Re: [Boost-users] [move] Move c-tor template

Subject: Re: [Boost-users] [move] Move c-tor template
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-03-30 11:52:02

On Fri, Mar 30, 2012 at 8:11 AM, Szymon Gatner <szymon.gatner_at_[hidden]>wrote:

> Hi,
>
> I am implementing custom smart pointer with move semantics and I am
> having problems with implementing move constructor with template
> param. In fact clone_ptr presented in docs suffers from that issue
> also. This code:
>
> class B {}
> class D: public B {}
>
> clone_ptr<D> makeD();
>
> void main()
> {
> clone_ptr<B> b = makeD()  // gives "error C2440: 'initializing' :
> cannot convert from 'clone_ptr<T>' to 'clone_ptr<T>'"
> }
>
>
> adding
>
>  template <class U>
>  clone_ptr(BOOST_RV_REF(clone_ptr<U>) p)            //Move constructor
>    : ptr(p.ptr) { p.ptr = 0; }
>
> to clone_ptr class does not help. Additional c-tor seems to be
> ignored. How to fix that?
>

Unfortunately, you can't catch rvalues of a move-emulation-enabled type in
a deduced context as you've tried to above (in C++03). If you want some
ideas on how to get something like this to work, search the Boost developer
list under the messages subject "[boost] [move] new rvalue reference
emulation".

The easiest solution, though, is simply to split the construction into two
steps:

clone_ptr<D> d = makeD();
clone_ptr<B> b = boost::move(d);

(This still requires the additional constructor you've given above!) I
*think*, due to RVO, there shouldn't be any overhead with the above pair of
statements compared with rolling everything into a single statement.

If this doesn't fulfill your needs, I suggest reading through the thread
given above in the Boost developer list.

HTH,

- Jeff