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Improving the Enum Class Bitmask


Back in 2016, I wrote a blog post about enum class bitfields that was influenced by Anthony William’s blog post about the very same topic: how to painlessly enable bitmask operators with the enum class feature that was introduced with C++11 for increased type safety and convenience. After using this in production code for a while, the amount of programming errors related to implicit conversions has diminished as usage has increased and legacy code has slowly converted. Yet, even by using this tool, I managed to write buggy code such as the following.

auto someMask = f();
if (someMask == myEnum::someEnumerator)

That code worked for more than 95% of the cases… which is not good enough. Fixing it was very easy once the bug was spotted. Simply correct this way :

auto someMask = f();
if (someMask & myEnum::someEnumerator)

Still, I wished this mistake could be detected at compile time. So I started working on a replacement for the previous solution that would make it impossible to use the operator== and operator!= operators to compare a bitmask with an enumerator. As someone pointed out on the Cpplang Slack channel, there are occasions when you want to make sure a mask has a single bit set and none other. This is one case where mask == enumerator is actually correct. I’ll get back to this concern very shortly.

Defining concepts

In order to make mask == enumerator a compilation error, two concepts are required : masks and enumerators. An enumerator’s purpose is to give a name to a specific bit when it is set. A mask, on the other hand, represents the state of every bit (and this way, of every enumerator), whether they are set or cleared. The language already provides the enum class type which represents enumerators. From this point, I’ll refer to enum class as type E. In order to represent masks, I define the bitmask<E> type.

If we get back to the mask == enumerator case that is considered valid. Why is it considered valid? Because the programmer really meant to use operator== because no other bit should be set. Well that violates the definition from the previous paragraph for the enumerator concept. The real intent when comparing a mask with an enumerator constant is to use that enumerator as a mask for the comparison. This means we require a convenient and intuitive way to promote E to bitmask<E>, like so:

template <typename E> bitmask<E> make_bitmask(E);

if (mask == make_bitmask(myEnum::someEnumerator))

This makes the intent of this comparison very explicit, it is not a mistake that a single enumerator was promoted as a bitmask.

Bitwise operations

Now let’s talk about bitmask operations. The bitmask operations are & (AND), | (OR), ^ (exclusive OR, XOR), ~ (complement), &= (AND assignment), |= (OR assignment) and ^= (XOR assignment). All of those operators are binary, except for the bitwise complement operator (~) which is unary. There are two different types every parameter can have, which results in 4 different permutations for the two parameters of the binary operators and 2 different types for the single parameter of the unary operation. Here is a summary the result types for every operation and permutation of parameters.

Binary bitwise operators

  E, E E, bitmask<E> bitmask<E>, E bitmask<E>, bitmask<E>
operator& E E E bitmask<E>
operator| bitmask<E> bitmask<E> bitmask<E> bitmask<E>
operator^ bitmask<E> bitmask<E> bitmask<E> bitmask<E>
operator&= bitmask<E> bitmask<E> bitmask<E> bitmask<E>
operator|= bitmask<E> bitmask<E> bitmask<E> bitmask<E>
operator^= bitmask<E> bitmask<E> bitmask<E> bitmask<E>

Unary bitwise operators

  E bitmask<E>
operator~ bitmask<E> bitmask<E>

Comparison operators

  E, E E, bitmask<E> bitmask<E>, E bitmask<E>, bitmask<E>
operator== bool static_assert static_assert bool
operator!= bool static_assert static_assert bool

As you may have noticed, there are only three bitwise operations that result with type E. Everything else returns the type bitmask<E>. Moreover, those three cases are three of the four permutations for a single operator : operator&. So the only way you can get back an enumerator from a bitmask is to apply a bitwise AND using at least one enumerator argument.

This design successfully enforces comparison of enumerator types with other enumerators of the same type, and of bitmask types with other bitmasks of the same type. Accidentally comparing an enumerator with a bitmask now triggers a static_assert that makes it explicit why this should be avoided.

Bool conversion

There is still one annoying issue. It should be possible to convert bitmask<E> to bool by testing whether it is non-zero. Likewise, an enumerator E should be convertible to bool so that the following statement is valid:

if (mask & enumerator)

Providing this conversion is done with the following function signature:

constexpr explicit operator bool() const;

The explicit on the bool operator allows it to be contextually convertible in five contexts:

  1. controlling expression of if, while, for;
  2. the logical operators !, && and ||;
  3. the conditional operator ?:;
  4. static_assert;
  5. noexcept.

This is how we’ll get implicit bool conversion for our if statements, while preventing other unwanted implicit conversions.

Bool conversion (continued)

The explicit operator bool can be implemented on the bitmask<E> template, but it cannot work for type E. In order to make this utility as convenient as possible, I have also defined type enumerator<T> so that the result of operator& with at least one enumerator argument results in a type which is contextually convertible to bool. This made the tables up there a bit more complex. Rather than the original 4 overloads for every operator, there are now 9 required overloads to cover every parameter permutation:

  1. T, T
  2. enumerator<T>, enumerator<T>
  3. bitmask<T>, bitmask<T>
  4. T, enumerator<T>
  5. enumerator<T>, T
  6. T, bitmask<T>
  7. bitmask<T>, T
  8. enumerator<T>, bitmask<T>
  9. bitmask<T>, enumerator<T>

Now that bitwise operations that produce an enumerator<T> are also contextually convertible to bool, the only thing left that is not is the original type T. This is actually a good thing as converting a single enumerator is always true. The only way it could be false is if you had an enumerator to represent 0, but that would violate the concepts defined at the beginning of this article : the value 0 is a mask, not an enumerator.

The complete source code can be found in my github repository.

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