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How to create an boost::asio::ip::address_v6 from sockaddr_in6 without making an extra copy?

The problem is that address_v6 class accepts raw data strictly as bytes_type class:

typedef array< unsigned char, 16 > bytes_type;
but sockaddr_in6 struct doesn't have that, it has C-style arrays, that can't be converted to std::array without copying. So I have to create an std::array, copy data there, and pass that array to address_v6, which copies data from that std::array to its internal buffer.
I wish I could use or implement some C-style array viewer class but I wouldn't be able to pass it to the constructor anyway, because it's not a template function.
Is there some way to create an address_v6 without making an extra copy of the data?
                Why are you concerned about copying just 16 bytes (two machine words on many modern systems)?
– Davis Herring
                May 31, 2021 at 6:32
                @DavisHerring due to my bad memory I forgot that array doesn't allocate its memory on heap, thus there's no overhead for that. So yeah it's fast anyway, but I was still curious how to avoid extra copying.
– Nikita128
                May 31, 2021 at 7:04
The std::array is required to be struct that contains raw array as its first and only non-static data member. As the raw array contains unsigned chars it is therefore standard layout class. So I can not find a reason from standard why following code would not work:
auto& bytearray = reinterpret_cast<std::array<unsigned char,16>&>(ipv6socket->sin6_addr.s6_addr); 
Replace auto& with plain auto if you want to copy.
                That's exactly what I wanted! I've tried to do a similar cast, but due to my lack of experience couldn't pull it off, had mistakes here and there. But now it works, thanks!
– Nikita128
                May 31, 2021 at 6:58
                It’s undefined behavior to use that reference for anything.  Being standard-layout doesn’t mean you can pretend an object is there if it’s not.
– Davis Herring
                May 31, 2021 at 7:22
                @DavisHerring why is it UB? If I'm fully aware of the insides of the class, and I'm sure they correspond with the data I'm casting from, it's still UB?
– Nikita128
                May 31, 2021 at 8:45
                @DavisHerring I do agree that std::array is specified very carefully to be bit-compatible with C-style arrays (imagine the riots from game devs, had they standardized otherwise). It should not be hard to find the authoritative section on this
– sehe
                May 31, 2021 at 12:16
                @Nikita128: [basic.lval]/11 provides the spirit of the rule, but actually applies only to scalars.  [expr.static.cast]/13 describes the failure of the reinterpret_cast to find an appropriate object (not just type!), and then eventually [expr.ref]/6.2 or [expr.call]/2 produces undefined behavior by omission for want of an object with the given member (function).
– Davis Herring
                May 31, 2021 at 15:39
                Yeah, you've just reminded me that std::array has no memory allocations on heap, so there is no overhead for that. But still I wanted to figure out how to avoid extra copying.
– Nikita128
                May 31, 2021 at 7:00
        
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