Re: [boost] [move] Declarations of the copy constructor and copy-assign operator produced by BOOST_MOVABLE_BUT_NOT_COPYABLE

Subject: Re: [boost] [move] Declarations of the copy constructor and copy-assign operator produced by BOOST_MOVABLE_BUT_NOT_COPYABLE
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-04-14 20:24:35

On Thu, Apr 14, 2011 at 9:23 AM, Joseph Trebbien <jtrebbien_at_[hidden]>wrote:

> Looking through the sources of Boost.Move in trunk, I am wondering
> about a couple of lines in <boost/move/move.hpp> that are part of the
> expansion of BOOST_MOVABLE_BUT_NOT_COPYABLE when
> BOOST_NO_RVALUE_REFERENCES is defined:
>
> #define BOOST_MOVABLE_BUT_NOT_COPYABLE(TYPE)\
>    private:\
>    TYPE(TYPE &);\
>    TYPE& operator=(TYPE &);\
>    public:\
>    operator ::boost::rv<TYPE>&() \
>    {  return *static_cast< ::boost::rv<TYPE>* >(this);  }\
>    operator const ::boost::rv<TYPE>&() const \
>    {  return *static_cast<const ::boost::rv<TYPE>* >(this);  }\
>    private:\
> //
>
> Why do the declarations of the copy constructor and copy-assign
> operator overload not take a const-reference to TYPE?  When
> BOOST_NO_RVALUE_REFERENCES is not defined, they do:
>
> #define BOOST_MOVABLE_BUT_NOT_COPYABLE(TYPE)\
>   public:\
>   typedef int boost_move_emulation_t;\
>   private:\
>   TYPE(const TYPE &);\
>   TYPE& operator=(const TYPE &);\
> //
>

In C++03 (when BOOST_NO_RVALUE_REFERENCES is defined), to prevent
TYPE-rvalues from binding to the copy constructor and copy assignment
operator, we need to declare the copy constructor's and copy assignment
operator's argument as reference-to-non-const (TYPE&).  This forces
TYPE-rvalues to bind to the user-provided move constructor and move
assignment operators (whose arguments are boost::rv< TYPE >&) via the
implicit conversion to boost::rv< TYPE >&.

- Jeff