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

"L. David Baron" <[email protected]> Wed, 11 Nov 2015 14:07:31 +0800

Newsgroups	gmane.comp.mozilla.devel.layout
Message-ID	<[email protected]>

Jesse suggested using enum classes for the values of enumerated CSS
properties, and I tend to agree that this is a good idea.  It gives
both stricter typechecking than the current uint8_t (etc.) values,
and better warnings for switch() statements omitting values.

I decided to try this out for box-sizing to see what it would look
like.  Patches are linked from:
https://bugzilla.mozilla.org/show_bug.cgi?id=1223653#c1

Interestingly, the typechecking actually caught a real bug, where we
were passing an nscoord (size inside the box-sizing) to a function
that expected a uint8_t (enumerated value of box-sizing property),
because we actually intended to call a different function.

But I'm mainly interested in feedback on:

1. Naming
=========

Do the names I chose seem reasonable?  I replaced:
  -#define NS_STYLE_BOX_SIZING_CONTENT       0
  -#define NS_STYLE_BOX_SIZING_PADDING       1
  -#define NS_STYLE_BOX_SIZING_BORDER        2
with:
  +namespace mozilla {
  +
  +enum class StyleBoxSizing : uint8_t {
  +  Content,
  +  Padding,
  +  Border
  +};
  +
  +} // namespace mozilla
so that most caller changes look like:
  -    case NS_STYLE_BOX_SIZING_BORDER:
  +    case StyleBoxSizing::Border:
or like:
     if (parent &&
  -      parent->StylePosition()->mBoxSizing != NS_STYLE_BOX_SIZING_BORDER) {
  +      parent->StylePosition()->mBoxSizing != StyleBoxSizing::Border) {

2. Casts
========

Does anybody see a good way to eliminate the casts I needed to use
in the main patch (described in the patch header)?

3. General
==========

Do others agree this is a good idea?  And are people ok with it
being done gradually?

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)

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