Re: instantiating a template on an incomplete type?
On Jun 12, 1:22 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:
Alf P. Steinbach wrote:
Granted. I should have said non-static data members. (The
standard says in [9.4.2/2]:
The declaration of a static data member in its class
definition is not a definition and may be of an incomplete
type other than cv-qualified void.)
The question about whether std::vector<xyz> is instantiated in
class xyz;
typedef std::vector<xyz> xyz_list;
class xyz {
xyz_list more_of_mine;
};
is really whether xyz_list in the declaration
xyz_list more_of_mine;
is allowed to be incomplete, i.e, whether something like
class def;
class abc {
def some_member;
};
is legal. I don't think it is and my compiler complains about
it. However, as of now, I cannot prove from the standard that
the snippet is ill-formed.
What about =A79.2/8: "Non-static members that are class objects
shall be objects of previously defined objects." Why they avoid
the expression "incomplete type" in this context is beyond me.
It would seem a lot clearer to me had they said that non-static
members (or non-static data members) cannot be incomplete types,
especially as it isn't legal to have a non-static data member
with type void or an array type with unknown dimensions either.
The modivation here is simple: the class definition determines
the layout, and in order to the determine the layout, the
compiler needs to know the size of each member. And it only
knows the size for complete types.
With regards to templates, of course, the rule (=A714.7.1) is "the
class template specialization is implicitly instantiated when
the specialization is referenced in a context that requires a
completely-defined object type or when the completeness of the
class type affects the semantics of the program [e.g. delete of
a pointer to the type]." So std::vector<xyz> will be
instantiated in the line which defines xyz::more_of_mine (but
not in the typedef); since the standard says that std::vector
requires a complete type when instantiated, the code is illegal.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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