Re: [boost] Re: problem with shared_ptr and member templates

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-06-24 04:32:41

From: "Markus Schöpflin" <markus.schoepflin_at_[hidden]>
> Peter Dimov wrote:
> >
> > From: "Markus Schöpflin" <markus.schoepflin_at_[hidden]>
> > > Hello all,
> > >
> > > I have a problem with MSVC6 and shared_ptr. The compiler
> > > is hopelessly confused by the template constructor:
> > >
> > > template<typename Y> explicit shared_ptr(Y * p)
> > >
> > > As a result, it fails to compile:
> > >
> > > boost::shared_ptr<int> p(0);
> >
> > This is as it should be. Either use the default constructor, or cast the
> > zero to the correct pointer type.
>
> Ok, thanks for the clarification. Is there anywhere some design
> rationale besides the FAQ where I can look up what should work
> and what not and why it works the way it does?

First let me clarify that this is not a problem with MSVC 6. 'Y *' can not
be deduced from '0'.

No, there is no rationale document yet. Sorry.

The reason that the zero initialization doesn't work (1.28+) is that the two
constructors are subtly different; ...

> For me this behaviour is kind of surprising as auto_ptr does
> have such a constructor.

... in the auto_ptr/scoped_ptr case, the default constructor _is_
zero-initialization. shared_ptr's default constructor, however, does not
require a complete type, and does not invoke 'delete' on destruction. The
constructor that takes a raw pointer does require a complete type for the
pointed-to object, and does invoke 'delete p' on destruction, where 'p' is
the value passed to the constructor, complete with its original type.

So if you say

shared_ptr<void> px(static_cast<X*>(0));

when 'px' is destructed, it will invoke

delete static_cast<X*>(0);

I could have made the simple case

shared_ptr<int> px(0);

work, but it's better not to; it is important to understand the difference.
And of course,

shared_ptr<void> px(0);

won't work anyway, since void is incomplete.