Boost shared_ptr: How to use custom deleters and allocators

Free function allocate_shared can be used with any standard compliant allocator. But what about shared_ptr's constructor and reset method.

template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);
template<class Y, class D, class A> void reset(Y * p, D d, A a);

The manu开发者_如何学JAVAal says that D should provide a call operator which will be used to delete the pointer and A must be a standard compliant allocator. If so, why D is needed? Can't A do both allocation and delocation? Don't you think that the requirement to provide a deleter for every custom allocator makes the above methods pretty much useless? When I use custom allocators, I go for allocate_shared. How do I know what is the proper way to free memory allocated by a custom allocator?

EDIT: After some experimentation with a verbatim allocator and a deleter I figured out that the allocator passed to the constructor of shared_ptr and to the factory function allocate_shared is used to allocate the internal structure of shared_ptr only. allocate_shared never uses the passed allocator to allocate the shared object. I think that the boost manual could have explained how the allocator is used more explicitly.

The allocator is intended to be used to allocate and deallocate internal shared_ptr details, not the object.

That is, while the deleter gives us full control over our shared object (because we control how it's acquired and released), the allocator parameter gives us control over the internal details of our object's shared nature.

If you look at N2351, at the end of the allocator proposal they note that Boost has implemented the feature, and link to an example that was made to demonstrate its use.

Here is that example, verbatim:

#include <boost/config.hpp>

//  shared_ptr_alloc2_test.cpp
//
//  Copyright (c) 2005 Peter Dimov
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)


#include <boost/detail/lightweight_test.hpp>
#include <boost/shared_ptr.hpp>
#include <memory>
#include <cstddef>

// test_allocator

struct test_allocator_base
{
    int id_;

    static int last_global_id_;
    static int count_;

    explicit test_allocator_base( int id ): id_( id )
    {
    }
};

int test_allocator_base::last_global_id_ = 0;
int test_allocator_base::count_ = 0;

template<class T> class test_allocator: public test_allocator_base
{
public:

    typedef T * pointer;
    typedef T const * const_pointer;
    typedef T & reference;
    typedef T const & const_reference;
    typedef T value_type;
    typedef std::size_t size_type;
    typedef std::ptrdiff_t difference_type;

private:

    static T * last_pointer_;
    static std::size_t last_n_;
    static int last_id_;

public:

    template<class U> struct rebind
    {
        typedef test_allocator<U> other;
    };

    pointer address( reference r ) const
    {
        return &r;
    }

    const_pointer address( const_reference s ) const
    {
        return &s;
    }

    explicit test_allocator( int id = 0 ): test_allocator_base( id )
    {
    }

    template<class U> test_allocator( test_allocator<U> const & r ): test_allocator_base( r )
    {
    }

    template<class U> test_allocator & operator=( test_allocator<U> const & r )
    {
        test_allocator_base::operator=( r );
        return *this;
    }

    void deallocate( pointer p, size_type n )
    {
        BOOST_TEST( p == last_pointer_ );
        BOOST_TEST( n == last_n_ );
        BOOST_TEST( id_ == last_id_ );

        --count_;

        ::operator delete( p );
    }

    pointer allocate( size_type n, void const * )
    {
        T * p = static_cast< T* >( ::operator new( n * sizeof( T ) ) );

        last_pointer_ = p;
        last_n_ = n;
        last_id_ = id_;

        last_global_id_ = id_;
        ++count_;

        return p;
    }

    void construct( pointer p, T const & t )
    {
        new( p ) T( t );
    }

    void destroy( pointer p )
    {
        p->~T();
    }

    size_type max_size() const
    {
        return size_type( -1 ) / sizeof( T );
    }
};

template<class T> T * test_allocator<T>::last_pointer_ = 0;
template<class T> std::size_t test_allocator<T>::last_n_ = 0;
template<class T> int test_allocator<T>::last_id_ = 0;

template<class T, class U> inline bool operator==( test_allocator<T> const & a1, test_allocator<U> const & a2 )
{
    return a1.id_ == a2.id_;
}

template<class T, class U> inline bool operator!=( test_allocator<T> const & a1, test_allocator<U> const & a2 )
{
    return a1.id_ != a2.id_;
}

template<> class test_allocator<void>: public test_allocator_base
{
public:

    typedef void * pointer;
    typedef void const * const_pointer;
    typedef void value_type;

    template<class U> struct rebind
    {
        typedef test_allocator<U> other;
    };

    explicit test_allocator( int id = 0 ): test_allocator_base( id )
    {
    }

    template<class U> test_allocator( test_allocator<U> const & r ): test_allocator_base( r )
    {
    }

    template<class U> test_allocator & operator=( test_allocator<U> const & r )
    {
        test_allocator_base::operator=( r );
        return *this;
    }
};

//

struct X
{
    static int instances;

    X()
    {
        ++instances;
    }

    ~X()
    {
        --instances;
    }

private:

    X( X const & );
    X & operator=( X const & );
};

int X::instances = 0;

int main()
{
    BOOST_TEST( X::instances == 0 );

    boost::shared_ptr<void> pv( new X, boost::checked_deleter<X>(), std::allocator<X>() );

    BOOST_TEST( X::instances == 1 );

    pv.reset( new X, boost::checked_deleter<X>(), test_allocator<float>( 42 ) );

    BOOST_TEST( X::instances == 1 );

    BOOST_TEST( test_allocator_base::last_global_id_ == 42 );
    BOOST_TEST( test_allocator_base::count_ > 0 );

    pv.reset();

    BOOST_TEST( X::instances == 0 );
    BOOST_TEST( test_allocator_base::count_ == 0 );

    pv.reset( new X, boost::checked_deleter<X>(), test_allocator<void>( 43 ) );

    BOOST_TEST( X::instances == 1 );
    BOOST_TEST( test_allocator_base::last_global_id_ == 43 );

    pv.reset( new X, boost::checked_deleter<X>(), std::allocator<void>() );

    BOOST_TEST( X::instances == 1 );

    pv.reset();

    BOOST_TEST( X::instances == 0 );

    return boost::report_errors();
}