Re: [Boost-users] boost::variant & c++11

Subject: Re: [Boost-users] boost::variant & c++11
From: Bryan Catanzaro (bcatanzaro_at_[hidden])
Date: 2013-05-15 17:13:15

Thanks for taking a look.

I checked, and on my system, you're right: adding a non-const conversion
operator allows the code to compile with c++11 and boost 1.53.0. Although
I'm not sure why adding a non-const conversion operator would be expected
to ensure the implicit conversion happens. I would expect the non-const
conversion operator couldn't be called on a temporary object (maybe things
are different when we have rvalue references?)

In any case, the "convertible" in this reproducer is standing in for a
class from Boost.Python, so I can't just add the extra non-const conversion
operator to work around the problem.

- bryan

On Wed, May 15, 2013 at 1:54 PM, Antony Polukhin <antoshkka_at_[hidden]>wrote:

> 2013/5/15 Bryan Catanzaro <bcatanzaro_at_[hidden]>:
> > Hi Antony -
> > Perhaps it is compiler dependent. Which version of MSVC did you reproduce
> > the error with? Does that version have c++11 support enabled (in
> particular,
> > rvalue references)
> > As I said, with Boost 1.48.0, this code works with g++ 4.6 and 4.7. With
> > 1.53.0, it works with g++ 4.6 and 4.7 only if c++11 support is turned
> off.
> > If c++11 support is turned on, the code does not work.
> > I've attached a shell script that compiles the attached code with the
> cross
> > product of {(g++4.6, g++4.7), (boost1.48, boost1.53), (noc++11,
> yesc++11)}.
> > As you can see from test.txt, the error happens only with boost 1.53 and
> > c++11 support.
> >
> >
> > - bryan
>
> Exactly the same things happen on GCC-4.5 and Clang-3.0.
>
> But what is more interesting here:
>     convertible c;
>     const convertible& cc = c;
>
>     // Always compiles, using any flags, compiler and Boost version
>     my_variant y = cc;
>     my_variant y1 = static_cast<const convertible&>(c);
>     my_variant y2 = static_cast<const convertible>(c);
>
>     // Always fail to compile, using any flags and Boost version
> //    my_variant z = c;
> //    my_variant y3 = static_cast<convertible&>(c);
>
> Tested on clang, gcc4.5, gcc 4.6, gcc4.7 on Boost 1.48 and Boost 1.53
> with and without C++11 flags.
>
> If we modify `convertible`:
>
> struct convertible {
>     operator my_variant() {
>         return my_variant();
>     }
>
>     operator my_variant() const {
>         return my_variant();
>     }
> };
>
> Then ALL the tests pass well, even the commented out ones. Even
> `my_variant y4 = static_cast<convertible&&>(c);` works well.
>
> I'll test it tomorrow on Visual Studio 11 once more and investigate
> issue futher.
>
> --
> Best regards,
> Antony Polukhin
>