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: stackoverflow.com/a/12444983/126769 — nos
– nos, Commented Apr 6, 2017 at 13:10

Bathsheba · Accepted Answer · 2017-04-06 13:15:03Z

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 6, 2017 at 13:15

Bathsheba

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7 Comments

NathanOliver Over a year ago

Isn't it UB to add these overloads into the std namespace?

Bathsheba Over a year ago

Not, I think, for template specialisations; even partial ones. Cf std::hash. Perhaps an expert can confirm.

Slava Over a year ago

I think that who worked on standard should think about using result of std::equal_range in for range loop.

Lightness Races in Orbit Over a year ago

@NathanOliver: [C++14: 17.6.4.2.1/1] looks ok iff T is user-defined.

Jarod42 Over a year ago

@BoundaryImposition: But we cannot add this restriction on T (via std::enable_iffor example :-/)

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