boost::shared_ptr - NULL - c++

* Christopher:
> If a method is declared to return a type boost::shared_ptr<sometype>,
> how can the method be changed to do the equivalent of returning NULL
> when it was declared to return a raw pointer?

Return boost::shared_ptr<sometype>().

Hm.

I'm amazed about some of the questions here. Please read the manual
before posting. Or at least, /glance/ at it.


Cheers, & hth.,

- Alf

--
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?

On Dec 1, 1:29 pm, "Alf P. Steinbach" <al...@start.no> wrote:
> * Christopher:
>
> > If a method is declared to return a type boost::shared_ptr<sometype>,
> > how can the method be changed to do the equivalent of returning NULL
> > when it was declared to return a raw pointer?
>
> Return boost::shared_ptr<sometype>().
>
> Hm.
>
> I'm amazed about some of the questions here. Please read the manual
> before posting. Or at least, /glance/ at it.
>
> Cheers, & hth.,
>
> - Alf
>
> --
> 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?

Are you referring to the boost manual?
http://www.boost.org/libs/smart_ptr/scoped_ptr.htm

I did and all I see is a reset function which sets the internal
pointer to NULL, but it doesn't say how to check if it is NULL, and it
says that behavior is undefined if you dereference it while the
internal pointer is NULL.

I've got a method in a class that allocates an object of a different
type and it has been deemed "unsafe" to return a raw pointer to it.
However, if the allocation or preliminary steps before allocation
fail, I want to return NULL.

* Christopher:
> On Dec 1, 1:29 pm, "Alf P. Steinbach" <al...@start.no> wrote:
>> * Christopher:
>>
>>> If a method is declared to return a type boost::shared_ptr<sometype>,
>>> how can the method be changed to do the equivalent of returning NULL
>>> when it was declared to return a raw pointer?
>> Return boost::shared_ptr<sometype>().
>>
>> Hm.
>>
>> I'm amazed about some of the questions here. Please read the manual
>> before posting. Or at least, /glance/ at it.
>
> Are you referring to the boost manual?
> http://www.boost.org/libs/smart_ptr/scoped_ptr.htm

Looks like it, yes.

Btw., please don't quote signatures (corrected).


> I did and all I see is a reset function which sets the internal
> pointer to NULL, but it doesn't say how to check if it is NULL, and it
> says that behavior is undefined if you dereference it while the
> internal pointer is NULL.
>
> I've got a method in a class that allocates an object of a different
> type and it has been deemed "unsafe" to return a raw pointer to it.
> However, if the allocation or preliminary steps before allocation
> fail, I want to return NULL.

Perhaps take a second glance at the docs.


Cheers, & hth.,

- Alf

--
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?

On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
Christopher <cpisz@austin.rr.com> wrote,
>Are you referring to the boost manual?
>http://www.boost.org/libs/smart_ptr/scoped_ptr.htm
>
>I did and all I see is a reset function which sets the internal
>pointer to NULL, but it doesn't say how to check if it is NULL,

How about
if (retvalue == boost::shared_ptr<sometype>()) {

Think!

>and it
>says that behavior is undefined if you dereference it while the
>internal pointer is NULL.

Yes, don't do that.

On Dec 1, 9:19 pm, David Harmon <sou...@netcom.com> wrote:
> On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
> Christopher <cp...@austin.rr.com> wrote,

> >Are you referring to the boost manual?
> >http://www.boost.org/libs/smart_ptr/scoped_ptr.htm

> >I did and all I see is a reset function which sets the internal
> >pointer to NULL, but it doesn't say how to check if it is NULL,

> How about
> if (retvalue == boost::shared_ptr<sometype>()) {

How about:
if ( retvalue == NULL ) { ... }

I've not looked at boost::shared_ptr in this regard, but all of
the intelligent pointers I've written accept it.

In practice, if I think there's the slightest chance of using
intelligent pointers, I'll write something like:

template< typename T >
bool
isValid( T const* p )
{
return p != NULL ;
}

and use it. Adding the necessary overloads for the intelligent
pointers as needed. (This allows easily switching back, when
you realize that the smart pointer doesn't work.) But as I
said, a correctly designed intelligent pointer should support
all of the idioms a raw pointer does: "if( ptr == NULL )", and
even "if ( ptr )" (although I've never seen a coding guideline
that would allow use of the latter).

And if worse comes to worse, there's always:
if ( ptr.get() == NULL ) ...

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

"James Kanze" <james.kanze@gmail.com> wrote in message
news:c9f11afd-5ae2-4004-a800-2153caf4fb19@e10g2000prf.googlegroups.com...
On Dec 1, 9:19 pm, David Harmon <sou...@netcom.com> wrote:
> On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
> Christopher <cp...@austin.rr.com> wrote,

> >Are you referring to the boost manual?
> >http://www.boost.org/libs/smart_ptr/scoped_ptr.htm

> >I did and all I see is a reset function which sets the internal
> >pointer to NULL, but it doesn't say how to check if it is NULL,

> How about
> if (retvalue == boost::shared_ptr<sometype>()) {

How about:
if ( retvalue == NULL ) { ... }

I've not looked at boost::shared_ptr in this regard, but all of
the intelligent pointers I've written accept it.

In practice, if I think there's the slightest chance of using
intelligent pointers, I'll write something like:

template< typename T >
bool
isValid( T const* p )
{
return p != NULL ;
}

and use it. Adding the necessary overloads for the intelligent
pointers as needed. (This allows easily switching back, when
you realize that the smart pointer doesn't work.) But as I
said, a correctly designed intelligent pointer should support
all of the idioms a raw pointer does: "if( ptr == NULL )", and
even "if ( ptr )" (although I've never seen a coding guideline
that would allow use of the latter).

And if worse comes to worse, there's always:
if ( ptr.get() == NULL ) ...


Yes, I tryed if( retval == NULL ) and it would not compile. However, the
get method does work. I've got it now, I just don't like it much over using
raw pointers. I've always been of the opinion that, if I make a method that
allocates something, I just document it and count on the caller to know what
to do. I don't think I like trying to idiot proof things, because I am just
introducing more nuiances. If I've never used these smart pointers and don't
know how, I suspect the callers of my methods won't either. I really doubt
they will take the time to learn how the shared_ptr works, what can and
can't be done with it etc., no matter how much I do or don't. Ah well.

On Dec 1, 12:19 pm, David Harmon <sou...@netcom.com> wrote:
> On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
> Christopher <cp...@austin.rr.com> wrote,
>
> >Are you referring to the boost manual?
> >http://www.boost.org/libs/smart_ptr/scoped_ptr.htm
>
> >I did and all I see is a reset function which sets the internal
> >pointer to NULL, but it doesn't say how to check if it is NULL,
>
> How about
> if (retvalue == boost::shared_ptr<sometype>()) {

How about the more succinct:

if (not retval) {

or, alternately:

if (retval == false) {

Greg

James Kanze a écrit :
> On Dec 1, 9:19 pm, David Harmon <sou...@netcom.com> wrote:
>> On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
>> Christopher <cp...@austin.rr.com> wrote,
>
>>> Are you referring to the boost manual?
>>> http://www.boost.org/libs/smart_ptr/scoped_ptr.htm
>
>>> I did and all I see is a reset function which sets the internal
>>> pointer to NULL, but it doesn't say how to check if it is NULL,
>
>> How about
>> if (retvalue == boost::shared_ptr<sometype>()) {
>
> How about:
> if ( retvalue == NULL ) { ... }
>
> I've not looked at boost::shared_ptr in this regard, but all of
> the intelligent pointers I've written accept it.

Boost share_ptr doesn't provide a operator==(smart_ptr<T>& smart,T*ptr).

I could not find the exact reason in the original proposal but I guess
it is an efficiency matter: it would be equivalent to
'(smart.use_count()?smart.ptr:NULL) == ptr' when the intended operations
in the case ptr=NULL would have been '(use_count()==0) ||
(smart.ptr==NULL)'.

>
> In practice, if I think there's the slightest chance of using
> intelligent pointers, I'll write something like:
>
> template< typename T >
> bool
> isValid( T const* p )
> {
> return p != NULL ;
> }
>
> and use it. Adding the necessary overloads for the intelligent
> pointers as needed. (This allows easily switching back, when
> you realize that the smart pointer doesn't work.) But as I
> said, a correctly designed intelligent pointer should support
> all of the idioms a raw pointer does: "if( ptr == NULL )", and
> even "if ( ptr )" (although I've never seen a coding guideline
> that would allow use of the latter).
>
> And if worse comes to worse, there's always:
> if ( ptr.get() == NULL ) ...

But smart-ptr does provide conversion to unspecified-bool-type.

The doc is quite explicit:

*conversions*

operator unspecified-bool-type () const; // never throws

Returns: an unspecified value that, when used in boolean contexts,
is equivalent to get() != 0.

Throws: nothing.

Notes: This conversion operator allows shared_ptr objects to be
used in boolean contexts, like if (p && p->valid()) {}. The actual
target type is typically a pointer to a member function, avoiding many
of the implicit conversion pitfalls.

[The conversion to bool is not merely syntactic sugar. It allows
shared_ptrs to be declared in conditions when using dynamic_pointer_cast
or weak_ptr::lock.]

Michael

On Dec 3, 6:36 am, Michael DOUBEZ <michael.dou...@free.fr> wrote:
> James Kanze a écrit :
> > On Dec 1, 9:19 pm, David Harmon <sou...@netcom.com> wrote:
> >> On Sat, 1 Dec 2007 11:36:51 -0800 (PST) in comp.lang.c++,
> >> Christopher <cp...@austin.rr.com> wrote,

> >>> Are you referring to the boost manual?
> >>>http://www.boost.org/libs/smart_ptr/scoped_ptr.htm

> >>> I did and all I see is a reset function which sets the internal
> >>> pointer to NULL, but it doesn't say how to check if it is NULL,

> >> How about
> >> if (retvalue == boost::shared_ptr<sometype>()) {

> > How about:
> > if ( retvalue == NULL ) { ... }

> > I've not looked at boost::shared_ptr in this regard, but all of
> > the intelligent pointers I've written accept it.

> Boost share_ptr doesn't provide a operator==(smart_ptr<T>& smart,T*ptr).

I'm not sure that that's what is wanted. In my own smart
pointers, I use something like:

template< typename T >
class SmartPtr
{
struct Hidden {} ;
public:
// ...
friend operator==( SmartPtr< T > const&, Hidden const* ) ;
friend operator!=( Hidden const*, SmartPtr< T > const& ) ;
// ...
} ;

Null pointer constants convert to Hidden const*, but the user
can't get one any other way.

> I could not find the exact reason in the original proposal but I guess
> it is an efficiency matter: it would be equivalent to
> '(smart.use_count()?smart.ptr:NULL) == ptr' when the intended operations
> in the case ptr=NULL would have been '(use_count()==0) ||
> (smart.ptr==NULL)'.

I can't really believe that the difference in efficiency would
make a difference here.

> > In practice, if I think there's the slightest chance of using
> > intelligent pointers, I'll write something like:

> > template< typename T >
> > bool
> > isValid( T const* p )
> > {
> > return p != NULL ;
> > }

> > and use it. Adding the necessary overloads for the intelligent
> > pointers as needed. (This allows easily switching back, when
> > you realize that the smart pointer doesn't work.) But as I
> > said, a correctly designed intelligent pointer should support
> > all of the idioms a raw pointer does: "if( ptr == NULL )", and
> > even "if ( ptr )" (although I've never seen a coding guideline
> > that would allow use of the latter).

> > And if worse comes to worse, there's always:
> > if ( ptr.get() == NULL ) ...

> But smart-ptr does provide conversion to unspecified-bool-type.

Yes. But this forces you do use something like:

if( ptr ) ...

Gratuous obfuscation, and forbidden by all of the coding
guidelines I've seen. (Never the less, I'd support it as well.
Because raw pointers do.)

Of course, from a QoI point of view, you wouldn't use bool, but
"Hidden const*", to avoid any chance of mis-use.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34