operator inheritance problem

[Juha Tenhosaari]

Hello,

I wonder if anyone could tell me why the operator= is not inherited into the
derived class in the following code:

class A
{
public:
void operator=(int i){a=i;}
int a;
}

class B : public A
{
public:
int b;
}

void main(void)
{
B bb;
bb=3; // trying to use the overloaded operator
cout << bb.a;
}

When I try to compile this, I get the error message "could not find match
for B::operator=(in t)". If I replace the overloaded operator by an ordinary
function (i.e. operator=(int) => set(int)), it works fine.

What am I doing wrong?


Juha

[John Carson]

Re: operator inheritance problem

"Juha Tenhosaari" <j.t@invalid.co m> wrote in message
news:dPdkb.2997 4$mU6.78385@new sb.telia.net[color=blue]
> Hello,
>
> I wonder if anyone could tell me why the operator= is not inherited
> into the derived class in the following code:
>
> class A
> {
> public:
> void operator=(int i){a=i;}
> int a;
> }
>
> class B : public A
> {
> public:
> int b;
> }
>
> void main(void)
> {
> B bb;
> bb=3; // trying to use the overloaded operator
> cout << bb.a;
> }
>
> When I try to compile this, I get the error message "could not find
> match for B::operator=(in t)". If I replace the overloaded operator by
> an ordinary function (i.e. operator=(int) => set(int)), it works fine.
>
> What am I doing wrong?
>
>
> Juha[/color]


Simple. Assignment operators are not inherited --- just like constructors
are not inherited.


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[Tim Threlfall]

Re: operator inheritance problem

13.5.3 of the standard:

Because a copy assignment operator operator= is implicitly declared for
a class if not declared by the user (12.8), a base class assignment
operator is always hidden by the copy assignment operator of the
derived class.

[grejdanospam@pacbell.net]

Re: operator inheritance problem

On Sun, 19 Oct 2003 02:36:44 +1000, John Carson
<donaldquixote@ datafast.net.au > wrote:
[color=blue]
> "Juha Tenhosaari" <j.t@invalid.co m> wrote in message
> news:dPdkb.2997 4$mU6.78385@new sb.telia.net[color=green]
>> Hello,
>>
>> I wonder if anyone could tell me why the operator= is not inherited
>> into the derived class in the following code:
>>
>> class A
>> {
>> public:
>> void operator=(int i){a=i;}
>> int a;
>> }
>>
>> class B : public A
>> {
>> public:
>> int b;
>> }
>>
>> void main(void)
>> {
>> B bb;
>> bb=3; // trying to use the overloaded operator
>> cout << bb.a;
>> }
>>
>> When I try to compile this, I get the error message "could not find
>> match for B::operator=(in t)". If I replace the overloaded operator by
>> an ordinary function (i.e. operator=(int) => set(int)), it works fine.
>>
>> What am I doing wrong?
>>
>>
>> Juha[/color]
>
>
> Simple. Assignment operators are not inherited --- just like constructors
> are not inherited.
>
>[/color]
They are inherited but hiddem , see response from Tim.

Adding using declaration to class B :

using A::operator =(int);

will bring it back to the public/protected/private view.

--
grzegorz

[John Carson]

Re: operator inheritance problem

<grejdanospam@p acbell.net> wrote in message
news:oprw8z44uk 6u0rcr@news.sf. sbcglobal.net[color=blue]
> On Sun, 19 Oct 2003 02:36:44 +1000, John Carson
> <donaldquixote@ datafast.net.au > wrote:
>[color=green]
> > "Juha Tenhosaari" <j.t@invalid.co m> wrote in message
> > news:dPdkb.2997 4$mU6.78385@new sb.telia.net[color=darkred]
> > > Hello,
> > >
> > > I wonder if anyone could tell me why the operator= is not
> > > inherited into the derived class in the following code:
> > >
> > > class A
> > > {
> > > public:
> > > void operator=(int i){a=i;}
> > > int a;
> > > }
> > >
> > > class B : public A
> > > {
> > > public:
> > > int b;
> > > }
> > >
> > > void main(void)
> > > {
> > > B bb;
> > > bb=3; // trying to use the overloaded operator
> > > cout << bb.a;
> > > }
> > >
> > > When I try to compile this, I get the error message "could not
> > > find match for B::operator=(in t)". If I replace the overloaded
> > > operator by an ordinary function (i.e. operator=(int) =>
> > > set(int)), it works fine.
> > >
> > > What am I doing wrong?
> > >
> > >
> > > Juha[/color]
> >
> >
> > Simple. Assignment operators are not inherited --- just like
> > constructors are not inherited.
> >
> >[/color]
> They are inherited but hiddem , see response from Tim.
>
> Adding using declaration to class B :
>
> using A::operator =(int);
>
> will bring it back to the public/protected/private view.
>
> --
> grzegorz[/color]



I think this satisfies my definition of "not inherited" --- and apparently
Stroustrup's too (see TC++PL, p. 307). Nevertheless, the technique for
making it accessible to the derived class is worth pointing out. However,
the using declaration should not specify the operator's argument. It should
be:

using A::operator = ;

Overload resolution will then occur in the usual way.


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[Juha Tenhosaari]

Vs: operator inheritance problem


<grejdanospam@p acbell.net> wrote:[color=blue][color=green]
> > Adding using declaration to class B :
> >
> > using A::operator =(int);
> >
> > will bring it back to the public/protected/private view.[/color][/color]

John Carson wrote:[color=blue]
> the using declaration should not specify the operator's argument. It[/color]
should[color=blue]
> be:
>
> using A::operator = ;[/color]

Thanks for the answers.

I modified the derived class this way:

class B : public A
{
using A::operator=;
public:
int b;
}


....but my compiler gave me error message "Identifier expected". I wonder
why.

This, however, did work:

class B : public A
{
public:
void operator=(int i){A::operator= (i);}
int b;
}


Juha

[John Carson]

Re: operator inheritance problem

"Juha Tenhosaari" <juha.news@teli a.com> wrote in message
news:qWukb.3379 7$dP1.106935@ne wsc.telia.net[color=blue]
>
> Thanks for the answers.
>
> I modified the derived class this way:
>
> class B : public A
> {
> using A::operator=;
> public:
> int b;
> }
>
>
> ...but my compiler gave me error message "Identifier expected". I
> wonder why.[/color]


You don't say the line at which this error message occurs. I believe that by
putting the using declaration at the start of the class, you have implicitly
made the operator private. Try moving it after public:
Note that compiler implementation of using declarations are very buggy, so I
would not be surprised if it didn't work.

After moving the using declaration to after public:, the code compiles find
using Dinkum Exam.

http://www.dinkumware.com/exam/dinkumExam.aspx

--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)