Problem with inheritance

**Josephine Schafer** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

"Victor Chew" <vchew@post1.co m> wrote in message
news:bga29i$er3 $1@mawar.singne t.com.sg...[color=blue]
> Can someone tell me why the following code doesn't work:
>[color=green]
> > TestClass.cpp
> > -------------
> > class A
> > {
> > public:
> > virtual void read(wchar_t& ch) { read(&ch, 0, 1); }
> > virtual void read(wchar_t* buf, int off, int len) = 0;
> > };
> >
> > class B : public virtual A
> > {
> > public:
> > virtual void read(wchar_t* buf, int off, int len) {}
> > };
> >
> > int main(int argc, char* argv[])
> > {
> > wchar_t ch;
> > B myclass;
> > myclass.read(ch );
> > }[/color]
>
> I have tried both gcc 2.96 and gcc 3.2. I get:
>[color=green]
> > TestClass.cpp: In function `int main(int, char**)':
> > TestClass.cpp:2 1: no matching function for call to `B::read(wchar_ t&)'
> > TestClass.cpp:1 4: candidates are: virtual void B::read(wchar_t *, int,[/color][/color]
int)[color=blue]
>
> Shouldn't B have inherited read(wchar_t& ch) from A?[/color]

Your base class function virtual void read(wchar_t& ch) has been hidden by
the derived class
function virtual void read(wchar_t* buf, int off, int len).

**ES Kim** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

"Victor Chew" <vchew@post1.co m> wrote in message
news:bga29i$er3 $1@mawar.singne t.com.sg...[color=blue]
> Can someone tell me why the following code doesn't work:
>[color=green]
> > TestClass.cpp
> > -------------
> > class A
> > {
> > public:
> > virtual void read(wchar_t& ch) { read(&ch, 0, 1); }
> > virtual void read(wchar_t* buf, int off, int len) = 0;
> > };
> >
> > class B : public virtual A
> > {
> > public:
> > virtual void read(wchar_t* buf, int off, int len) {}
> > };
> >
> > int main(int argc, char* argv[])
> > {
> > wchar_t ch;
> > B myclass;
> > myclass.read(ch );
> > }[/color]
>
> I have tried both gcc 2.96 and gcc 3.2. I get:
>[color=green]
> > TestClass.cpp: In function `int main(int, char**)':
> > TestClass.cpp:2 1: no matching function for call to `B::read(wchar_ t&)'
> > TestClass.cpp:1 4: candidates are: virtual void B::read(wchar_t *, int, int)[/color]
>
> Shouldn't B have inherited read(wchar_t& ch) from A?
>[/color]

B::read(wchar_t * buf, int off, int len) hides A::read(wchar_t & ch).
If you override an overloaded base class functions,
redefine full set of the functions.

--
ES Kim

**Alf P. Steinbach** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew@post1.co m> wrote:
[color=blue]
>Can someone tell me why the following code doesn't work:
>[color=green]
> > TestClass.cpp
> > -------------
>> class A
>> {
>> public:
>> virtual void read(wchar_t& ch) { read(&ch, 0, 1); }
>> virtual void read(wchar_t* buf, int off, int len) = 0;
>> };
>>
>> class B : public virtual A
>> {
>> public:[/color][/color]

using A::read;
[color=blue][color=green]
>> virtual void read(wchar_t* buf, int off, int len) {}
>> };
>>
>> int main(int argc, char* argv[])
>> {
>> wchar_t ch;
>> B myclass;
>> myclass.read(ch );
>> }[/color]
>
>I have tried both gcc 2.96 and gcc 3.2. I get:
>[color=green]
>> TestClass.cpp: In function `int main(int, char**)':
>> TestClass.cpp:2 1: no matching function for call to `B::read(wchar_ t&)'
>> TestClass.cpp:1 4: candidates are: virtual void B::read(wchar_t *, int, int)[/color]
>
>Shouldn't B have inherited read(wchar_t& ch) from A?[/color]

It has, but without the 'using' it's hidden by the other read.
Now don't ask me _why_ someone thought that would be sensible.

**Victor Chew** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

I don't get it. Isn't overriding based on method signatures? There is
clearly a difference between read(wchar_t&) and read(wchar_t*, int,
int)! Why can't I selectively override one of the methods from the base
class? What is the workaround?

Alf P. Steinbach wrote:
[color=blue]
> On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew@post1.co m> wrote:
>
>[color=green]
>>Can someone tell me why the following code doesn't work:
>>
>>[color=darkred]
>>>TestClass.cp p
>>>-------------
>>>class A
>>>{
>>>public:
>>> virtual void read(wchar_t& ch) { read(&ch, 0, 1); }
>>> virtual void read(wchar_t* buf, int off, int len) = 0;
>>>};
>>>
>>>class B : public virtual A
>>>{
>>>public:[/color][/color]
>
>
> using A::read;
>
>[color=green][color=darkred]
>>> virtual void read(wchar_t* buf, int off, int len) {}
>>>};
>>>
>>>int main(int argc, char* argv[])
>>>{
>>> wchar_t ch;
>>> B myclass;
>>> myclass.read(ch );
>>>}[/color]
>>
>>I have tried both gcc 2.96 and gcc 3.2. I get:
>>
>>[color=darkred]
>>>TestClass.cp p: In function `int main(int, char**)':
>>>TestClass.cp p:21: no matching function for call to `B::read(wchar_ t&)'
>>>TestClass.cp p:14: candidates are: virtual void B::read(wchar_t *, int, int)[/color]
>>
>>Shouldn't B have inherited read(wchar_t& ch) from A?[/color]
>
>
> It has, but without the 'using' it's hidden by the other read.
> Now don't ask me _why_ someone thought that would be sensible.
>[/color]

**Kevin Goodsell** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

ES Kim wrote:
[color=blue]
>
>
> B::read(wchar_t * buf, int off, int len) hides A::read(wchar_t & ch).
> If you override an overloaded base class functions,
> redefine full set of the functions.
>[/color]

There is no overriding in this case. Also, redefining the entire set of
functions would be a pointless waste of time. A using declaration brings
the base class's overloads into scope nicely.

-Kevin

**Victor Chew** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

Do you mind posting a short code segment showing me how to do this?

Thanks!

Kevin Goodsell wrote:
[color=blue]
> ES Kim wrote:
>[color=green]
>>
>>
>> B::read(wchar_t * buf, int off, int len) hides A::read(wchar_t & ch).
>> If you override an overloaded base class functions,
>> redefine full set of the functions.
>>[/color]
>
> There is no overriding in this case. Also, redefining the entire set of
> functions would be a pointless waste of time. A using declaration brings
> the base class's overloads into scope nicely.
>
> -Kevin
>[/color]

**John Harrison** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

"Victor Chew" <vchew@post1.co m> wrote in message
news:bgaanf$f9i $1@mawar.singne t.com.sg...[color=blue]
> Do you mind posting a short code segment showing me how to do this?
>
> Thanks!
>[/color]

class B : public virtual A
{
public:
virtual void read(wchar_t& ch) { A::read(ch); }
virtual void read(wchar_t* buf, int off, int len) {}
};

john

**Makis Papapanagiotou** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

Hello,

You can use polymorphism in order to have the workaround solution.
Even though I wouldn't call it workaround, anyway...

A* myclass = new B;
myclass.read(ch );

Then everything will work properly, since late binding will take part, and
it will resolve the correct functions.

"Victor Chew" <vchew@post1.co m> wrote in message
news:bga9h4$f89 $1@mawar.singne t.com.sg...[color=blue]
> I don't get it. Isn't overriding based on method signatures? There is
> clearly a difference between read(wchar_t&) and read(wchar_t*, int,
> int)! Why can't I selectively override one of the methods from the base
> class? What is the workaround?
>
> Alf P. Steinbach wrote:
>[color=green]
> > On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew@post1.co m> wrote:
> >
> >[color=darkred]
> >>Can someone tell me why the following code doesn't work:
> >>
> >>
> >>>TestClass.cp p
> >>>-------------
> >>>class A
> >>>{
> >>>public:
> >>> virtual void read(wchar_t& ch) { read(&ch, 0, 1); }
> >>> virtual void read(wchar_t* buf, int off, int len) = 0;
> >>>};
> >>>
> >>>class B : public virtual A
> >>>{
> >>>public:[/color]
> >
> >
> > using A::read;
> >
> >[color=darkred]
> >>> virtual void read(wchar_t* buf, int off, int len) {}
> >>>};
> >>>
> >>>int main(int argc, char* argv[])
> >>>{
> >>> wchar_t ch;
> >>> B myclass;
> >>> myclass.read(ch );
> >>>}
> >>
> >>I have tried both gcc 2.96 and gcc 3.2. I get:
> >>
> >>
> >>>TestClass.cp p: In function `int main(int, char**)':
> >>>TestClass.cp p:21: no matching function for call to `B::read(wchar_ t&)'
> >>>TestClass.cp p:14: candidates are: virtual void B::read(wchar_t *, int,[/color][/color][/color]
int)[color=blue][color=green][color=darkred]
> >>
> >>Shouldn't B have inherited read(wchar_t& ch) from A?[/color]
> >
> >
> > It has, but without the 'using' it's hidden by the other read.
> > Now don't ask me _why_ someone thought that would be sensible.
> >[/color]
>[/color]

**Kevin Goodsell** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

Victor Chew wrote:
[color=blue]
> Do you mind posting a short code segment showing me how to do this?
>
> Thanks!
>[/color]

Please do not top-post. Re-read section 5 of the FAQ for posting
guidelines: http://www.parashift.com/c++-faq-lite/how-to-post.html

See Alf's reply for the example you requested.

-Kevin

**David White** · Jul 19 '05, 03:55 PM

Re: Problem with inheritance

"Victor Chew" <vchew@post1.co m> wrote in message
news:bga9h4$f89 $1@mawar.singne t.com.sg...[color=blue]
> I don't get it. Isn't overriding based on method signatures? There is
> clearly a difference between read(wchar_t&) and read(wchar_t*, int,
> int)![/color]

Please read all replies, such as my other one, if you see it there. It has a
link to a thread that explains the rule and why.

DW

**Alf P. Steinbach** · Jul 19 '05, 03:57 PM

Re: Problem with inheritance

On Thu, 31 Jul 2003 13:45:46 +0800, Victor Chew <vchew@post1.co m> wrote:
[color=blue]
>I don't get it. Isn't overriding based on method signatures? There is
>clearly a difference between read(wchar_t&) and read(wchar_t*, int,
>int)! Why can't I selectively override one of the methods from the base
>class?[/color]

You can, and you just did.

It is just as static type B that one 'read' method is hidden. The other
is still there. E.g., you can cast it to A& and access the other 'read'.

David White here provided a URL to an earlier discussion where Russel
Hanneken provided a URL to an even earlier discussion where Chris Newton
tried to explain the original thinking, see [http://tinyurl.com/hlts].

[color=blue]
> What is the workaround?[/color]

'using', as shown in my first reply.

**Victor Chew** · Jul 19 '05, 03:57 PM

Re: Problem with inheritance

Thanks for all your replies. I support Chris' comment that "I personally
regard this decision as unfortunate". If there is a perfect method
signature match in the base class which is not overridden in the
subclass, then it should simply be used. It's the principle of least
surprise.

Alf P. Steinbach wrote:
[color=blue]
> On Thu, 31 Jul 2003 13:45:46 +0800, Victor Chew <vchew@post1.co m> wrote:
>
>[color=green]
>>I don't get it. Isn't overriding based on method signatures? There is
>>clearly a difference between read(wchar_t&) and read(wchar_t*, int,
>>int)! Why can't I selectively override one of the methods from the base
>>class?[/color]
>
>
> You can, and you just did.
>
> It is just as static type B that one 'read' method is hidden. The other
> is still there. E.g., you can cast it to A& and access the other 'read'.
>
> David White here provided a URL to an earlier discussion where Russel
> Hanneken provided a URL to an even earlier discussion where Chris Newton
> tried to explain the original thinking, see [http://tinyurl.com/hlts].
>
>
>[color=green]
>>What is the workaround?[/color]
>
>
> 'using', as shown in my first reply.
>[/color]

Problem with inheritance

Problem with inheritance

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