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

15 May 2013

      2013/5/15 Bryan Catanzaro :
...
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(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(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(c);` works well.

I'll test it tomorrow on Visual Studio 11 once more and investigate
issue futher.

--
Best regards,
Antony Polukhin