Attributes

[Joseph S. Myers]

I refer you to the various issues I have raised with attributes proposals 
in WG14 in the past, some at least of which have also been forwarded to 
WG21.  The committees have generally chosen not to listen to compatibility 
concerns, except insofar as WG14 ended up not adopting general attribute 
syntax at all but instead adding new keywords for some particular uses of 
attributes.

I also quote here something I sent off-reflector to Herb Sutter 
(Microsoft) last September (see in particular the first paragraph 
regarding treating [[]] as having clearly separate rules from 
__attribute__):

  [...] I'm also concerned about things for users.  If I implement the 
  proposals in GCC, compatibility with existing code would inevitably mean 
  that __attribute__ follows existing rules for how it binds to syntax 
  constructs, and [[]] follows the different C1x rules, and 
  __attribute__((noreturn)) is part of the type system, and [[noreturn]] 
  isn't.  This avoids breaking existing code, but is bound to confuse users 
  when they try to change syntax (likely with macros such as those you 
  mention) if they happened to be using cases where the two syntaxes bind 
  differently.  When you have two similar but incompatible ways of doing 
  something, you have a no win situation for users - and it's not WG14's job 
  to create work for front-end writers implementing the different modes for 
  compatibility with different code, they should be helping users by 
  avoiding needing the different modes.

  The fact that there are some problems with existing implementations 
  (including bugs where it may not be at all clear what is actually the best 
  thing to do), such as questions of exactly how attributes should interact 
  with the type system, may mean that a standard version cannot be perfectly 
  compatible with existing implementations in all cases.  But by studying 
  what implementations actually do, and what real code using these features 
  does, and what problems have been noted in implementation over the years, 
  it ought to be possible to come up with something friendlier to users (and 
  less ambitious) than the existing proposals.  So far however the proposers 
  of attributes standardization do not seem to have been paying any 
  attention to my points [...]

  Issues we've observed in practice with how attributes interact with the 
  type system include those described in 
  <http://gcc.gnu.org/ml/gcc/2006-10/msg00318.html> and 
  <http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01056.html>.

-- 
Joseph S. Myers
joseph@codesourcery.com