Hi. The example "move.cpp" could not be compiled on MSVC 7.1. Reasons: 1. The obvious one X& operator=(X rhs) { std::cout << "ASSIGN #" << id << (owner?"":" (EMPTY)") << " <== #" << rhs.id << (rhs.owner?"":" (EMPTY)") << std::endl; owner = rhs.owner; rhs.owner = false; assert(owner); //MISSING RETURN VALUE; } 2. error C2664: 'sink' : cannot convert parameter 1 from 'const X' to 'X' Definitions: X const z7; void sink(X) Error location: std::cout << " ------ test 15, pass const lvalue by-value ------- " << std::endl; sink(z7); I think compiler is wrong here. It should call template <class T> X(T& rhs, typename enable_if_same<X const,T>::type = 0) constructor. Or may be I wrong, and missed something Roman
-----Original Message----- From: David Abrahams [mailto:dave@boost-consulting.com] Sent: Wednesday, April 14, 2004 5:05 PM To: boost@lists.boost.org Subject: [boost] Yo, move semantics anyone?
Hi,
I'm just wondering if the deafening silence indicates a lack of interest or whether something else is going on? Feedback would be appreciated.
Thanks!
David Abrahams <dave@boost-consulting.com> writes:
I have to stop playing with this stuff and get back to work now, but over the weekend I started a library that allows a "reasonably convenient, useful amount" of move semantics. Many compilers implement enough (N)RVO that implicit moving from rvalues is unneccessary, and those seem to coincide roughly with the compilers where we have to internally shut off some capability because of the compiler's interpretation of 8.5.3/5 (**). So it seems you can pretty much eliminate all needless copies.
Here's what you can do with the current state of the move library, which supplies macros (eeeeew!) to avoid code duplication:
struct X : boost::movable<X> { BOOST_LVALUE_COPY_CTOR( X, (rhs)(: initializers...), { // copy ctor body })
BOOST_LVALUE_ASSIGN( X, (rhs), { // copy assign body })
X(move_from<X> rhs) { // move ctor logic }
X& operator=(move_from<X> rhs) { // move assign logic } };
boost::movable<X> just encapsulates the conversion to move_from<X> and makes the copy ctor/assignment private, so you can always replace it with a written-out conversion to move_from<X> if you don't like the "MI non-EBO" (or other) impact of library-mandated inheritance.
This work owes inspiration to Rani Sharoni's "improved auto_ptr" (http://tinyurl.com/2rxpj) and of course one must acknowledge Andrei Alexandrescu for being crazy enough to even try doing this in a library in the first place (Mojo). Eric Friedman had an earlier boost/move.hpp in the sandbox based on Mojo (which I replaced with his permission), and I adapted his tests, in addition to my own, to work with the new code.
It might make sense to use this library to optimize selected Boost components; shared_ptr, any, and dynamic_bitset come to mind, just off the top of my head.
You can find the code in boost/move.hpp and tests in libs/move/test/*.cpp. It "basically works" with all MSVC versions, Borland, gcc-2.95, gcc-3.3.1, comeau (strict warnings mode only, until I find out how to detect that we're in strict errors mode), and all intel-win32 versions. I don't know how to make gcc-3.2 work yet. The places where it fails, certain things seem to compile which one would prefer to cause an error.
Enjoy, Dave
(**) see http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2004/n1610.html -- Dave Abrahams Boost Consulting www.boost-consulting.com
-- Dave Abrahams Boost Consulting http://www.boost-consulting.com _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost