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From: "J.Barfurth" <techview@bfk-net.de>
Subject: Re: auto_ptr copy semantics
Date: 1999/04/15
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sirwillard@my-dejanews.com schrieb in Nachricht
<7f0i8a$sc9$1@nnrp1.dejanews.com>...
>The long and short of it is that you should really never pass an auto_pt=
r
into
>a function (well, never is awfully strong, but if you read on you'll see
that
>it's highly unlikely that you want to ever do this).  Let's look at both
ways
>of passing an auto_ptr, by reference and by value.
>
>First, by reference:
[snip]
Agreed, can almost never be correct

>As for the other way, we've already discussed the problem.  Ownership is
>passed, and thus we're left with a "dangling auto_ptr" in the calling
>function.
>So, we're left to conclude that we should only ever pass real pointers:

We are also left with a (potentially) dangling pointer, if the specificat=
ion
of a function [say g(T*)] states, that the call transfers ownership. In t=
his
case, if the caller uses an auto_ptr<T> for exception safety, it would ha=
ve
to call it as g(ptr.release()). That's easy to forget. Using an auto_ptr =
as
the type of the parameter works properly in both cases: when either a bar=
e
pointer or an auto_ptr is used as the argument.

>Disclaimer:  There are rare circumstances when you want to pass the
>ownership, and in such circumstances it may be valid to pass an auto_ptr=
 by
>value. However, some may contend that this is bad practice, as it leads =
to
>code that's harder to understand and maintain.


IME it isn't that rare to pass ownership, but this of course is a matter =
of
(design) style. But when ownership is taken by a function, I always make
that explicit in the function's signature, by using some kind of smart
pointer as parameter type (auto_ptr being the least smart of these).
Admittedly this does make it harder to understand client code - you have =
to
look up the callee's signature to see that such a transfer is taking plac=
e.
But ownership semantics must be well documented anyways, and (as someone
else recently mentioned on this group) good naming can make up for a lot =
of
sins.

-- J=F6rg Barfurth
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