Ion Gaztañaga schrieb:
klaus triendl wrote:
So, my question is whether this is well-known and this feature was deliberately excluded from Boost.Move or nobody has discovered it yet. It would be great to have support for copy- and (implicit) move-semantics.
I didn't know about this feature. Could you elaborate a bit? And it would be even better if you could modify the library to support this feature ;-)
Well, it's actually very simple: add not only a user-defined conversion to rv<T>& but to const rv<T>& as well and have a ctor/assignment operator handling the const rv<T>&. Compilers will choose then a conversion operator based on the const-qualification of an object. This technique is working on msvc9. I let a short code example speak: <code> template<typename T> class rv: public T {}; class copyable_and_movable { typedef copyable_and_movable type; public: // non-const lvalues bind here copyable_and_movable(copyable_and_movable& other) {} // const lvalues and const rvalues bind here copyable_and_movable(const rv<type>& other) {} // non-const rvalues bind here - temporaries! copyable_and_movable(rv<type>& other) {} // ... assignment operators ditto operator rv<type>&() { return static_cast<rv<type>& >(*this); } operator const rv<type>&() const { return static_cast<const rv<type>& >(*this); } }; </code> I'm not quite sure yet how Boost.Move could support this - the caveat lies in having essentially two copy constructors. A possible idea is: Extend the BOOST_ENABLE_MOVE_EMULATION macro to include the const conversion operator. Provide a macro for the const-copy ctor: #if !defined(BOOST_HAS_RVALUE_REFS) # define BOOST_CONST_RV_REF(TYPE) const boost::rv< TYPE >& #else # define BOOST_CONST_RV_REF(TYPE) const TYPE& #endif When writing copyable_and_movable guard the non-const-copy ctor with the BOOST_HAS_RVALUE_REFS macro. In code: <code> class copyable_and_movable { typedef copyable_and_movable type; public: #if !defined(BOOST_HAS_RVALUE_REFS) // non-const lvalues bind here copyable_and_movable(copyable_and_movable& other) {} #endif // const lvalues and const rvalues bind here copyable_and_movable(BOOST_CONST_RV_REF(type) other) {} // non-const rvalues bind here - temporaries! copyable_and_movable(BOOST_RV_REF(type) other) {} // ... assignment operators ditto BOOST_ENABLE_MOVE_EMULATION(type) }; </code> Attached you find a sample project just showing my tests without using Boost.Move. Best, Klaus