RE: [boost] new version of smart pointer class templates

From: Brey, Edward D (EdwardDBrey_at_[hidden])
Date: 2002-01-25 16:16:16

The static argument example is a good one.  I now see how you could want to
change the deleter within the lifetime of a shared_ptr type and even an
object.  Without use a Loki-style design, the function object would need to
be kept whether this flexibility is used or not, which isn't perfect, but is
quite reasonable.

I think a nice feature would be to add a second template parameter allowing
the default deleter to be changed on a type basis.  This would allow
creation of a type like shared_cfile, while still allowing heterogeneous
deleters when desired.

If I'm not mistaken, the deleter template parameter would exactly parallel
the Compare template parameter in std::set et al.

 -----Original Message-----
From: 	Peter Dimov [mailto:pdimov_at_[hidden]] 
Sent:	Friday, 25 January 2002 2:22 PM
To:	boost_at_[hidden]
Subject:	Re: [boost] new version of smart pointer class templates

From: "Brey, Edward D" <EdwardDBrey_at_[hidden]>
> I don't think this will work.  There is no implicit conversion between 0
> shared_ptr<FILE>, which is a good thing.

Yes, you could (should?) do this

shared_ptr<FILE> my_fopen(const char* name, const char* mode)
{
    if (FILE* p = fopen(name, mode))
        return shared_ptr<FILE>(p, fclose);
    else
        throw open_error(name, errno);
}

instead. Of course this is simply an example.

> On a larger question, what is the design rational for not making the
deleter
> a template parameter?

Both approaches have their uses. It is generally expected that the Loki
smart pointer, or some other policy-based design, will cover the template
parameterized case, where you encode the deallocation policy into the type
of the pointer.

shared_ptr<> can be used in situations where you don't want to have the
deallocation policy encoded in the type. For example, if you maintain a
library that has

shared_ptr<Object> createObject();

and you want to change the allocator used for object creation (or even make
some objects static) without breaking compatibility.

Info: http://www.boost.org  Send unsubscribe requests to:
<mailto:boost-unsubscribe_at_[hidden]> 

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/