CODEWITHSUNDEEP
c++enumslanguage-lawyerc++20reinterpret-cast
jacobsa
jacobsa

Reputation: 6609

Can a pointer to a C++ enum's underlying type be turned into a pointer to the enum value?

Is there a legal way, according to the C++20 standard, to turn a pointer to an unscoped enumeration type's underlying type into a pointer to the enumeration type? In other words:

enum Enum : int {
  FOO = 0,
  BAR = 1,
}

// How do I implement this without undefined behavior (and ideally without
// implementation-defined behavior)?
const Enum* ToEnum(const int* p);

I'm surprised to find that it's not listed as a legal use of reinterpret_cast.

If you're interested in why I want this: in a templated API I'm trying to work around the fact that protocol buffers provide repeated enum fields as a proto2::RepeatedField<int>, i.e. an array of ints, despite the fact that there is a strongly-typed enum associated with the field. I would like to be able to turn this into a std::span<Enum> without needing to copy the values.

Upvotes: 1

Views: 203

Answers (1)

Davis Herring
Davis Herring

Reputation: 39808

No, this is not one of the very few exceptions to the aliasing rule ([basic.lval]/11): you can construct such a std::span, but attempting to use its elements (e.g., s.front()=FOO) has undefined behavior.

Upvotes: 2

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