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Sebastian Faust
Guest
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 Posted: Sat Sep 27, 2003 7:49 pm Post subject: using friend and template class |
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Hi,
is a construction like the following possible:
template<class view_model>
class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
clase friend_class
{
friend_class() {}
virtual ~friend_class{}
friend template_class;
};
My compile ( Visual C++ 7.0 ) doesn't accept this code. Is it possible and
if yes how, to use a template class as a friend class?
Thanks in advance
Sebastian
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WW
Guest
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| Posted: Sat Sep 27, 2003 8:50 pm Post subject: Re: using friend and template class |
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Sebastian Faust wrote:
| Quote: | Hi,
is a construction like the following possible:
template
class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
clase friend_class
{
friend_class() {}
virtual ~friend_class{}
friend template_class;
};
My compile ( Visual C++ 7.0 ) doesn't accept this code. Is it
possible and if yes how, to use a template class as a friend class?
|
There is no such thing as template class. There are class templates. Read
this two sentences until you grasp the difference. Class templates are
*only* templates. They are not classes. So they cannot be made friends.
Furthermore I do not understand that why do you call the class giving friend
access friend_class? In C++ we call friend class the class *being given*
the friendship. Furthermore you have template_clase and template_class. I
assume this is a typo.
--
WW aka Attila
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Viatcheslav L. Gorelenkov
Guest
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| Posted: Sun Sep 28, 2003 1:53 am Post subject: Re: using friend and template class |
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Hi,
This code is works (VC++ 7.1):
template<class view_model> class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
class friend_class
{
friend_class() {}
virtual ~friend_class() {}
friend class template_clase<class view_model>; // this is a
template!!!!
};
:-))
Have fun,
Viatcheslav.
"Sebastian Faust" <sfaust_deleteme (AT) logic-software (DOT) de> wrote
| Quote: | Hi,
is a construction like the following possible:
template
class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
clase friend_class
{
friend_class() {}
virtual ~friend_class{}
friend template_class;
};
My compile ( Visual C++ 7.0 ) doesn't accept this code. Is it possible and
if yes how, to use a template class as a friend class?
Thanks in advance
Sebastian
|
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| Back to top |
|
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Viatcheslav L. Gorelenkov
Guest
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| Posted: Sun Sep 28, 2003 2:28 am Post subject: Re: using friend and template class |
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"WW" <wolof (AT) freemail (DOT) hu> wrote
| Quote: | Sebastian Faust wrote:
Hi,
is a construction like the following possible:
template<class view_model
class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
clase friend_class
{
friend_class() {}
virtual ~friend_class{}
friend template_class;
};
My compile ( Visual C++ 7.0 ) doesn't accept this code. Is it
possible and if yes how, to use a template class as a friend class?
There is no such thing as template class. There are class templates.
Read
this two sentences until you grasp the difference. Class templates are
*only* templates. They are not classes. So they cannot be made friends.
|
1. At least at definition we call it "template )
2. Both "template classes" and "classes" are data abstractions.
3. Both abstractions has visibility rules (public, protected, private) for
data/methods.
4. For some other abstractions (does not matter - "template classes",
"classes", "functions") we can allow to neglect visibility rules. (This is
what "friend"(s) are for  ) )
5. Therefore: there is no logical reason that template classes can not made
friends.
Viatcheslav.
| Quote: |
Furthermore I do not understand that why do you call the class giving
friend
access friend_class? In C++ we call friend class the class *being given*
the friendship. Furthermore you have template_clase and template_class.
I
assume this is a typo.
--
WW aka Attila
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WW
Guest
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| Posted: Sun Sep 28, 2003 7:30 am Post subject: Re: using friend and template class |
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Viatcheslav L. Gorelenkov wrote:
| Quote: | "WW" <wolof (AT) freemail (DOT) hu> wrote in message
There is no such thing as template class. There are class
templates. Read this two sentences until you grasp the difference.
Class templates are *only* templates. They are not classes. So
they cannot be made friends.
1. At least at definition we call it "template <typename X> class Y".
)
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Haha
| Quote: | 2. Both "template classes" and "classes" are data abstractions.
|
There is no such thing as template class.
| Quote: | 3. Both abstractions has visibility rules (public, protected,
private) for data/methods.
|
Class templates has none. Class template instances (which are classes)
have.
| Quote: | 4. For some other abstractions (does not matter - "template classes",
"classes", "functions") we can allow to neglect visibility rules.
(This is what "friend"(s) are for ) )
|
There is no such thing as template class.
| Quote: | 5. Therefore: there is no logical reason that template classes can
not made friends.
|
Except that there is no such thing as template class.
Only class templates exist.
--
WW aka Attila
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WW
Guest
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| Posted: Sun Sep 28, 2003 7:35 am Post subject: Re: using friend and template class |
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Viatcheslav L. Gorelenkov wrote:
[SNIP]
| Quote: | friend class template_clase<class view_model>; // this is a
template!!!!
|
Wrong again. The above is *not* a template. It is a concrete type, a
concrete instance of a class template. For newbies: this is a beautiful way
of how *not* to learn anything. With trial and error, not even knowing what
a class template is, trying to insert and remove things until it works - and
finally still *not knowing anything* and state that a concrete class is a
template.
If you ever decide to learn anything do it right. Learn what the
terminology is, because that defines already a lot. And if you did not
learn it and you are warned about using the worng word(s) here: think about
it. Unless you want to make a fool of yourself as the above person did.
--
WW aka Attila
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Viatcheslav L. Gorelenkov
Guest
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| Posted: Sun Sep 28, 2003 2:04 pm Post subject: Re: using friend and template class |
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"WW" <wolof (AT) freemail (DOT) hu> wrote
| Quote: | Viatcheslav L. Gorelenkov wrote:
[SNIP]
friend class template_clase<class view_model>; // this is a
template!!!!
Wrong again. The above is *not* a template. It is a concrete type, a
concrete instance of a class template. For newbies: this is a beautiful
way
of how *not* to learn anything. With trial and error, not even knowing
what
a class template is, trying to insert and remove things until it works -
and
finally still *not knowing anything* and state that a concrete class is a
template.
1. The above IS template. This is NOT a concreate instance. Take a look what |
is inside angle brackets ( <..> ).
| Quote: | If you ever decide to learn anything do it right.
|
2. Completely agree.
Viatcheslav.
| Quote: | Learn what the
terminology is, because that defines already a lot. And if you did not
learn it and you are warned about using the worng word(s) here: think
about
it. Unless you want to make a fool of yourself as the above person did.
--
WW aka Attila
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WW
Guest
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| Posted: Sun Sep 28, 2003 5:07 pm Post subject: Re: using friend and template class |
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Viatcheslav L. Gorelenkov wrote:
| Quote: | "WW" <wolof (AT) freemail (DOT) hu> wrote in message
news:bl6301$nfc$1 (AT) phys-news1 (DOT) kolumbus.fi...
Viatcheslav L. Gorelenkov wrote:
[SNIP]
friend class template_clase<class view_model>; // this is a
template!!!!
Wrong again. The above is *not* a template. It is a concrete type,
a concrete instance of a class template. For newbies: this is a
beautiful way of how *not* to learn anything. With trial and error,
not even knowing what a class template is, trying to insert and
remove things until it works - and finally still *not knowing
anything* and state that a concrete class is a template.
1. The above IS template. This is NOT a concreate instance. Take a
look what is inside angle brackets ( <..> ).
|
I have missed the class keyword there. Blame it on my decomposing mind.
Unfortunately the aboveis not yet C++, but it is proposed!
I thought you wanted to make one *instance* of that template a friend, the
one instantiated from friend_class. I guess I have to work harder on
eliminating my preconceptions when reading a post. And especially when
answering to it. :-(
| Quote: | If you ever decide to learn anything do it right.
2. Completely agree.
|
The "template friend" you are looking for it (right now) not part of C++.
AFAIK there is a proposal to add it - but I cannot find it.
--
WW aka Attila
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Shane Beasley
Guest
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| Posted: Sun Sep 28, 2003 6:28 pm Post subject: Re: using friend and template class |
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"Sebastian Faust" <sfaust_deleteme (AT) logic-software (DOT) de> wrote
| Quote: | template<class view_model
class template_clase
{
protected:
template_clase() {}
virtual ~template_clase() {}
};
clase friend_class
{
friend_class() {}
virtual ~friend_class{}
|
WRONG:
| Quote: | friend template_class;
|
RIGHT:
template
It's a template, so tell that to the compiler. Also, the keyword
"class" is required after "friend" if it's a class or class template.
| Quote: | };
My compile ( Visual C++ 7.0 ) doesn't accept this code. Is it possible and
if yes how, to use a template class as a friend class?
|
Visual C++ 7.0 does allow the above code. (6.0, however, does not; it
does not allow template friends at all.)
- Shane
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WW
Guest
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| Posted: Sun Sep 28, 2003 6:52 pm Post subject: Re: using friend and template class |
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WW wrote:
[SNIP my junk]
I have to sleep more. Or less. I have been lost in the timeline, back to
1995. Of course you can make all instances of a class template friend. See
Shane's post.
--
WW aka Attila
Shaving hair off again, preparing ash
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Shane Beasley
Guest
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| Posted: Sun Sep 28, 2003 7:02 pm Post subject: Re: using friend and template class |
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"Viatcheslav L. Gorelenkov" <vlg315 (AT) boo (DOT) net> wrote
| Quote: | friend class template_clase<class view_model>; // this is a
template!!!!
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Nope. You're giving friendship to template_clase<view_model>, where
view_model is a class, not template_clase itself. This is similar to
the following:
template <typename T> class foo {
public:
typedef typename T::type type;
};
class bar {
private:
typedef void type;
// give friendship to foo<bar>, where bar is a class
friend class foo<bar>;
};
// explicitly instantiate foo<bar> (for testing purposes)
template class foo<bar>;
Only foo<bar> is friends with foo, not foo<int>, etc. If you want all
instantiations of foo to be given friendship, do as thus:
template <class> friend class foo;
This makes friends with foo, which is a class template which takes one
parameter.
- Shane
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WW
Guest
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| Posted: Sun Sep 28, 2003 7:04 pm Post subject: Re: using friend and template class |
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Shane Beasley wrote:
[SNIP]
| Quote: | template <class> friend class foo;
This makes friends with foo, which is a class template which takes one
parameter.
|
Yep. (Hopefully) temporary insanity from my side.
--
WW aka Attila
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Viatcheslav L. Gorelenkov
Guest
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| Posted: Sun Sep 28, 2003 9:37 pm Post subject: Re: using friend and template class |
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"Shane Beasley" <sbeasley (AT) cs (DOT) uic.edu> wrote
| Quote: | "Viatcheslav L. Gorelenkov" <vlg315 (AT) boo (DOT) net> wrote
friend class template_clase<class view_model>; // this is a
template!!!!
|
1. This is a template. ( !!!!! )) ) See source code in my 1-st posting. I
do not give friendship to instantiated template_clase<view_model>, rather it
is friendship to class template_clase<class view_model>. See what is between
the angle brackets <...>. Your can change view_model to C, T, whatever - it
still will compile and work the same way.
| Quote: | Nope. You're giving friendship to template_clase<view_model>, where
view_model is a class, not template_clase itself. This is similar to
the following:
template <typename T> class foo {
public:
typedef typename T::type type;
};
class bar {
private:
typedef void type;
// give friendship to foo<bar>, where bar is a class
friend class foo<bar>;
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2. To give friendship to template class foo :
friend class foo<class TTT>; // CLASS TTT
This is true for MS VC++ compiler ( see original posting).
Viatcheslav.
| Quote: | };
// explicitly instantiate foo<bar> (for testing purposes)
template class foo<bar>;
Only foo<bar> is friends with foo, not foo<int>, etc. If you want all
instantiations of foo to be given friendship, do as thus:
template <class> friend class foo;
This makes friends with foo, which is a class template which takes one
parameter.
- Shane
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Shane Beasley
Guest
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| Posted: Mon Sep 29, 2003 3:25 am Post subject: Re: using friend and template class |
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"Viatcheslav L. Gorelenkov" <vlg315 (AT) boo (DOT) net> wrote
| Quote: | friend class template_clase<class view_model>; // this is a
template!!!!
1. This is a template. ( !!!!! )) ) See source code in my 1-st posting.
|
I thought the above code was yours... Did I copy it incorrectly?
| Quote: | I
do not give friendship to instantiated template_clase<view_model>, rather it
is friendship to class template_clase<class view_model>. See what is between
the angle brackets <...>. Your can change view_model to C, T, whatever - it
still will compile and work the same way.
|
Completely and utterly wrong. Observe:
template <class> class template_clase;
class friend_class {
private:
static void f () { }
#ifdef YOURCODE
friend class template_clase<class view_model>;
#else
template <class> friend class template_clase;
#endif
};
template <class view_model>
class template_clase {
public:
void f () { friend_class::f(); } // 18
};
class view_model { };
template class template_clase<view_model>;
template class template_clase<friend_class>; // 23
If YOURCODE is defined, Comeau gives me the following:
"ComeauTest.c", line 18: error: function "<unnamed>::friend_class::f"
is inaccessible
void f () { friend_class::f(); } // 18
^
detected during instantiation of "void
<unnamed>::template_clase<view_model>::f() [with
view_model=<unnamed>::friend_class]" at line 23
In other words, template_clase<view_model> has access to
friend_class::f(), but not template_clase<friend_class>. Therefore,
your code gives friendship only to template_clase<view_model>.
If YOURCODE is *not* defined, Comeau accepts the code. That means that
only my code gives friendship to both template_clase<view_model> and
template_clase<friend_class> -- in fact, it gives friendship to all
instantiations of template_clase.
| Quote: | 2. To give friendship to template class foo :
friend class foo<class TTT>; // CLASS TTT
This is true for MS VC++ compiler ( see original posting).
|
No, it is not. Visual C++ .NET (aka 7.0) gives friendship as ISO C++,
Comeau, GCC 3.2, and I say it should. It accepts (and rejects) the
above code exactly as all these compilers do.
(NB: Visual C++ 6.0 is broken and cannot give friendship to templates.
It rejects the above code regardless of whether YOURCODE is defined.)
- Shane
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David Rubin
Guest
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| Posted: Mon Sep 29, 2003 5:47 pm Post subject: Re: using friend and template class |
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WW wrote:
| Quote: |
Viatcheslav L. Gorelenkov wrote:
"WW" <wolof (AT) freemail (DOT) hu> wrote in message
There is no such thing as template class. There are class
templates. Read this two sentences until you grasp the difference.
Class templates are *only* templates. They are not classes. So
they cannot be made friends.
1. At least at definition we call it "template <typename X> class Y".
)
Haha
2. Both "template classes" and "classes" are data abstractions.
There is no such thing as template class.
|
A class template is a pattern for a class. A template class is an
instantiation of a class template. Read the FAQ!
/david
--
Andre, a simple peasant, had only one thing on his mind as he crept
along the East wall: 'Andre, creep... Andre, creep... Andre, creep.'
-- unknown
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