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Subject: Re: [std-proposals] Allow reinterpret_cast (and static_cast from void*) in a constant expression in limited cases
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2019-10-31 09:23:25


On Thu, Oct 31, 2019 at 10:14 AM connor horman via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I propose to allow reinterpret_cast from pointer to integral, as well as
> the no-op/identity reintepret_cast in constant expressions. This particular
> version is useful in implementing hashcode algorithms at compile time, and
> to my knowledge bit_cast does not work for pointer types at compile time.
>

I think this is a non-starter. Consider:

// https://godbolt.org/z/Xd48ZV
const int i = 42;
constexpr int is_64_aligned(const int *p) {
    return reinterpret_cast<intptr_t>(p) % 64 == 0;
}
constexpr bool x = is_64_aligned(&i);
int main(int argc, char **argv) {
    bool y = is_64_aligned(argc ? &i : nullptr);
    assert(x == y);
}

How is the compiler supposed to know *at compile time* what bits are in the
runtime bit-pattern of `p`?

-Arthur



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