Re: [patch] std::unique_ptr improvements

[Lawrence Crowl]

On 12/28/12, Jonathan Wakely <jwakely....@gmail.com> wrote:
> On 28 December 2012 01:51, Lawrence Crowl wrote:
>> I'm not getting errors when converting from derived to base.
>> E.g. the following compiles, when it should not.
>>
>> std::unique_ptr<const base []> acb_ad(new derived[3]);
>
> I get an error:
>
> shm$ cat up.cc
> #include <memory>
> struct base { };
> struct derived : base { virtual ~derived() = default; };
> std::unique_ptr<const base []> acb_ad(new derived[3]);
> shm$
> shm$ g++11 up.cc -c
> up.cc:4:53: error: use of deleted function ‘std::unique_ptr<_Tp [],
> _Dp>::unique_ptr(_Up*) [with _Up = derived; <template-parameter-2-2> =
> void; _Tp = const base; _Dp = std::default_delete<const base []>]’
>  std::unique_ptr<const base []> acb_ad(new derived[3]);
>                                                      ^
> In file included from /home/redi/gcc/4.x/include/c++/4.8.0/memory:81:0,
>                  from up.cc:1:
> /home/redi/gcc/4.x/include/c++/4.8.0/bits/unique_ptr.h:343:2: error:
> declared here
>   unique_ptr(_Up* __p) = delete;
>   ^

That was pilot error on my part.  However, I've been having trouble
when the argument to the constructor or reset has a conversion
operator.  The code does distinquish between a safe conversion to
base and an unsafe conversion to derived.

Here is a simplified version of the problem.  The code as is fails
to reject the last two calls to accept.  The primary reason is that
is_convertable permits both the invocation of the conversion operator
and the derived to base conversion.  At present, I see no way to
get a handle on the 'natural' return type of the conversion operator.

#include <type_traits>

struct base { };
struct derived : base { };

template <typename T, typename F>
typename std::enable_if<
                std::is_convertible< F, T* >::value,
                T*
        >::type
accept(F arg) { return arg; }

template <typename T, typename F>
typename std::enable_if<
                !std::is_convertible< F(*)[], T(*)[] >::value,
                T*
        >::type
accept(F* arg) = delete;

struct cvt_b {
  operator base*() { return 0; }
};

struct cvt_d {
  operator derived*() { return 0; }
};

int main()
{
  // should PASS
  accept< base >( (base*)0 );
  accept< const base >( (base*)0 );
  accept< base >( cvt_b() );
  accept< const base >( cvt_b() );
  // should FAIL
  accept< base >( (derived*)0 );
  accept< const base >( (derived*)0 );
  accept< base >( cvt_d() );
  accept< const base >( cvt_d() );
}

-- 
Lawrence Crowl