[boost] Yo, move semantics anyone - errors in move.cpp example

From: Roman Yakovenko (roman.yakovenko_at_[hidden])
Date: 2004-04-14 23:45:17

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_at_[hidden]]
> Sent: Wednesday, April 14, 2004 5:05 PM
> To: boost_at_[hidden]
> 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_at_[hidden]> 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
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