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Comment by int_19h | Hacker News Reader

You don't need to cast anything - just placement-new it inside the array. So long as the array is properly aligned, this is fine. After that, it would be UB to peek at the bytes of the object via the array because of aliasing issues, but I don't see why it would be improper to use the pointer returned by new.

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After the object is constructed by placement-new, the class methods still needs to reinterpret_cast the char array to an object pointer to access the object.

I don't think in this specific case there is an UB involved, but I'm not language standard lawyer so I'm not sure. I feel the standard's specification on what is allowed to reinterpret_cast and what isn't is arcane (or at least far from straightforward to understand).

Not too terrible to create a private method that does the reinterpret_cast for you.

I've had to use this technique in the past and therefore dived into the standard for quite a while. I don't recall encountering any UB concerns.

You will get the properly typed pointer to object from new, so if you want to play it completely safe wrt UB, it can be stashed away alongside the array in the public class; this adds sizeof(T*) to the latter, but avoids casts entirely.

But, yes, you do technically need std::launder to get it directly via the array.

> After that, it would be UB to peek at the bytes of the object via the array because of aliasing issues

Do you have a source for this? IIRC char and std::byte have a specific aliasing exception. I.e. char* and std::byte are allowed to alias anything.

You obviously aren't allowed to modify the char or std::byte array (because that would violate the struct/class's aliasing rule).

I'm not so sure anymore, after looking at the standard again. It says that it's okay to access any object via a glvalue of type char, unsigned char, or std::byte; note that this does not include arrays of the same. If you have a field of array type, and you subscript it, the expression does involve a glvalue of that array type as one of the operands - but does this constitute "access"?

A little late, but yes, subscripting an array yields a gvalue of that type, but that is not an access unless you use the glvalue in some other computation (I assume). But even if you do access it (i.e. make a bitwise copy of the entire array) that should still not be UB.

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