Innocent Bystander - 6 months ago 30

C++ Question

Given

`enum class val { foo = 1, bar = 2, baz = 4 };`

It is possible to define:

`val operator|(val x, val y)`

{

return static_cast<val>(static_cast<int>(x) | static_cast<int>(y));

}

However, is it semantically correct to do so?

I am leaning towards

`int convert(val x)`

{

switch(x)

{

case val::foo: return 42;

case val::bar: return 53;

case val::baz: return 64;

}

}

Calling

`convert(val::foo | val::bar)`

`0`

Here is g++ version. And here is clang++ version.

My question is two-fold:

- Is it semantically correct to store values in an enum that are not represented by an enumerator? Excerpts from the standard are most welcome.

1.a Which compiler is correct in the above linked examples, g++ or clang++?

- Is there a standard (or proposed) way to represent flags in C++?

I can think of several possible implementations:

`enum class val { foo, bar, baz, size };`

using val_flags = std::set<val>; // (1)

using val_flags = std::vector<bool>; // (2)

using val_flags = std::bitset<val::size>; // (3)

using val_flags = std::underlying_type<val>::type; // (4)

Thank you all for your answers. I ended up resurrecting my old enum operator template. In case anybody is interested, it can be found here: github.com

Answer

following, seemingly well-behaving example:

It's not, but make one minor change:

```
int convert(val x)
{
switch(x)
{
case val::foo: return 42;
case val::bar: return 53;
case val::baz: return 64;
}
return 9; // ADDED THIS LINE
}
```

and all will be well. An alternate fix would be to use a `default:`

case and return there.

Your existing code triggers undefined behavior^{1} by reaching the closing brace of a function with a non-`void`

return type. Because it is undefined behavior, both compilers are correct.

The semantics of holding values in an `enum`

type which are bitwise OR combinations of enumerator values are well-defined and guaranteed. The standard requires that instances of the `enum`

can store any integer value with no more bits used than any of the enumerator values defined, which includes all bitwise-OR combinations. The formal language used to say this is a bit messy, but here it is (note that your case is an `enum class`

, these always have fixed underlying type and the first sentence applies):

For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type. Otherwise, for an enumeration where e

_{min}is the smallest enumerator and e_{max}is the largest, the values of the enumeration are the values in the range b_{min}to b_{max}, deﬁned as follows: Let K be 1 for a two’s complement representation and 0 for a ones’ complement or sign-magnitude representation. b_{max}is the smallest value greater than or equal to max(|e_{min}| − K, |e_{min}|) and equal to 2^{M}− 1, where M is a non-negative integer. b_{min}is zero if e_{min}is non-negative and −(b_{max}+ K) otherwise. The size of the smallest bit-field large enough to hold all the values of the enumeration type is max(M, 1) if b_{min}is zero and M + 1 otherwise. It is possible to deﬁne an enumeration that has values not defined by any of its enumerators. If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a single enumerator with value 0.

(from n4582, section 7.2 `[dcl.enum]`

)

^{1} From 6.6.3 `[stmt.return]`

:

Flowing off the end of a constructor, a destructor, or a function with a

cv`void`

return type is equivalent to a return with no operand. Otherwise, flowing off the end of a function other than main (3.6.1) results in undeﬁned behavior.