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Internet Message Format | 1996-08-06 | 3.3 KB

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From: [e-mail] (Lars Farm)
Newsgroups: comp.std.c++
Subject: Re: sample auto_ptr template
Date: 12 Apr 1996 08:53:43 PDT
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Approved: [e-mail]
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Originator: [e-mail]

In article <[e-mail]>,
[e-mail] (J. Kanze) wrote:

>I believe that John Skaller had a proposal with something like six
>different pointer types.  The committee found this a little too

He said a while ago that he proposed two classes(*). (1) auto_ptr for
lifetime control within a block (no copy/assign) and (2) a ref counted
class for shared objects. There is a need for both but only one was
accepted and that is the real problem.

auto_ptr<> is slowly being mutated into a mixture of the two classes. It
has moved away from its original goal and is becoming a cheap shared_ptr<>.
This requires very strange and disturbing use of op=/copy.

void f( auto_ptr<X> x ) { ...will delete x }

  // promise: x const, x will _not_ change, x will _not_ become invalid
void g( const auto_ptr<X>&x ) {
  // ... x ok
  f(x);
  // ... oops
}
void h() {
   auto_ptr<X> p( new X );
   // p valid
   g(x);
   // p bad. Undetected by the compiler because the standard
   // library class auto_ptr subverted the C++ type system
}

IMHO the only reasonable solution is to have two classes. If the committee
only accepts one then users will have to write the other. That's ok, as
long as it is obvious that auto_ptr isn't a general purpose smart_ptr. With
the current weird auto_ptr copy/op= perhaps users will have to write both
the original and the ref counted class on their own. Or worse, they will
actually use auto_ptr as if it was the rejected ref_counted ptr. This is a
reasonable assumption because auto_ptr is the only standard smart ptr and
users should be allowed to assume it is safe. It would be safe with private
op=/copy as the original.

When designing a class for mass consumption it should not only support the
intended idioms. It should also prevent missuse in common situations. The
original does that. The current auto_ptr doesn't.

Please reconsider John Max Skallers original proposal.

(*) he has also mentioned other smart pointers here. holding arrays -
block/ref-counted. copying pointers and cloning pointers using varying
implementation techniques but AFAIK they were never proposed as standard
classes.

--
Lars Farm, [e-mail]
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