Passing enum to overload function

Question

I'm trying to pass an enum which is basically a byte to a overload resolution, but it doesn't work.

Here is the enum:

enum class WriteOp : uint8_t {
  kAdd = 0,
  kDelete
};

Here are the overloads:

 void copy_out(uint8_t& v) { getn((char*) &v, 1); }
 void copy_out(uint16_t& v) { getn((char*) &v, 2); }
 void copy_out(uint32_t& v) { getn((char*) &v, 4); }
 void copy_out(uint64_t& v) { getn((char*) &v, 8); }

And here is a typical error messages I'm getting:

main.cpp:164:8: note: candidate function not viable: no known conversion from 'WriteOp' to 'uint8_t &' (aka 'unsigned char &') for 1st argument
void copy_out(uint8_t& v) { getn((char*) &v, 1); }

Any simple and elegant way to do that?

You have a *strongly typed enum*, the point of this is to make it a different type (ie not implicitly casted). You should either cast it or make an overload. — Fantastic Mr Fox, Aug 19 '15 at 20:56
One specific feature of `enum class` is that it is not implicitly convertible. If you want that, use an ordinary `enum`. — Bo Persson, Aug 19 '15 at 20:56
@Frank You have explicitly to convert a scoped enumerator to a desired type. — Vlad from Moscow, Aug 19 '15 at 20:56
See also [converting-enum-to-underlying-type](http://stackoverflow.com/questions/14589417/can-an-enum-class-be-converted-to-the-underlying-type). — , Aug 19 '15 at 21:03
_"I'm trying to pass an enum"_ No you're not. _"which is basically a byte"_ No it's not. — Lightness Races in Orbit, Aug 19 '15 at 21:06

score 2 · Answer 1 · answered Aug 19 '15 at 20:59

2

You have a strongly typed enum the whole point of these are they are not implicitly convertible and can have scope. You can do 1 of 3 things:

Create an overload for your type: void copy_out(WriteOp& v) { getn((char*) &v, 1); }
Explicitly cast your enum copy_out(&(uint8_t)my_enum);
Use a standard enum.

enum WriteOp {
  kAdd = 0,
  kDelete
};

answered Aug 19 '15 at 20:59

Fantastic Mr Fox

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Note that many compilers support sized non-class enums. Oh, and are you certain that an eum can be implicitly cast to a reference to its underlying type? Or that its underlying type is one of those types? – Yakk - Adam Nevraumont Aug 19 '15 at 21:20

score 2 · Accepted Answer · answered Aug 19 '15 at 21:24

template<class E,
  std::enable_if_t<std::is_enum<E>{}>* = nullptr
>
void copy_out(E& e){
  getn(reinterpret_cast<char*>(&e),sizeof(e));
}

if you are taking an enum, take an enum.

The above takes any enum, and does a getn.

With a bit of refactoring, you could move all calls to getn and its unsafe casts into one function.

Passing enum to overload function

2 Answers2