How to detect NULL for boost:shared_ptr

Hi,

I am playing with boost pointer and try to wrap the following codes

A* func(){
...
if(condition 1 ){
return a;
}
else
return NULL;

}

Now I wrap a as shared_ptr(new A()) and change funcion as share_ptr<A>
func();
The problem is that shared_ptr will not accept NULL.

How should I do it in this case?

Thanks

Chris

re: How to detect NULL for boost:shared_ptr

tradevol@yahoo.com wrote:What does it mean that it "will not accept" it? Do you get a compiler
error? Do you get a run-time exception? If the latter, catch it. If
the former, deal with it (see FAQ 5.8).
Read the FAQ.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

re: How to detect NULL for boost:shared_ptr

tradevol@yahoo.com schrieb:Did you try to return a default-constructed pointer like this?

return shared_ptr<A>();

--
Thomas

re: How to detect NULL for boost:shared_ptr

On 4 Sie, 21:37, "Thomas J. Gritzan" <phygon_antis...@gmx.dewrote:Well, I don't know exactly what you mean, since shared_ptr is there to
avoid the ordinary pointers. However, if I understand correctly what
this is all about you can do it this way:

shared_ptr<Afunc()
{
shared_ptr<Ar; //automatically set to point to NULL
if(condition 1 )
{
shared_ptr<Ar(new A);
return r; //I read somewhere to avoid creating new shared_ptrs
"on the fly" like "return shared_ptr<A>(new A);"
}
else
{
shared_ptr<Ar; //set to point to NULL automatically
return r;
}

}

// ... and then, somewhere in code:
shared_ptr<Atest = func();
if(test.get() == NULL)
do sth



I hope it helps...

re: How to detect NULL for boost:shared_ptr

On Aug 4, 5:01*pm, sylvester.zal...@googlemail.com wrote:yes. This what I asked for. sorry for the confusion.
Thank you very much

re: How to detect NULL for boost:shared_ptr

sylvester.zaluga@googlemail.com wrote:
This is an unnecessary stack variable. Same with below. You can call
reset(T * p) to assign to the pointer declared in function scope.

re: How to detect NULL for boost:shared_ptr

On Aug 4, 10:03 pm, trade...@yahoo.com wrote:
Are you sure? I've not much experience with boost::shared_ptr,
but all of the smart pointers I've use do accept constructing
them with NULL.
In this particular case, I'd probably write something like:

shared_ptr< A >
func()
{
return shared_ptr< A >(
someCondition
? new A
: NULL ) ;
}

In more complicated cases, however:

shared_ptr< A >
func()
{
shared_ptr< A result ;
if ( someCondition ) {
result = new A ;
}
return result ;
}

I'd be very surprised if either of them failed to work with
boost::shared_ptr. Just as I'd be very surprised if, given the
definition of result, above "result = NULL ;" wouldn't compile;
for that matter, there's no reason why "if ( result == NULL )"
can't be made to work as expected, either.

--
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

re: How to detect NULL for boost:shared_ptr

On 2008-08-05 04:17:07 -0400, James Kanze <james.kanze@gmail.comsaid:
Yes, it's a problem. The constructor that takes a raw pointer is a template:

template <class Tclass shared_ptr {
template <class Yexplicit shared_ptr(Y *p);
...
};

with a requirement that Y* must be convertible to T*. So a normal null
pointer doesn't work. For a shared_ptr<intyou have to use something
like (int*)0, or use the default constructor.

In C++0x there's a new constructor that takes an argument of type nullptr_t.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

re: How to detect NULL for boost:shared_ptr

On Aug 5, 12:51 pm, Pete Becker <p...@versatilecoding.comwrote:And there's not a non-templated version alongside of it?
Sounds like something got forgotten along the way.
But it would be easy to add a constructor which would work with
NULL. Which is what most users would expect.

Hmmm. I don't see in Boost documentation where they support
comparison with NULL, either. Which would seem a rather
frequent operation, and certainly necessary if you're doing like
the original poster, and converting from raw pointers.

--
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

re: How to detect NULL for boost:shared_ptr

Hmmm. I don't see in Boost documentation where they supportYou may not be able to compare to NULL explicitly (this is unconfirmed by
me), but shared_ptr has the operator unspecified_bool_type( ) so you can
test the pointer directly, so it's really not that big a deal (OTHER than
the fact that a shared_ptr is not syntactically a "drop-in" replacement for
a conventional pointer).

shared_ptr< MyClass myVar = func( );

if( myVar ) // is the pointer not null?
DoSomething( );

re: How to detect NULL for boost:shared_ptr

On Aug 6, 2:32 am, "Kevin Frey" <kevin_g_f...@hotmail.comwrote:In other words, it supports the misfeatures of standard
pointers, but not the correct way of using them. (Obviously,
if you're going to call it a smart pointer, you have to support
the misfeatures as well. But not supporting the standard idiom
is simply not acceptable.)

--
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