Re: smart pointer clarifications

From:
"Phil Bouchard" <phil@fornux.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 21 Aug 2008 05:40:30 -0700
Message-ID:
<g8jnnv$s33$1@aioe.org>
"mlimber" <mlimber@gmail.com> wrote in message
news:2e3e012e-a35c-43e2-9fe0-f3678e88a417@d77g2000hsb.googlegroups.com...

On Aug 20, 12:57 pm, "Phil Bouchard" <p...@fornux.com> wrote:


[...]

Either we do it this way or we share the allocator across the container
and
smart pointer so that the smart pointer uses deallocate(void *):

1)
std::vector<shifted_ptr<int>, shifted_allocator<shifted_ptr<int> > > c1;

2)
std::vector<shifted_ptr<int, shifted_allocator<int> >,
shifted_allocator<shifted_ptr<int, shifted_allocator<int> > > > c2;

I don't even think option 2) is implementable.


I may just be dense, but I still don't see what you're getting at.
Can't you just use the deleter parameter to indicate special deletion?
Something like this (untested):

  class C { /*...*/ };

  template<class Alloc>
  class MyDeleter
  {
    Alloc& m_alloc;

    typedef typename Alloc::value_type T;

  public:
    MyDeleter( Alloc& alloc ) : m_alloc( alloc ) {}

    // Might want a const T* version also
    void operator()( T* const ptr )
    {
      m_alloc.destroy( ptr );
      m_alloc.deallocate( ptr, sizeof(T) );
    }
  };

  void Foo( MyAllocator<C>& alloc, const C& initVal )
  {
    typedef std::vector< std::tr1::shared_ptr<C> > VSPC;
    VSPC v( 10 );
    for( VSPC::iterator it=v.begin(); it != v.end(); ++it )
    {
      // Could hide these next lines in a factory function
      C* const c = alloc.allocate( sizeof(C) );
      alloc.construct( c, initVal );

      it->reset( c, MyDeleter( m_alloc ) );
    }
    // ...
  }

Now, when the vector "v" goes out of scope, the shared_ptrs use the
"alloc" object to deallocate their pointees.


What you are suggesting is very similar to option 2) I was anteriorly
explaining. In my case I was using the allocator passed in as a template
parameter as a deleter so that there is no need to repeatedly call the
overloaded constructor shared_ptr(T *, D *). I can't imagine calling this
special constructor for each node pointer inside a container in a flexible
way. If I forget calling it for one pointer the compiler will not report
any compilation error.

-Phil

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