Re: auto_ptr and copy ctr question

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 29 May 2008 09:24:20 -0400

Message-ID:

<g1maq5$lqe$1@news.datemas.de>

Soumen wrote:

If I've class with one of data members of auto_ptr type, how should I
write copy ctr or copy assignment operator if I don't intend to allow
transfer of ownership happen? Declaring that member const could be one
option I guess. Clarify me if this understanding is wrong. Do I need
to disable these (copy ctr and assignment operator) in such cases?
What happens when I don't write one and use the compiler generated
one? Does transfer of ownership still happens or it's same as
declaring member as const? Looks like disabling these two are best
design decision.

If you don't provide the "move semantics" offered by the auto_ptr
itself, you could do two things. One, as you mentioned, is to prohibit
the copying altogether. That's a bit harsh to me, but if your design
can endure that, it's the simplest of the two. The other is to provide
copying of the owned object by copy-creating a new one in case of the
constructor and copy-assigning the pointed-to item in case of the
assignment:

     class Has_auto_ptr {
         std::auto_ptr<sometype> myPtr;
     public:
         ...
         Has_auto_ptr(Has_auto_ptr const& other) :
             myPtr(new sometype(*other.myPtr)) {}
         Has_auto_ptr& operator =(Has_auto_ptr const& other) {
             *myPtr = *other.myPtr;
             return *this;
         }
     };

Disclaimer: the code is unchecked, provided for illustration purposes only.

V
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