I have the following program. The auto_ptr cause the compile error.
When I change auto_ptr to pointer, the error gone.

Would you please help me to understand what is wrong?

/*******************************************/
class iterator {
private:
std::auto_ptr<int> _iterator;
// int *_iterator;
};

class collection {
public:
iterator begin() {
return iterator();
}
};


int main() {
collection va;
iterator it(va.begin());
}
/***********************************/


The error message:

main_test.cc: In member function `iterator collection::begin()':
main_test.cc:49: error: no matching function for call to
`iterator::iterator(iterator)'
main_test.cc:40: note: candidates are: iterator::iterator(iterator&)
main_test.cc: In function `int main()':
main_test.cc:56: error: no matching function for call to
`iterator::iterator(iterator)'
main_test.cc:49: note: candidates are: iterator::iterator()
main_test.cc:40: note: iterator::iterator(iterator&)

* PengYu.UT@gmail.com:[color=blue]
> I have the following program. The auto_ptr cause the compile error.
> When I change auto_ptr to pointer, the error gone.
>
> Would you please help me to understand what is wrong?
>
> /*******************************************/
> class iterator {
> private:
> std::auto_ptr<int> _iterator;
> // int *_iterator;
> };
>
> class collection {
> public:
> iterator begin() {
> return iterator();[/color]

'iterator()' is a temporary. You can't bind a temporary to a reference
to non-const. But the only copy constructor you have available has
argument type 'iterator&', a reference to non-const (because that's the
only one that can be generated that can handle the auto_ptr member), and
so the compiler complains.

You could do this like

iterator begin()
{
iterator result;
return result;
}

but having that auto_ptr in there is most probably a design error,
because it seems to be 100% meaningless, and you would need to add
additional support to make the function useful to client code.

Do you really want an iterator refer to and own a dynamically allocated int?
[color=blue]
> }
> };[/color]


--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

Alf P. Steinbach wrote:[color=blue]
> * PengYu.UT@gmail.com:[color=green]
> > I have the following program. The auto_ptr cause the compile error.
> > When I change auto_ptr to pointer, the error gone.
> >
> > Would you please help me to understand what is wrong?
> >
> > /*******************************************/
> > class iterator {
> > private:
> > std::auto_ptr<int> _iterator;
> > // int *_iterator;
> > };
> >
> > class collection {
> > public:
> > iterator begin() {
> > return iterator();[/color]
>
> 'iterator()' is a temporary. You can't bind a temporary to a reference
> to non-const. But the only copy constructor you have available has
> argument type 'iterator&', a reference to non-const (because that's the
> only one that can be generated that can handle the auto_ptr member), and
> so the compiler complains.
>
> You could do this like
>
> iterator begin()
> {
> iterator result;
> return result;
> }[/color]

The compilor reports error, too. Is there anything else wrong?
[color=blue]
>
> but having that auto_ptr in there is most probably a design error,
> because it seems to be 100% meaningless, and you would need to add
> additional support to make the function useful to client code.[/color]

This is just a mini example reduced from a rather long segment of code.
Do take it too serious?[color=blue]
>
> Do you really want an iterator refer to and own a dynamically allocated int?
>[color=green]
> > }
> > };[/color]
>
>
> --
> A: Because it messes up the order in which people normally read text.
> Q: Why is it such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing on usenet and in e-mail?[/color]

Well, the design doesn't really make sense for a start. Why would you
have an iterator class with one member and no constructors or other
methods, and that points to int types? Maybe read up on auto_ptr, which
will give you an idea of its purpose in relation to raw pointers. A
default constructor, copy constructor and maybe even a destructor
wouldn't go astray, at least the code will compile.

If you want to write a container and iterator class, take a look at the
design and implementation of the STL which will help a great deal.
[color=blue]
> The error message:
>
> main_test.cc: In member function `iterator collection::begin()':
> main_test.cc:49: error: no matching function for call to
> `iterator::iterator(iterator)'[/color]

This refers to the constr...
[color=blue]
> main_test.cc:40: note: candidates are: iterator::iterator(iterator&)[/color]

This is what you have.
[color=blue]
> main_test.cc: In function `int main()':
> main_test.cc:56: error: no matching function for call to
> `iterator::iterator(iterator)'[/color]

You don't have this.
[color=blue]
> main_test.cc:49: note: candidates are: iterator::iterator()[/color]

You do have this.
[color=blue]
> main_test.cc:40: note: iterator::iterator(iterator&)[/color]

Regards,

Michael

Apart from what Alf has already pointed out regarding the temporary
returned by collection::begin(), a few comments. The design doesn't
really make sense for a start. Why would you have an iterator class
with one member and no constructors or other methods, and that points
to int types? Maybe read up on auto_ptr, which will give you an idea of
its purpose in relation to raw pointers. A default constructor, copy
constructor and maybe even a destructor wouldn't go astray, at least
the code will compile.

If you want to write a container and iterator class, take a look at the
design and implementation of the STL which will help a great deal.
[color=blue]
> The error message:
>
> main_test.cc: In member function `iterator collection::begin()':
> main_test.cc:49: error: no matching function for call to
> `iterator::iterator(iterator)'[/color]

This refers to the constr...
[color=blue]
> main_test.cc:40: note: candidates are: iterator::iterator(iterator&)[/color]

This is what you have.
[color=blue]
> main_test.cc: In function `int main()':
> main_test.cc:56: error: no matching function for call to
> `iterator::iterator(iterator)'[/color]

You don't have this.
[color=blue]
> main_test.cc:49: note: candidates are: iterator::iterator()[/color]

You do have this.
[color=blue]
> main_test.cc:40: note: iterator::iterator(iterator&)[/color]

Regards,

Michael