Can I legally reinterpret_cast between layout-compatible standard-layout types?
c++, c++14, standard-layout, standards
Solution
but I can't see anything in the standard that therefore allows me to `reinterpret_cast` between them, even though that seems like the reasonable interpretation of "value representation". Is this technically allowed by the standard?
No. The standard is clear (see [basic.lval] p10) about which types can be aliased, and layout-compatible types are not included.
If not, what does knowing the value representation of a type give you?
If the types are both trivially copyable and have the same value representation then you could memcpy from an object of one type to an object of the other type, and vice versa. If they're not trivially copyable then it doesn't give you much at all.
AFAICT the standard doesn't actually say what can and can't be done with layout-compatible types.
Problem
I'm writing a class that, assuming the answer to Are enumeration types layout compatible with their underlying type? is "yes", is layout-compatible `struct kevent` but uses `enum class`es for `filter`, `flags`, etc. with the proper underlying types for the relevant fields. It is also standard-layout (the fields are all `private` and all themselves standard layout, there are no `virtual` members, there are no base classes). From my reading of `n3690`, I can determine that my class and `struct kevent` have the same value representation, but I can't see anything in the standard that therefore allows me to `reinterpret_cast` between them, even though that seems like the reasonable interpretation of "value representation". Is this technically allowed by the standard? If not, what does knowing the value representation of a type give you? EDIT 2014/02/24 16:45 EST: In response to a comment, I should clarify that I want to `reinterpret_cast` the first class to a reference to the second, as of course you can't directly `reinterpret_cast` a non-pointer type to another non-pointer type.