Can I reinterpret_cast some byte range of a POD C-Array to std::array?

Question

I want to use fixed contiguous bytes of a long byte array s as keys in a std::map<std::array<char,N>,int>. Can I do this without copying by reinterpreting subarrays of s as std::array<char,N>?

Here is a minimal example:

#include <map>
int main() {
    std::map<std::array<char,10>,int> m;
    const char* s="Some long contiguous data";

    // reinterpret some contiguous 10 bytes of s as std::array<char,10>
    // Is this UB or valid? 
    const std::array<char,10>& key=*reinterpret_cast<const std::array<char,10>*>(s+5);

    m[key]=1;
}

I would say yes, because char is a POD type that does not require alignment to specific addresses (in contrast to bigger POD types, see https://stackoverflow.com/a/32590117/6212870). Therefore, it should be OK to reinterpret_cast to std::array<char,N> starting at every address as long as the covered bytes are still a subrange of s, i.e. as long as I ensure that I do not have buffer overflow.

Can I really do such reinterpret_cast or is it UB?

EDIT: In the comments, people correctly pointed to the fact that I cannot know for sure that for std::array<char,10> arr it holds that (void*)&arr==(void*)&arr[0] due to the possibility of padding of the internal c-array data member of the std::array template class, even though this typically should not be the case, especially since we are considering a char POD array. So I update my question:

Can I rely on the reinterpret_cast as done above when I check via static_assert that indeed there is no padding? Of coures the code won't compile anymore on compiler/platform combinations where there is padding, so I won't use this method. But I want to know: Are there other concerns apart from the padding? Or is the code valid with a static_assert check?

Answer 1

No—there is no object of type std::array<char,10> at that address, regardless of the layout of that type. (The special rules for char do not apply to a type that happens to have char subobjects.) As always, it is not the reinterpret_cast itself whose behavior is undefined, but rather the access through that non-object when using it as a map key. (What you are allowed to do in this case is merely cast it back to the real type, for use with C-like interfaces that require a fixed pointer type but do not actually use the object.)

This access also of course involves a copy; if your goal was to avoid copying at all, just make a

std::map<const char*,int,ten_cmp>

where ten_cmp is a functor type that compares 10 bytes starting from each address (via std::strncmp or std::string_view).

If you do want the map to own its key data, just std::memcpy from the string into a key; compilers often recognize that such temporary “buffers” don’t need to exist independently and actually read from the source in the fashion you hope to do with reinterpret_cast.