gmane.comp.lib.boost.user

Meryl Silverburgh | 14 Jul 23:41

how can I return a 'null' smart pointer?

Hi,

Can you please tell me how can I return a 'null' smart pointer?

typedef shared_ptr<MyA> MyAPtr;
typedef vector<MyAPtr>  MyAVector;

For example,I have this code, which return NULL in an error situation:

MyAPtr myfunction(MyAVector& v, int size) {
      if (index < v.size()) {
            return v[index];
       } else {
          // how can I return NULL here?

      }

}

How can I achieve that when I use boost smart pointer?

Thank you for any tip.

Nat Goodspeed | 14 Jul 23:47

Re: how can I return a 'null' smart pointer?

Meryl Silverburgh wrote:

> MyAPtr myfunction(MyAVector& v, int size) {
>       if (index < v.size()) {
>             return v[index];
>        } else {
>           // how can I return NULL here?

Does it work to return MyAPtr() ?

>       }
> 
> }

Marshall Clow | 14 Jul 23:50

Re: how can I return a 'null' smart pointer?

At 4:43 PM -0500 7/14/08, Meryl Silverburgh wrote:
>Hi,
>
>Can you please tell me how can I return a 'null' smart pointer?
>
>typedef shared_ptr<MyA> MyAPtr;
>typedef vector<MyAPtr>  MyAVector;
>
>For example,I have this code, which return NULL in an error situation:
>
>MyAPtr myfunction(MyAVector& v, int size) {
>       if (index < v.size()) {
>             return v[index];
>        } else {
>           // how can I return NULL here?
>       }
>}
>
>How can I achieve that when I use boost smart pointer?

I suggest that you throw an exception here, rather than returning 
some bogus value.
--

-- 
-- Marshall

Marshall Clow     Idio Software   <mailto:marshall <at> idio.com>

It is by caffeine alone I set my mind in motion.
It is by the beans of Java that thoughts acquire speed,
the hands acquire shaking, the shaking becomes a warning.

(Continue reading)

Thorsten Ottosen | 15 Jul 10:26

Re: how can I return a 'null' smart pointer?

Marshall Clow skrev:
> At 4:43 PM -0500 7/14/08, Meryl Silverburgh wrote:
>> Hi,
>>
>> Can you please tell me how can I return a 'null' smart pointer?
>>
>> typedef shared_ptr<MyA> MyAPtr;
>> typedef vector<MyAPtr>  MyAVector;
>>
>> For example,I have this code, which return NULL in an error situation:
>>
>> MyAPtr myfunction(MyAVector& v, int size) {
>>       if (index < v.size()) {
>>             return v[index];
>>        } else {
>>           // how can I return NULL here?
>>       }
>> }
>>
>> How can I achieve that when I use boost smart pointer?
> 
> I suggest that you throw an exception here, rather than returning some 
> bogus value.

I concur, but I suggest you simply add a precondition:

   assert( index < v.size() )

-Thorsten

(Continue reading)

Meryl Silverburgh | 16 Jul 02:06

Re: how can I return a 'null' smart pointer?

On Tue, Jul 15, 2008 at 3:26 AM, Thorsten Ottosen
<thorsten.ottosen <at> dezide.com> wrote:
> Marshall Clow skrev:
>>
>> At 4:43 PM -0500 7/14/08, Meryl Silverburgh wrote:
>>>
>>> Hi,
>>>
>>> Can you please tell me how can I return a 'null' smart pointer?
>>>
>>> typedef shared_ptr<MyA> MyAPtr;
>>> typedef vector<MyAPtr>  MyAVector;
>>>
>>> For example,I have this code, which return NULL in an error situation:
>>>
>>> MyAPtr myfunction(MyAVector& v, int size) {
>>>      if (index < v.size()) {
>>>            return v[index];
>>>       } else {
>>>          // how can I return NULL here?
>>>      }
>>> }
>>>
>>> How can I achieve that when I use boost smart pointer?
>>
>> I suggest that you throw an exception here, rather than returning some
>> bogus value.
>

You mention "I suggest that you throw an exception here, rather than

(Continue reading)

Steven Watanabe | 16 Jul 02:12

Re: how can I return a 'null' smart pointer?

AMDG

Meryl Silverburgh wrote:
> So when I do this:
>
> shared_ptr<MyClass> aPtr;
>
> and then I check for 'null' , like this:
>
> if (aPtr == NULL) {
> // will it return true here?
>   
Yes.  It will be true.

In Christ,
Steven Watanabe

Filip KonviÄka | 15 Jul 15:49

Re: how can I return a 'null' smart pointer?

Marshall Clow (14.7.2008 23:50):
> At 4:43 PM -0500 7/14/08, Meryl Silverburgh wrote:
>> Hi,
>>
>> Can you please tell me how can I return a 'null' smart pointer?
>>
>> typedef shared_ptr<MyA> MyAPtr;
>> typedef vector<MyAPtr>  MyAVector;
>>
>> For example,I have this code, which return NULL in an error situation:
>>
>> MyAPtr myfunction(MyAVector& v, int size) {
>>       if (index < v.size()) {
>>             return v[index];
>>        } else {
>>           // how can I return NULL here?
>>       }
>> }
>>
>> How can I achieve that when I use boost smart pointer?
> 
> I suggest that you throw an exception here, rather than returning some 
> bogus value.

What is so bad in general about returning uninitialized smart pointers? 
Is that more costly than throwing an exception? I mean, there is even 
some support for testing a smart pointer for validity built into 
shared_ptr, so why not use it where appropriate?

Cheers,

(Continue reading)

Nikolai N Fetissov | 15 Jul 15:59

Re: how can I return a 'null' smart pointer?

> Marshall Clow (14.7.2008 23:50):
>> At 4:43 PM -0500 7/14/08, Meryl Silverburgh wrote:
>>> Hi,
>>>
>>> Can you please tell me how can I return a 'null' smart pointer?
>>>
>>> typedef shared_ptr<MyA> MyAPtr;
>>> typedef vector<MyAPtr>  MyAVector;
>>>
>>> For example,I have this code, which return NULL in an error situation:
>>>
>>> MyAPtr myfunction(MyAVector& v, int size) {
>>>       if (index < v.size()) {
>>>             return v[index];
>>>        } else {
>>>           // how can I return NULL here?
>>>       }
>>> }
>>>
>>> How can I achieve that when I use boost smart pointer?
>>
>> I suggest that you throw an exception here, rather than returning some
>> bogus value.
>
> What is so bad in general about returning uninitialized smart pointers?
> Is that more costly than throwing an exception? I mean, there is even
> some support for testing a smart pointer for validity built into
> shared_ptr, so why not use it where appropriate?
>

(Continue reading)

Filip KonviÄka | 15 Jul 16:17

Re: how can I return a 'null' smart pointer?

>> What is so bad in general about returning uninitialized smart pointers?
>> Is that more costly than throwing an exception? I mean, there is even
>> some support for testing a smart pointer for validity built into
>> shared_ptr, so why not use it where appropriate?
>>
> 
> The only problem with returning a null pointer, or -1,
> or whatever in case of error is that it's easy to ignore
> at the calling scope, so the upper layers would not even know
> the error occurred. Exception, on the other hand, propagates
> up the stack enforcing more structured error handling.

I agree, but only if the case is an error. In my application, I have 
several places where the "null" value is a regular result. That's what I 
meant by saying "where appropriate".

Cheers,
Filip

gast128 | 16 Jul 23:25

Re: how can I return a 'null' smart pointer?

Nikolai N Fetissov <nikolai-boost <at> fetissov.org> writes:

> > What is so bad in general about returning uninitialized smart pointers?
> > Is that more costly than throwing an exception? 
I think it's even the other way around.

> > I mean, there is even
> > some support for testing a smart pointer for validity built into
> > shared_ptr, so why not use it where appropriate?
I think this is approriate, as u write in your email. For example finding 
customer orders which need not exist in an order table, is not an exceptional 
case and can be signalled with a NULL ptr.

> The only problem with returning a null pointer, or -1,
> or whatever in case of error is that it's easy to ignore
> at the calling scope, so the upper layers would not even know
> the error occurred. Exception, on the other hand, propagates
> up the stack enforcing more structured error handling.
Yes but not handling exceptions is fatal for your applications which can be a 
good thing if the case would corrupt your system anyway, or a bad thing if no 
serious bad things happen. If every Space Shuttle would crash on every fault, 
there would be no new austronauts in the USA.

Sutter has made a guideline for when to use exceptions and when not ("When and 
How to Use Exceptions", http://www.ddj.org/cpp/184401836). By using this 
guideline, programmers actually have to deal with two strategies instead of 
one, not to mention that writing excpetion safe code (without leaks) is very 
hard. If you get stuck you can always buy his books :)

Then again there was someone who wrote that writing exception safe code is

(Continue reading)

Marshall Clow | 16 Jul 23:48

Re: how can I return a 'null' smart pointer?

  gast128 <gast128 <at> hotmail.com> wrote:

>Sutter has made a guideline for when to use exceptions and when not ("When and
>How to Use Exceptions", http://www.ddj.org/cpp/184401836). By using this
>guideline, programmers actually have to deal with two strategies instead of
>one, not to mention that writing excpetion safe code (without leaks) is very
>hard. If you get stuck you can always buy his books :)

Actually, having read all of Bruce's books, and having talked to him 
at a couple of conferences, I find that writing exception safe code 
is quite simple - once you have embraced RAII everywhere. You do have 
to think about exception safety, but for most routines that thinking 
boils down to "What happens if something throws here? Um, .... 
nothing bad.", since RAII ensures that no locals leak, and I can look 
at the global/parameter cases quickly.

I find it also leads to simple, easy to write (and understand code).
--

-- 
-- Marshall

Marshall Clow     Idio Software   <mailto:marshall <at> idio.com>

It is by caffeine alone I set my mind in motion.
It is by the beans of Java that thoughts acquire speed,
the hands acquire shaking, the shaking becomes a warning.
It is by caffeine alone I set my mind in motion.

gast128 | 17 Jul 12:19

Re: how can I return a 'null' smart pointer?

Marshall Clow <marshall <at> idio.com> writes:

> 
>   gast128 <gast128 <at> hotmail.com> wrote:
> 
> >Sutter has made a guideline for when to use exceptions and when not ("When 
and
> >How to Use Exceptions", http://www.ddj.org/cpp/184401836). By using this
> >guideline, programmers actually have to deal with two strategies instead of
> >one, not to mention that writing excpetion safe code (without leaks) is very
> >hard. If you get stuck you can always buy his books :)
> 
> Actually, having read all of Bruce's books
Eh, isn't his name Herb...

>, and having talked to him 
> at a couple of conferences, I find that writing exception safe code 
> is quite simple - once you have embraced RAII everywhere. You do have 
> to think about exception safety, but for most routines that thinking 
> boils down to "What happens if something throws here? Um, .... 
> nothing bad.", since RAII ensures that no locals leak, and I can look 
> at the global/parameter cases quickly.
Not sure about that. I think you probably have to wrap every raw pointer in a 
smart pointer or ending up having try / catch everywhere. Writing everywhere 
try / catch doesn't improve the readability imo (which is also depreciated by 
people). Item 17-23 and item 29 of more exceptional c++ are all about case 
where exceptions occur, none of them are trivial (for me at least).

> I find it also leads to simple, easy to write (and understand code).
Sure? What if you make a pool of available devices like (COM ports) and they

(Continue reading)

Marshall Clow | 17 Jul 15:21

Re: how can I return a 'null' smart pointer?

>Marshall Clow <marshall <at> idio.com> writes:
>
>>
>>    gast128 <gast128 <at> hotmail.com> wrote:
>>
>>  >Sutter has made a guideline for when to use exceptions and when not ("When
>and
>>  >How to Use Exceptions", http://www.ddj.org/cpp/184401836). By using this
>>  >guideline, programmers actually have to deal with two strategies instead of
>>  >one, not to mention that writing excpetion safe code (without 
>>leaks) is very
>>  >hard. If you get stuck you can always buy his books :)
>>
>>  Actually, having read all of Bruce's books
>Eh, isn't his name Herb...

D'oh! 
[ OK - I don't know who I which "Bruce" I was thinking of ]

>  >, and having talked to him
>>  at a couple of conferences, I find that writing exception safe code
>>  is quite simple - once you have embraced RAII everywhere. You do have
>>  to think about exception safety, but for most routines that thinking
>>  boils down to "What happens if something throws here? Um, ....
>>  nothing bad.", since RAII ensures that no locals leak, and I can look
>>  at the global/parameter cases quickly.
>Not sure about that. I think you probably have to wrap every raw pointer in a
>smart pointer or ending up having try / catch everywhere.

I think that's what I said: "Embraced RAII everywhere". That means

(Continue reading)

Marshall Clow | 17 Jul 15:29

Re: how can I return a 'null' smart pointer?

At 6:21 AM -0700 7/17/08, Marshall Clow wrote:
>Of course anyone can make up examples where exceptions are good/bad, 
>so I won't bore you with mine.

Re-reading this, I realized that this could be read as saying that 
gast128's example was boring.

That was not my intent - rather that "dueling examples" would get boring.

P.S.	Bruce Sutter was a baseball player, and I think that's where 
I in my memory I grabbed the wrong first name from. 
--

-- 
-- Marshall

Marshall Clow     Idio Software   <mailto:marshall <at> idio.com>

It is by caffeine alone I set my mind in motion.
It is by the beans of Java that thoughts acquire speed,
the hands acquire shaking, the shaking becomes a warning.
It is by caffeine alone I set my mind in motion.

Joshua Perry | 17 Jul 00:24

Re: how can I return a 'null' smart pointer?

--------------------------------------------------
From: "gast128" <gast128 <at> hotmail.com>
Sent: Wednesday, July 16, 2008 3:25 PM
To: <boost-users <at> lists.boost.org>
Subject: Re: [Boost-users] how can I return a 'null' smart pointer?

> Nikolai N Fetissov <nikolai-boost <at> fetissov.org> writes:
>
>> > What is so bad in general about returning uninitialized smart pointers?
>> > Is that more costly than throwing an exception?
> I think it's even the other way around.
>
>> > I mean, there is even
>> > some support for testing a smart pointer for validity built into
>> > shared_ptr, so why not use it where appropriate?
> I think this is approriate, as u write in your email. For example finding
> customer orders which need not exist in an order table, is not an 
> exceptional
> case and can be signalled with a NULL ptr.
>

A function like you mention (maybe vector<OrderPtr> 
Customer::GetUnfulfilledOrders( void ); )  could return an vector or custom 
collection with 0 elements, however with a function like OrderPtr 
Customer::GetOrder(int ordern); to me it feels natural to throw an 
exception.

Sure shared_ptr does have a mechanism for validity checking but if you 
forget to validate it and dereference the pointer then you have an 
exceptional condition on your hands.  Is it easier to check the pointer for

(Continue reading)

Daryle Walker | 17 Jul 12:02

Re: how can I return a 'null' smart pointer?

On Jul 16, 2008, at 6:24 PM, Joshua Perry wrote:

> A function like you mention (maybe vector<OrderPtr>  
> Customer::GetUnfulfilledOrders( void ); )  could return an vector  
> or custom collection with 0 elements, however with a function like  
> OrderPtr Customer::GetOrder(int ordern); to me it feels natural to  
> throw an exception.

I think this is more like a find function, so you could return an  
iterator to the appropriate element, and return a one-past-the-end  
iterator if the order number wasn't found.  In your example, you're  
effectively trying to run a find and then return a dereferenced  
result.  I guess that you are trying to hide any container-ness about  
the order system, so either returning an order reference or throwing  
is probably better, but only if you expect the user to never try  
entering invalid order numbers.  If entering invalid numbers is a  
standard occurrence (pre-checking is too hard and/or not expected),  
then it wouldn't be a post-condition violation, and a null-pointer  
should be returned instead.

--

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com

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