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From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-04-16 15:06:53

Ion Gaztañaga wrote:

> For objects that have no such default-state, and have no resource
> reassignment functions (like reset() or similar) there is no problem
> because there is no interface to reuse the object.

I think I was with you until this point. It seems to me that the exact
opposite is true; objects that have no "empty" or "default" state cannot
implement a move, at least not easily, whereas reusing a moved-from object
via assignment or reset is no problem at all.

FWIW, I've changed the shared_ptr move constructors and assignment operators
to leave the source empty, but this doesn't mean that I'm convinced that the
general case needs to be constrained in such a way. In particular,

    T& operator=( T && r )
        swap( r );
        return *this;

seems a very reasonable implementation of move assignment, and has a slight
performance advantage (one call to ~T instead of two in a typical use case)

    T& operator=( T && r )
        T( move( r ) ).swap( *this );
        return *this;

which is what is needed for r to be left empty.

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