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Edmund Kapusniak
Guest
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 Posted: Fri Jan 21, 2005 10:28 am Post subject: Pointer to member polymorphism |
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Hi,
Why does the following not work?
class base
{
};
class derived
: public base
{
};
struct s
{
derived member;
};
int main( int argc, char* argv[] )
{
base s::* pointer_to_member = &s::member;
}
I realise that &s::member is of type derived s::* and not base s::*,
but with 'normal' pointers I can assign a pointer to a derived class to
a pointer of base class type with no problems. Is the same not true of
pointers to members?
If there isn't any way of achieving what I want, does anyone know the
reason why it's not allowed?
Thanks,
--
Edmund
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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dtmoore
Guest
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| Posted: Sat Jan 22, 2005 5:35 am Post subject: Re: Pointer to member polymorphism |
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Edmund Kapusniak wrote:
| Quote: | Hi,
Why does the following not work?
class base
{
};
class derived
: public base
{
};
struct s
{
derived member;
};
int main( int argc, char* argv[] )
{
base s::* pointer_to_member = &s::member;
}
|
You are trying to bind a pointer to part of an object that doesn't
exist. You need to create an object of type s somewhere, then you
should be able to bind its "member" object to a pointer to the base or
derived type.
HTH,
Dave Moore
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Thomas Maeder
Guest
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| Posted: Sat Jan 22, 2005 5:35 am Post subject: Re: Pointer to member polymorphism |
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Edmund Kapusniak <birotanker (AT) hotmail (DOT) com> writes:
| Quote: | class derived
: public base
{
};
struct s
{
derived member;
};
int main( int argc, char* argv[] )
{
base s::* pointer_to_member = &s::member;
}
I realise that &s::member is of type derived s::* and not base s::*,
but with 'normal' pointers I can assign a pointer to a derived class to
a pointer of base class type with no problems. Is the same not true of
pointers to members?
|
Yes. It's not true. The two pointer-to-member types in question are
unrelated.
| Quote: | If there isn't any way of achieving what I want, does anyone know the
reason why it's not allowed?
|
What problem are you trying to solve?
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Victor Bazarov
Guest
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| Posted: Sat Jan 22, 2005 11:25 am Post subject: Re: Pointer to member polymorphism |
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"Edmund Kapusniak" <birotanker (AT) hotmail (DOT) com> wrote...
| Quote: | Why does the following not work?
class base
{
};
class derived
: public base
{
};
struct s
{
derived member;
};
int main( int argc, char* argv[] )
{
base s::* pointer_to_member = &s::member;
}
I realise that &s::member is of type derived s::* and not base s::*,
but with 'normal' pointers I can assign a pointer to a derived class to
a pointer of base class type with no problems. Is the same not true of
pointers to members?
If there isn't any way of achieving what I want, does anyone know the
reason why it's not allowed?
|
It's been discussed here a couple of times, IIRC. The agreement was that
it wasn't needed, so nobody cared to figure it out, IIRC, again. Of course
it is quite possible that I don't RC, and there was some other reason.
Victor
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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