Re: Custom destructors -- memory management

"James Kanze" <>
3 Apr 2007 07:50:12 -0700
On Apr 3, 3:07 pm, Dave Rahardja
<> wrote:

On 2 Apr 2007 23:36:11 -0700, "Erik Wikstr=F6m" <eri...@student.chalmers.=



On 3 Apr, 07:45, Dave Rahardja
<> wrote:

I need to design a container that must hold a set of
references to a variety of object types, managed by
different memory managers. At some time, the container must
destroy each object. Currently I have the following design
in mind:

Maybe I missed some of the requirements but what's wrong with just
using the normal destructor? If the container uses any of the standard
containers then the destructor will be called automatically. If you
are using pointers then the destructor will be called on delete.

Have you perhaps been using some garbagecollected language like Java
or C# where you don't know when an object is destroyed? Well, in C++
you do, so all you have to worry about is to make sure that the object
is destroyed (going out of scope or using delete on a pointer) and
that the destructor cleans up the stuff that needs to be cleaned up.

I'm sorry if I wasn't clear--it wasn't the destructor call
that I was worried about. It is that each of the elements in
this container may be allocated by a _different_ memory
manager. Thus, using global delete may not be appropriate for
all elements.

How do you know which memory manager is responsible for a given
object? If it is purely a function of the type, then using a
common base class, and providing an operator delete member
function in the class should do the trick; you just use delete,
as normal, and the compiler takes care of the rest. Failing
that, you need some means of tracking which memory manager is
responsible for which bit of memory. The most frequent solution
for this is to use some hidden memory in front of the allocated
block. You then replace the global new and delete functions to
set this memory, and to use it when they are called. Say
something along the lines of:

    union BlockHeader
        MemoryManager* owner ;
        double dummyForAlignment ; // may need something more
                                    // for some processors...
    } ;

    void* operator new( size_t n )
        BlockHeader* result =
                static_cast< BlockHeader * >(
                    ::malloc( n + sizeof( BlockHeader ) ) ) ;
        if ( result == NULL ) {
            throw std::bad_alloc() ;
        result->owner = NULL ;
        return result + 1 ;

    void* operator new( size_t n, MemoryManager* from )
        BlockHeader* result =
                static_cast< BlockHeader * >(
                    from->alloc( n + sizeof( BlockHeader ) ) ) ;
        if ( result == NULL ) {
            throw std::bad_alloc() ;
        result->owner = from ;
        return result + 1 ;

    void operator delete( void* p )
        BlockHeader* block
                = static_cast< BlockHeader* >( p ) - 1 ;
        if ( block->owner == NULL ) {
            ::free( block ) ;
        } else {
            block->owner->free( block ) ;
    } ;

Again, this takes care of everything automatically. You
allocate with new or new(memmgr), and you delete as usual.

James Kanze (GABI Software)
Conseils en informatique orient=E9e objet/
                   Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

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