '17 at 13:13
or `for (auto&& e : ranges::view::counted(a, n))` with [range-v3](https://github.com/ericniebler/range-v3). — Jarod42, Apr 06 '17 at 17:19

score 1 · Answer 1 · answered Apr 06 '17 at 13:15

1

In reasonable code, I'd shy away from it. But C++ allows you to commit acts of sheer devilry. In that vein, I offer a solution:

At the expense of some considerable obfuscation, you can write some preparatory templates:

namespace std
{
    template <typename T> T* begin(std::pair<T*, T*> const& a)
    {
        return a.first;
    }

    template <typename T> T* end(std::pair<T*, T*> const& a)
    {
        return a.second;
    }
}

Then you can write something like

for (auto&& i : std::make_pair(a, a + n)){

}

The template stuff brings in suitable definitions of begin and end which is required by the range for loop.

answered Apr 06 '17 at 13:15

Bathsheba

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Isn't it UB to add these overloads into the `std` namespace? – NathanOliver Apr 06 '17 at 13:18
1

Not, I think, for template specialisations; even partial ones. Cf `std::hash`. Perhaps an expert can confirm. – Bathsheba Apr 06 '17 at 13:18
I think that who worked on standard should think about using result of `std::equal_range` in for range loop. – Slava Apr 06 '17 at 13:19
1

@NathanOliver: `[C++14: 17.6.4.2.1/1]` looks ok _iff_ `T` is user-defined. – Lightness Races in Orbit Apr 06 '17 at 13:45
@BoundaryImposition: But we cannot add this restriction on `T` (via `std::enable_if`for example :-/) – Jarod42 Apr 06 '17 at 14:07
@Jarod42: You can't get the compiler to enforce it for you, no. – Lightness Races in Orbit Apr 06 '17 at 16:57
`for (auto&& e : ranges::view::counted(a, n))` with [range-v3](https://github.com/ericniebler/range-v3). – Jarod42 Apr 06 '17 at 17:18

Can you cast a pointer, into an array?

1 Answers1