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mshetty
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 Posted: Sun Aug 29, 2004 10:46 pm Post subject: Confused with the "virtual base class" concept |
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Hi,
I get an error "Warning: b::a_method hides the virtual function
a::a_method()."
on compiling the following code..
#include <iostream.h>
class a
{
public:
virtual int a_method ()
{
cout << "a::a_method" << endl;
return 0;
}
};
class b : public a
{
public:
int a_method (int x)
{
cout << "b::a_method" << endl;
return x;
}
};
class c : public a
{
public:
int a_method ()
{
cout << "c::a_method" << endl;
return 100;
}
};
int main()
{
b b_var;
b_var.a_method(10);
return 0;
}
The warning goes off if done the following:
Change class b : public a
to
class b : public virtual a
Have not been to understand the reason why the warning is removed as
"b::a_method still hides a::a_method()"
Would help if I could get some input.
Thanks and Regards,
M Shetty
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ivan Korotkov
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| Posted: Mon Aug 30, 2004 10:50 am Post subject: Re: Confused with the "virtual base class" concept |
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| Quote: | I get an error "Warning: b::a_method hides the virtual function
a::a_method()."
class b : public a
{
public:
int a_method (int x)
{
cout << "b::a_method" << endl;
return x;
}
};
|
I don't know why virtual inheritance removes this, but when you declare an
overloaded function in derived class (b::a_method(int)) it hides all
functions with the same name from base class (a::a_method()). For example,
you can't call b.a_method(); If you include using a::a_method; in b
definition, a::a_method won't become hidden. Btw, you probably want to
declare a_method(int) as virtual, too, don't you?
--
Ivan
e-mail me at: korotkov2 at ztel dot ru
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Carl Barron
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| Posted: Mon Aug 30, 2004 10:53 am Post subject: Re: Confused with the "virtual base class" concept |
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mshetty <mshetty (AT) mail (DOT) com> wrote:
| Quote: | Hi,
I get an error "Warning: b::a_method hides the virtual function
a::a_method()."
on compiling the following code..
#include
class a
{
public:
virtual int a_method ()
{
cout << "a::a_method" << endl;
return 0;
}
};
class b : public a
{
public:
int a_method (int x)
{
cout << "b::a_method" << endl;
return x;
}
}
note well a::a_method() is a function with no args retuning int, and |
b::a_method(int) is a function with an int arg returning int. Since b
has no method a_method() but one of a_method(int); it will hide [not
have available] the virtual function a_method(), if want the
vritual-ness of a_method() you must have such a function in b, since you
already have a a_method(int) which hides the defaulting a::method() of
being used, If you really want b::a_method_a(int) as well, then you need
a forwarding member function
void a_method() {a::a_method();} as well.
Virtual bases are used to provide just one base class of a type rather
then multiples in cases like this:
class Base{};
class Inbetween_1:public virtual Base{};
class Inbetween_2:public virtual Base{};
class TheWinner:public Inbetween_1,public Inbetween_2{};
this makes one instande of Base in an instance of TheWinner;
if the Inbetween's above were to each inherit Base non-virtually then
TheWinner would have two instances of Base namely
Inbetween_1::Base and Inbetween_2::Base. The class diagram of the
virtual inheritance case looks like a rhombus, and the non virtual
inheritance diagram looks like a Y.
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Maxim Yegorushkin
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| Posted: Mon Aug 30, 2004 11:46 pm Post subject: Re: Confused with the "virtual base class" concept |
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mshetty <mshetty (AT) mail (DOT) com> wrote:
[]
| Quote: | The warning goes off if done the following:
Change class b : public a
to
class b : public virtual a
Have not been to understand the reason why the warning is removed as
"b::a_method still hides a::a_method()"
|
It looks like poor diagnostic.
--
Maxim Yegorushkin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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David Michell
Guest
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| Posted: Mon Aug 30, 2004 11:51 pm Post subject: Re: Confused with the "virtual base class" concept |
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I have added a line in class b.
read it.
hth
david michell
#include <iostream.h>
class a
{
public:
virtual int a_method ()
{
cout << "a::a_method" << endl;
return 0;
}
};
class b : public a
{
public:
virtual int a_method (){} /*ADD THIS LINE becaue
class c has to know that this method is virtual. It will not be taken
as default by inheritance*/
int a_method (int x)
{
cout << "b::a_method" << endl;
return x;
}
};
class c : public a
{
public:
int a_method ()
{
cout << "c::a_method" << endl;
return 100;
}
};
int main()
{
b b_var;
b_var.a_method(10);
return 0;
}
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Vladislav Lazarenko
Guest
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| Posted: Tue Aug 31, 2004 10:04 am Post subject: Re: Confused with the "virtual base class" concept |
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[email]mshetty (AT) mail (DOT) com[/email] (mshetty) wrote in message news:<bfbb8fd4.0408290554.551900f8 (AT) posting (DOT) google.com>...
| Quote: | Hi,
I get an error "Warning: b::a_method hides the virtual function
a::a_method()."
on compiling the following code..
#include
class a
{
public:
virtual int a_method ()
{
cout << "a::a_method" << endl;
return 0;
}
};
class b : public a
{
public:
int a_method (int x)
{
cout << "b::a_method" << endl;
return x;
}
};
class c : public a
{
public:
int a_method ()
{
cout << "c::a_method" << endl;
return 100;
}
};
int main()
{
b b_var;
b_var.a_method(10);
return 0;
}
The warning goes off if done the following:
Change class b : public a
to
class b : public virtual a
Have not been to understand the reason why the warning is removed as
"b::a_method still hides a::a_method()"
Would help if I could get some input.
|
Hi, mshetty. It is a silly warning, you can simply ignore it. For
example the Visual Studio .NET with 4 level (highest) of warning
messages did not generate it. But it is pretty-clear allusion to bad
design of your program.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ivan Korotkov
Guest
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| Posted: Tue Aug 31, 2004 7:57 pm Post subject: Re: Confused with the "virtual base class" concept |
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| Quote: | public:
virtual int a_method (){} /*ADD THIS LINE becaue
class c has to know that this method is virtual. It will not be taken
as default by inheritance*/
|
class C inherits A directly, or is it a typo? Anyway, overriding a_method
as empty is not excellent, it's better to import it from A by using A::f;
in class B. Btw, which compiler did you use? VC++ understands that
a_method is virtual:
#include <iostream>
using namespace std;
class A { public : virtual void f() { cout << "In A::fn"; } };
class B : public A { public: void f(int) {} };
class C : public B { public: void f() { cout << "In C::fn"; }};
int main()
{
C c;
static_castf();
return 0;
}
yields
In C::f
--
Ivan
e-mail me at: korotkov2 at ztel dot ru
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Mike Bland
Guest
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| Posted: Wed Sep 01, 2004 2:55 am Post subject: Re: Confused with the "virtual base class" concept |
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[email]mshetty (AT) mail (DOT) com[/email] (mshetty) wrote in message news:<bfbb8fd4.0408290554.551900f8 (AT) posting (DOT) google.com>...
| Quote: | Hi,
I get an error "Warning: b::a_method hides the virtual function
a::a_method()."
on compiling the following code..
|
[snip]
| Quote: | The warning goes off if done the following:
Change class b : public a
to
class b : public virtual a
Have not been to understand the reason why the warning is removed as
"b::a_method still hides a::a_method()"
|
As others have pointed out, b::a_method() does hide a::a_method(), and
it seems it should hide it whether "a" is a virtual base of "b" or
not.
Intrigued by this, I tried your code under both GCC 3.4.1 and Sun CC
5.3 (WorkShop 6 Update 2) on Solaris 8, and was astonished by the
results:
`g++ -Wall -ansi -pedantic` compiled both the virtual and nonvirtual
versions without so much as a single warning (after changing
iostream.h to iostream and adding "using std::cout" and "using
std::endl" at the top). I tried this multiple times out of disbelief,
but the result was indeed the same each time; no mistyping on my part.
`CC +w2` produced the following results:
--BEGIN--
# CC +w2 virtual_base.cxx
"virtual_base.cxx", line 25: Warning: b::a_method hides the virtual
function a::a_method().
1 Warning(s) detected.
# vim virtual_base.cxx
[change base to virtual]
# CC +w2 virtual_base.cxx
"virtual_base.cxx", line 25: Warning: b::a_method Hides the virtual
function a::a_method() in a virtual base.
1 Warning(s) detected.
--END--
So now I'm even more confused. The Sun compiler seems to be right on
with this one, and GCC is oblivious, which is exactly backwards from
what I might've expected. What compiler and platform are you using,
M? Can anyone else provide some insight into what's going on?
Mike
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Mike Bland
Guest
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| Posted: Fri Sep 03, 2004 11:03 am Post subject: Re: Confused with the "virtual base class" concept |
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[email]mbland (AT) acm (DOT) org[/email] (Mike Bland) wrote in message news:<b808d7c1.0408311035.2edde96e (AT) posting (DOT) google.com>...
| Quote: | So now I'm even more confused. The Sun compiler seems to be right on
with this one, and GCC is oblivious, which is exactly backwards from
what I might've expected.
|
Whoops, after looking at the GCC info page, I realized that what I
need to use is -Woverloaded-virtual. Wonder why it's not
automatically a part of -Wall.
Still, I'd be interested in knowing which compiler the OP was using.
Mike
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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mshetty
Guest
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| Posted: Wed Sep 08, 2004 8:17 am Post subject: Re: Confused with the "virtual base class" concept |
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[email]mbland (AT) acm (DOT) org[/email] (Mike Bland) wrote in message news:<b808d7c1.0409020721.46a5216c (AT) posting (DOT) google.com>...
| Quote: | mbland (AT) acm (DOT) org (Mike Bland) wrote in message news:<b808d7c1.0408311035.2edde96e (AT) posting (DOT) google.com>...
So now I'm even more confused. The Sun compiler seems to be right on
with this one, and GCC is oblivious, which is exactly backwards from
what I might've expected.
Whoops, after looking at the GCC info page, I realized that what I
need to use is -Woverloaded-virtual. Wonder why it's not
automatically a part of -Wall.
Still, I'd be interested in knowing which compiler the OP was using.
|
Hi,
Details of the compiler and system I am using
Compiler : CC: Sun WorkShop 6 update 2 C++ 5.3 2001/05/15
OS : SunOS pcspsun3 5.7 Generic_106541-34 sun4u sparc SUNW,Ultra-5_10
Regards,
M Shetty
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Old Wolf
Guest
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| Posted: Thu Sep 09, 2004 9:58 am Post subject: Re: Confused with the "virtual base class" concept |
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[email]mbland (AT) acm (DOT) org[/email] (Mike Bland) wrote:
| Quote: | mbland (AT) acm (DOT) org (Mike Bland) wrote:
So now I'm even more confused. The Sun compiler seems to be right on
with this one, and GCC is oblivious, which is exactly backwards from
what I might've expected.
Whoops, after looking at the GCC info page, I realized that what I
need to use is -Woverloaded-virtual. Wonder why it's not
automatically a part of -Wall.
|
No diagnostic is required by the Standard for the case of a function
hiding a base function (it's merely a quality-of-implementation issue).
Also -Wall is unfortunately-named, it should be -Wa-few-common-problems.
You should probably read the g++ documentation and add in lots more
-W settings to your Makefile.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Mike Bland
Guest
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| Posted: Fri Sep 10, 2004 1:34 pm Post subject: Re: Confused with the "virtual base class" concept |
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[email]mshetty (AT) mail (DOT) com[/email] (mshetty) wrote in message news:<bfbb8fd4.0409071938.1df9c1c4 (AT) posting (DOT) google.com>...
| Quote: | Details of the compiler and system I am using
Compiler : CC: Sun WorkShop 6 update 2 C++ 5.3 2001/05/15
OS : SunOS pcspsun3 5.7 Generic_106541-34 sun4u sparc SUNW,Ultra-5_10
|
Interesting. We're using the same Sun compiler, but I'm on patch
111685-20 dated 2004/03/19. You're using Solaris 7 and I'm on Solaris
8. Could be an issue with the compiler patchlevel, too. Try hitting
http://access1.sun.com/sundev/fdp6u2-patches.html to upgrade your
compiler patchlevel and see what happens.
Mike
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Mike Bland
Guest
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| Posted: Fri Sep 10, 2004 1:40 pm Post subject: Re: Confused with the "virtual base class" concept |
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[email]oldwolf (AT) inspire (DOT) net.nz[/email] (Old Wolf) wrote in message news:<843a4f78.0409081529.7ce9d3f2 (AT) posting (DOT) google.com>...
| Quote: | No diagnostic is required by the Standard for the case of a function
hiding a base function (it's merely a quality-of-implementation issue).
|
That's kinda surprising. Do you know if there are any plans to add
such a requirement?
| Quote: | Also -Wall is unfortunately-named, it should be -Wa-few-common-problems.
You should probably read the g++ documentation and add in lots more
-W settings to your Makefile.
|
LOL That's how I realized, eventually, that -Wall didn't contain
-Woverloaded-virtual. I mean, I've known for a long time that -Wall
didn't really turn on *all* warnings, but I was quite suprised when I
looked at the info page after my initial posting and realized it
didn't include this one by default.
Incidentally, I use a "pedantic" script that precompiles our sources
with g++ with various warnings explicitly turned on before compiling
them with Sun CC. It's worked out quite well for shaking out all
sorts of nasty errors and warnings.
Mike
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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