Re: using enum classes for enumerated CSS property values in nsStyleConsts.h

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On 11/10/2015 10:40 PM, L. David Baron wrote:
>> One more thing may need to be done before more conversion comes:
>> we need a typesafe way to declare bitmasks which we are using for
>> several properties. I filed bug 1217241 for something similiar
>> several weeks ago.
> 
> I think mfbt/TypedEnumBits.h already does this.

As a heads-up, there's one slightly-awkward thing I've run into with
TypedEnumBits.h's "MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS" magic:
there's no implicit conversion to bool, when you try to put the result
of a bitmask operation into a variable or function-arg.

Specifically -- suppose I have "MyMaskType foo", and I want to test if
it has the bit MyMaskType::SomeBit set. The compiler will happily
accept this:
  if (foo | MyMaskType::SomeBit)

But, the compiler rejects this:
  bool isBitSet = foo | MyMaskType::SomeBit;

...and more importantly, this:
  SomeFunctionWhichTakesABoolArg(foo | MyMaskType::SomeBit);

The error looks like this, in clang:
error: no viable conversion from
'mozilla::CastableTypedEnumResult<MyMaskType>' to 'bool'

The compiler forces me to use "!!" or "bool(...)" to make the boolean
conversion explicit in these latter two cases, which makes these
type-safe masks a bit clumsier to work with.

~Daniel

P.S. (I can work around this by adding an "operator bool" to the
CastableTypedEnumResult type-definition in mfbt/TypedEnumBits.h. Maybe
that's something we should do.)
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