CODEWITHSUNDEEP
c++arrayspointerscastingreference
fredoverflow
fredoverflow

Reputation: 263320

Can I reverse the process of array-to-pointer decay?

Is it legal to cast a pointer to the first element of an array to a pointer to the entire array?

template<typename T, size_t N>
void whatever(T(&)[N])
{
    std::cout << N << '\n';
}

int main()
{
    int a[10];
    int * p = a;
    whatever(*(int(*)[10])(p));   // <-- legal?
}

This prints 10 on my compiler, but I'm not sure if the C++ standard allows it.

Upvotes: 4

Views: 418

Answers (2)

AProgrammer
AProgrammer

Reputation: 52324

I remember having concluded that

  • converting a pointer to the first element of a POD-struct into a pointer to the POD-struct
  • converting a pointer to one element of an array into a pointer to an array of size sufficiently small

are both well defined. But if the first conclusion was quite direct (i.e. there was a section in the standard saying so in quite plain words), the second took several steps (i.e. you needed to put together several part of the standard and draw a conclusion) and I've no time to redo the search now. If nobody gives citations one way or the other, I'll try to come back later with some.

Edit: I've not found my reasoning back. Currently all what I've is that under the assumption that reinterpret_cast gives the same result as the array decay, the round-trip from "pointer to array" to "pointer to first element" to "pointer to array" is guaranteed (per 5.2.10/7 in C++98) as the alignment requirement for an array can't be less strict than the alignment requirement for the element.

Upvotes: 0

Armen Tsirunyan
Armen Tsirunyan

Reputation: 133092

No, it's not legal(as in it's Undefined Behavior). A pointer to the whole array is &a not p. Basically, you're casting one pointer to another. The standard describes all the allowed conversions and this one is not among them.

Upvotes: 2

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