"Johan Råde" <rade@maths.lth.se> wrote in message news:ft10ag$6nn$1@ger.gmane.org...

Your idea certainly could be useful. It should probably be combined with the Null Object Pattern.

Yes, it would be a way to enforce the Null Object Pattern. Although one should only use null objects where they make sense. Null objects should not be used to pass additional state information. If you create a find function that returns a null object if no items are found, you really gain nothing from using the pattern and the code will probably be more prone to error.

However, having many different smart pointers would make it more difficult for different software components to interact with each other.

Yes I think this variant of smart pointer will encounter resistance from the community because it is much easier to have only one shared_ptr to worry about. This is why we have the boost::shared_ptr and not a policy based one. The nullability attribute would fit nicely into a policy based smart pointer as the policy design avoids the complexity explosion of many separate classes. Thinking about this some more, I think there may be a more fundamental problem at play here. I would suggest that smart pointers as we have come to know them are poorly designed. It's only natural that they evolved to reflect the capabilities of the C++ pointer (indeed they are called smart POINTERS), but are the semantics of C++ pointers a good idea in terms of a class interface? I would argue that they are not. A smart pointer fails the One Responsibility Rule regarding the its nullability. Allowing a smart pointer to be null is an extra state carried by the class when it could be maintained by other means. If we remove null capability from all smart pointers, essentially turning them into smart references, what do we do when we absolutely need to represent a reference that does not exist (or dare I say optional)? Well boost already has the answer. We use boost::optional; a template for representing optional values. Using boost::optional would free us from a proliferation of smart pointers (nullable and not) and usability concerns. I'm sure no one is going argue that optional<int> creates too much interface friction because int is used commonly as component interface type. Therefore optional<WidgetRef> should not create problems for components that use WidgetRef. I know it is an insane idea to suggest changing the entire boost::smart_ptr library to a boost::smart_ref libary, but I think it may actually be the right thing to do. Does anyone agree? -Ross

Perhaps there is even a backwards compatible solution. Could we employ an optional null object pattern in methods and or constructors of shared_ptr that might establish a raw nill state? Perhaps something like this obviously oversimplified example could be use. template < typename T> void shared_ptr < T > :: reset ( T * p = 0 ) { if ( p ) { /* what happens in shared_ptr now */ } else { /* new behavior in shared_ptr, when it detects a nill raw pointer */ static NullObjectTypes < T > nullObject; *this = shared_ptr < T > ( &nullObject, NullObjectTypes < T > :: deleter () ); } } Jeff -- View this message in context: http://boost.2283326.n4.nabble.com/Why-no-non-null-smart-pointers-tp2642959p... Sent from the Boost - Dev mailing list archive at Nabble.com.

On Mon, Apr 28, 2014 at 9:42 PM, Jeff Hill <johill@lanl.gov> wrote:

Maybe there /is/ even a backwards compatible solution.

Perhaps an optional null object pattern could be employed in methods, and or constructors, of shared_ptr that might establish a raw nill state? Perhaps something like this obviously oversimplified example could be use.

template < typename T> void shared_ptr < T > :: reset ( T * p = 0 ) { if ( p ) { /* what happens in shared_ptr now */ } else { /* new behavior in shared_ptr, when it detects a nill raw pointer */ *this = NullObjectTypes < T > :: factory (); } }

Please, don't change shared_ptr (or any other *_ptr) behavior. They are intended to behave as pointers, and pointers do have a null state. This is a useful feature in many cases. I'd prefer the functionality you're looking for to be implemented as a separate component/library. It may be based on pointers internally though.

You've overlooked the use case of reclaiming ownership from smart_ptr.

Sorry, apparently a bit more explanation is needed. In the code above I have this assignment. *this = NullObjectTypes < T > :: factory (); My idea is that shared_ptr <T> is returned from this factory function so that it can be assigned to *this, and so ownership /is/ properly maintained. The default templated version of NullObjectTypes < T > :: factory () would return a default constructed shared_ptr <T>. This is necessary of course for backwards compatibility. template < class T > struct NullObjectTypes shared_ptr < ChessPiece > factory () { return shared_ptr < T > (); } }; The benefit comes when T is an interface class. In this situation we can specialize NullObjectTypes < T > as follows. Lets say that T is the ChessPiece class, and we will implement the null object pattern for it. struct ChessPiece { virtual void move() = 0; }; struct ChessPieceNull : public ChessPiece { void move() {} }; template <> NullObjectTypes < ChessPiece > shared_ptr < ChessPiece > factory (); }; template <> shared_ptr < ChessPiece > NullObjectTypes < ChessPiece > :: factory () { static ChessPieceNull chessPieceNull; return shared_ptr < ChessPiece > ( &chessPieceNull, nullDeleter () ); } -- View this message in context: http://boost.2283326.n4.nabble.com/Why-no-non-null-smart-pointers-tp2642959p... Sent from the Boost - Dev mailing list archive at Nabble.com.

On Tue, Apr 29, 2014 at 07:44:27AM -0700, Jeff Hill wrote:

So I deleted the wrong post. This is the version of the proposed change that I intended to keep.

Note that mailing lists (which this actually is) doesn't _do_ deletion. If you want to rescind a post, reply to it and inform people that the parent should be unconsidered. Any operation that your particular groups/news-alike gateway promises is a "delete" is a completely local thing. No-one outside of your gateway will ever see it, maybe not even outside of you. -- Lars Viklund | zao@acc.umu.se

static ChessPieceNull chessPieceNull; static shared_ptr < ChessPiece > nullsp( &chessPieceNull, nullDeleter () ); return nullsp;

Thanks for your suggestion. Agree this is usually preferable, but since the code is in the specialization (within a user snap-in) then of course the user can decide which approach he prefers. -- View this message in context: http://boost.2283326.n4.nabble.com/Why-no-non-null-smart-pointers-tp2642959p... Sent from the Boost - Dev mailing list archive at Nabble.com.

On Mon, Apr 28, 2014 at 8:20 AM, Jeff Hill <johill@lanl.gov> wrote:

I also find the nill state of shared_ptr to be a liability when designing interfaces. The primary purpose of shared_ptr when used in an interface context is to convey ownership, and almost always it feels wrong to also provide simultaneously an option to convey ownership of nothing. The poor user is left with the responsibility of testing every shared_ptr he has received for null state before using it, but its easy to forget and the compiler provides no assistance. Ultimately, the library interface based on shared_ptr is more error prone, less likely to be easy to use, and therefore arguably suboptimal.

Of course no one suggests that shared_ptr should (or could) be changed in ways that impacts backwards compatibility, but I have to agree with the author of the post that this issue can be identified as a legitimate cause for concern.

Jeff

There is usefullness to null and non-null smart pointers. Just so we're not repeating thoughts here too much, this was also discussed at the end of last year: http://lists.boost.org/Archives/boost/2013/10/206732.php I think the general consensus is that people want non-null smart pointers (myself included), but there are many subtleties that people disagree on. Be sure to read that thread going down paths that have been traveled before. -- -Matt Calabrese