Boost Optional with Boost Thread compilation issue

Question

My environment is Visual Stuido 2013, VC12, Boost 1.59. The following code (a minimal repro of the real code):

#include "boost/thread.hpp"
#include "boost/optional.hpp"

class MyClass
{
public:

    template <typename T>
    operator const T& () const;

};

boost::optional<MyClass> foo()
{
    boost::optional<MyClass> res;
    return res;
}

int main(int argc)
{
    foo();
}

Doesn't compile, the error:

    1>------ Build started: Project: TestBoostOptional, Configuration: Debug x64 ------
    1>  main.cpp
    1>c:\workspace\third_party\boost_1_59_0\boost/optional/optional.hpp(297): error C2664: 'void boost::optional_detail::optional_base::construct(MyClass &&)' : cannot convert argument 1 from 'boost::detail::thread_move_t' to 'const MyClass &'
    1>          with
    1>          [
    1>              T=MyClass
    1>          ]
    1>          Reason: cannot convert from 'boost::detail::thread_move_t' to 'const MyClass'
    1>          with
    1>          [
    1>              T=MyClass
    1>          ]
    1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
    1>          c:\workspace\third_party\boost_1_59_0\boost/optional/optional.hpp(292) : while compiling class template member function 'boost::optional_detail::optional_base::optional_base(boost::optional_detail::optional_base &&)'
    1>          with
    1>          [
    1>              T=MyClass
    1>          ]
    1>          c:\workspace\third_party\boost_1_59_0\boost/optional/optional.hpp(873) : see reference to function template instantiation 'boost::optional_detail::optional_base::optional_base(boost::optional_detail::optional_base &&)' being compiled
    1>          with
    1>          [
    1>              T=MyClass
    1>          ]
    1>          c:\workspace\third_party\boost_1_59_0\boost/optional/optional.hpp(766) : see reference to class template instantiation 'boost::optional_detail::optional_base' being compiled
    1>          with
    1>          [
    1>              T=MyClass
    1>          ]
    1>          main.cpp(14) : see reference to class template instantiation 'boost::optional' being compiled

Note the #include "boost/thread.hpp". When removing this include the code compiles. Anything that can be done to workaround?

Answer 1

You must define BOOST_THREAD_USES_MOVE before you use any boost header.

#define BOOST_THREAD_USES_MOVE

More information are located here. This define emulates a move by Boost.Move which is necessary here.

In order to implement Movable classes, move parameters and return types Boost.Thread uses the rvalue reference when the compiler support it. On compilers not supporting it Boost.Thread uses either the emulation provided by Boost.Move or the emulation provided by the previous versions of Boost.Thread depending whether BOOST_THREAD_USES_MOVE is defined or not. This macros is unset by default when BOOST_THREAD_VERSION is 2. Since BOOST_THREAD_VERSION 3, BOOST_THREAD_USES_MOVE is defined.

Also see Boost.Move:

Boost.Thread uses by default an internal move semantic implementation. Since version 3.0.0 you can use the move emulation emulation provided by Boost.Move.

When BOOST_THREAD_VERSION==2 define BOOST_THREAD_USES_MOVE if you want to use Boost.Move interface. When BOOST_THREAD_VERSION==3 define BOOST_THREAD_DONT_USE_MOVE if you don't want to use Boost.Move interface.