Re: smart pointer clarifications

mlimber <>
Wed, 20 Aug 2008 13:35:34 -0700 (PDT)
On Aug 20, 12:57 pm, "Phil Bouchard" <> wrote:

"mlimber" <> wrote in message
On Aug 20, 4:26 am, "Phil Bouchard" <> wrote:


I still don't get it. Let's say you're designing std::vector, which
has an allocator template parameter. It doesn't matter if the user-
defined class or a smart pointer (as long as it has value semantics).
Given this code:

 class C { */...*/ };
 // ...
 std::vector<C> v1( 10 );
 std::vector< std::tr1::shared_ptr<C> > v2( 10 );

Under the hood, each of the latter two lines will use vector's
(default) allocator to grab memory for the 10 instances of the
contained type and then placement-new to construct the instances in
that memory. (Note that all the shared_ptr's pointees are null at this
point.) Why does the allocator care if it's making a C or a

Cheers! --M

Indeed in that case it won't matter because the pools used to allocate no=


from the container and to deallocate objects from the smart pointer are n=


shared. Sometimes it is in our favor to share to same pool used by the
container and the smart pointer. For example shifted_ptr<> uses this t=


prevent cyclicism (still requires STL modifications).

Either we do it this way or we share the allocator across the container a=


smart pointer so that the smart pointer uses deallocate(void *):

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

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;

    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.

Cheers! --M

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