CODEWITHSUNDEEP
c++c++11language-lawyer
Daniel Jour
Daniel Jour

Reputation: 16156

static_cast to enum class from underlying type value and switch for compiler assistance

Suppose I have an enum:

enum class Thing : std::uint8_t {
  Foo = 21,
  Bar = 42,
  Baz = 1
};

I want to convert a "raw" value (of the underlying type) to a value of that enum, with error handling catching values which are not "a thing". I could use checks like if (raw_value == static_cast<std::uint8_t>(Thing::Foo)) and then an else with the error handling, but then I could forget for example Thing::Baz. However, when I switch over a value of type Thing, then my compiler (as probably most modern compilers) warns me about unhandled enumeration values ("enumeration value Baz not handled in switch").

So I came up with this:

Thing thingFromUInt8(std::uint8_t const b) {
  Thing const t = static_cast<Thing>(b);
  switch (t) { // If I'd add Thing::Frob but forget it here I get a warning
    case Thing::Foo:
    case Thing::Bar:
    case Thing::Baz:
      return t;
  }
  throw std::runtime_error("That's not a thing...");
}

Questions:

Upvotes: 6

Views: 1296

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275370

It is legal C++.

The drawback is the DRY violation, but avoiding it is difficult.

In we'll have reflection and be able to generate equivalent code (without having to rely on compiler warnings to make sure we didn't miss any). Reflection syntax is still a bit in flux, but every version I have read over was able to handle that problem.

Upvotes: 3

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