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      ISO C++

      auto_ptr

   Explaining all of the fun and delicious things that can

   happen with misuse of the auto_ptr class

   template (called AP here) would take some

   time. Suffice it to say that the use of AP

   safely in the presence of copying has some subtleties.

     The AP class is a really

      nifty idea for a smart pointer, but it is one of the dumbest of

      all the smart pointers -- and that's fine.

     AP is not meant to be a supersmart solution to all resource

      leaks everywhere.  Neither is it meant to be an effective form

      of garbage collection (although it can help, a little bit).

      And it can notbe used for arrays!

     AP is meant to prevent nasty leaks in the

     presence of exceptions.  That's all.  This

     code is AP-friendly:

    // Not a recommend naming scheme, but good for web-based FAQs.

    typedef std::auto_ptr<MyClass>  APMC;

    extern function_taking_MyClass_pointer (MyClass*);

    extern some_throwable_function ();

    void func (int data)

    {

        APMC  ap (new MyClass(data));

        some_throwable_function();   // this will throw an exception

        function_taking_MyClass_pointer (ap.get());

    }

   When an exception gets thrown, the instance of MyClass that's

      been created on the heap will be delete'd as the stack is

      unwound past func().

   Changing that code as follows is not AP-friendly:

        APMC  ap (new MyClass[22]);

   You will get the same problems as you would without the use

      of AP:

        char*  array = new char[10];       // array new...

        ...

        delete array;                      // ...but single-object delete

     AP cannot tell whether the pointer you've passed at creation points

      to one or many things.  If it points to many things, you are about

      to die.  AP is trivial to write, however, so you could write your

      own auto_array_ptr for that situation (in fact, this has

      been done many times; check the mailing lists, Usenet, Boost, etc).

  All of the containers

      described in the standard library require their contained types

      to have, among other things, a copy constructor like this:

    struct My_Type

    {

        My_Type (My_Type const&);

    };

     Note the const keyword; the object being copied shouldn't change.

     The template class auto_ptr (called AP here) does not

     meet this requirement.  Creating a new AP by copying an existing

     one transfers ownership of the pointed-to object, which means that

     the AP being copied must change, which in turn means that the

     copy ctors of AP do not take const objects.

     The resulting rule is simple: Never ever use a

     container of auto_ptr objects. The standard says that

     undefined behavior is the result, but it is

     guaranteed to be messy.

     To prevent you from doing this to yourself, the

      concept checks built

      in to this implementation will issue an error if you try to

      compile code like this:

    #include <vector>

    #include <memory>

    void f()

    {

        std::vector< std::auto_ptr<int> >   vec_ap_int;

    }

Should you try this with the checks enabled, you will see an error.