[boost] Re: type_traits - almost there?

From: John Maddock (John_Maddock_at_[hidden])
Date: 2000-02-06 07:39:43

Steve,

>I think we're almost done!

>There's just one little change I'd like now. . .  While reviewing the
>definition of a POD type, I noticed that not all POD types have to have
>trivial default constructors.  Therefore, in our type_traits, I recommend
>that is_POD should not imply has_trivial_constructor:<

You had me worried there for a minute - but checking the standard I'm
fairly sure that if a type is a POD then the default constructor is
trivial:

From 9.0:

"A POD-struct is an aggregate  class
  that  has  no  non-static data members of type pointer to member, non-
  POD-struct, non-POD-union (or array of such types) or  reference,  and
  has  no  user-defined  copy  assignment  operator  and no user-defined
  destructor.  Similarly, a POD-union is an aggregate union that has  no
  non-static  data  members  of  type pointer to member, non-POD-struct,
  non-POD-union (or array of such types) or reference, and has no  user-
  defined  copy  assignment  operator and no user-defined destructor.  A
  POD class is a class that is either a POD-struct or a POD-union."

The definition of an aggregate is in 8.5.1:

"1 An aggregate is an array or a class (_class_)  with  no  user-declared
  constructors  (_class.ctor_),  no private or protected non-static data
  members (_class.access_), no base classes  (_class.derived_),  and  no
  virtual functions (_class.virtual_)."

Those definitions are actually pretty tight, I don't see how any POD-struct
type can fail to have a trivial default constructor given those rules - did
you have a specific case in mind?

- John.