[Boost-users] shared_ptr / auto_ptr

Subject: [Boost-users] shared_ptr / auto_ptr
From: Chard (boost_at_[hidden])
Date: 2010-02-23 11:11:13

We've recently been evaluating a move from boost 1.38 to 1.42 [MSVC71], and 
we had some code that breaks down to the following:

[1] struct T {};
[2] std::auto_ptr<T> ap;
[3] boost::shared_ptr<T> sp = ap;

Line [3] stopped compiling because of the 'explicit' introduced (in 
shared_ptr.hpp) at line [L] (below).

>From smart_ptr/shared_ptr.hpp:

[A]    template<class Y>
[B]    explicit shared_ptr(std::auto_ptr<Y> & r): px(r.get()), pn()
[C]    {
[D]        Y * tmp = r.get();
[E]        pn = boost::detail::shared_count(r);
[F]        boost::detail::sp_enable_shared_from_this( this, tmp, tmp );
[G]    }
[H]
[I]#if !defined( BOOST_NO_SFINAE ) && !defined( 
BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION )
[J]
[K]    template<class Ap>
[L]    explicit shared_ptr( Ap r, typename 
boost::detail::sp_enable_if_auto_ptr<Ap, int>::type = 0 ): px( r.get() ), 
pn()
[M]    {
[N]        typename Ap::element_type * tmp = r.get();
[O]        pn = boost::detail::shared_count( r );
[P]        boost::detail::sp_enable_shared_from_this( this, tmp, tmp );
[Q]    }

The fix is trivial, but what I am curious about is:

- the 'explicit' appears to have been removed at version 1.35 (i.e. it was 
there at 1.34) and then re-introduced at 1.39.  Is there some history to 
this?

and

- it looks like our code was falling into the second overload with 1.38 (the 
first overload was always explicit); with a fix it will now select the first 
overload, so for what case does the second overload now exist?

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