Re: using a class inside a class.
On Apr 29, 4:45 pm, Cynic <nik.n.n...@gmail.com> wrote:
On Apr 29, 5:00 pm, James Kanze <james.ka...@gmail.com> wrote:
On Apr 29, 10:57 am, Ian Collins <ian-n...@hotmail.com> wrote:
James Kanze wrote:
On Apr 28, 8:33 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:=
All I can tell you is I've done this exact same thing
in the past successfully in both Microsoft Visual C++
.net 2003 and DevC++. Here is something that compiles
sucessfully in Microsoft Visual C++ .net 2003:
#include <list>
class CFoo
{
private:
std::list<CFoo> m_children;
};
int main()
{
CFoo Foo;
}
No warnings, no errors.
That's what's so nice about undefined behavior. The fact that
it works doesn't prove anything.
I don't know why your version of gcc is complaining, I don't
think it should, AFAIK this is perfectly legal, although I
haven't read the standard on it.
=A717.4.3.6/2:
In particular, the effects are undefined in the
following cases:
[...]
-- if an incomplete type (3.9) is used as a template
argument when instantiating a template component.
A class is only a complete type after the closing braces, so
your code has undefined behavior. (The standard could have
allowed incomplete types for certain specific cases, but it
didn't.)
But isn't the template component being instantiated in main
after the class is complete?
The class itself will be instantiated anytime it is used in
a context where a complete type is required. (How could it
be otherwise?) Class members (functions, static data
members) will be instantiated when (and only when) they are
used. Since only complete types can be used as member
variables, the class itself must be instantiated in CFoo.
(Note that declaring the type as "std::list< CFoo >*
m_children" should work.)
this would create a pointer to a LIST?
Yes. And since declaring a pointer to the type doesn't require
the type (i.e. the instantiation of std::list) to be complete,
it doesn't trigger instantiation of the template.
and would it help then if the std::list is created outside the
class?
Yes. As long as the type in question is complete whenever the
class is instantiated. The formal rules as to when something is
intantiated are fairly complex, but a rough approximation,
sufficient for most concerns, is simply that the class itself
will be instantiated anytime the compiler needs a complete type,
or something in the class, and the class members will be
instantiated anytime they are used.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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