Re: Distinguish between pointers created with 'new' and created with references.

Goran <>
Thu, 15 Jul 2010 17:59:53 CST
On Jul 15, 2:03 pm, dailos <> wrote:

Hi all,
I am struggling with a design problem. It showed up because of some
architectures constraints.
I have a group of classes which derive from an abstract one.
I have a vector with pointers to the generic abstract class (neither
vector of references nor vector of classes are allowed since it's
A member function that loads objects into the vector, lets say from
file, has to do it creating object with new operator since it couldn't
be done by references to objects created locally.
A public functions lets the user to add single objects too but it
could use new or reference as an argument.
The problem is when I need to call the destructor, and erase all of
the objects created within the vector.
I thought of using delete operator for each element in the vector, but
what if some other elements has been added from a reference. delete
&something ## failure.

I guess there is no way to distinguish between object address created
with new and created with a reference.
How a to write a proper destructor then??

This problem of your crops up here in various forms, but does it

There is no other good solution to your problem but to take care of
the lifetime of your objects yourself. This is because C++, just like
C, are languages where you manage heap manually. If you browse through
here, you might find some "solutions", but they are all non-standard,
wrong, of limited use, or a combination thereof.

In your main(), lifetime of theOrange is the block ({}) created by
main. You don't do anything about it, language runtime destroys it
when block is done. Lifetime of objects created in loadFruits is
whatever you decide, because these are on the heap, and you have to
manage heap manually. You have to delete them once you won't ever try
to use them again.

Now, as to what could work... You could declare addFruit like this:

void addFruit(auto_ptr<Fruit> p);

Given what auto_ptr does, it's clear that you can only addFruit
allocated on the heap. So you can take pointer out of "p" and put it
in Basket, and Basket is the owner of it's fruits (meaning, when
Basket is destroyed, all it's fruit is deleted).

That's not unreasonable lifetime handling strategy.

You could make it more complicated (warning: compiled with head-
debugger, debugged with head-debugger and tested with head-test-

class FruitHolder
  FruitHolder(Fruit& f) : _owns(false), _f(&f) {}
  FruitHolder(Fruit* f) : _owns(true), _f(f) {}
  FruitHolder() : _owns(false), _f(NULL) {} // standard containers
need this
  FruitFolder& operator=(FruitFolder& rhs)
    if (this == &rhs) return rhs;
    if (_owns) delete _f;
    _f = rhs._f;
    if (rhs._owns)
      _owns = true;
      rhs._owns = false;
  ~FruitHolder() { if (_owns) delete f; }

  operator Fruit*() { return _f; }
  operator const Fruit*() const { return _f; }

  bool _owns; // Perhaps "mutable" and operator=(const FruitFolder&
  Fruit* _f;
  FruitHolder(const FruitHolder&); // Not needed and potentially
vector<FruitHolder> fruitList;

void addFruit(auto_ptr<Fruit> p)
  FruitHolder fh(p.release());
void addFruit(Fruit& f)
  FriutHolder fh(f);

Idea here is that whatever comes in as a reference is not owned by
Basket, all else is owned by basket. Fruit Holder is similar to
auto_ptr in a sense that it "moves" Fruit pointers around on
assignment (provided that my op= up here works), but it doesn't
necessarily delete them (_owns flag).

I'd be surprised if there wasn't a template library that does the
above already (Loki, perhaps? Boost?).

All in all, there you have two ( perhaps working ;-) ) solutions. None
of them are generic, but I am adamant that a generic one doesn't


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