Re: [boost] Movable but not copyable bug?

25 Aug 2014

      El 25/08/2014 18:09, Mostafa escribió:
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On Mon, 25 Aug 2014 03:23:59 -0700, Ion Gaztañaga 
wrote:
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El 25/08/2014 2:29, Mostafa escribió:
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I had posted this on the user's list but I think this should be brought
to the attention of the developers. The following code contains a
templated copy constructible Boost.Move enabled class (Bar) with a
single non-copy constructible Boost.Enabled member (Foo). If Bar's copy
constructor is not explicitly defined then the code compiles with both
g++ 4.8.2 and VS2005, else if it's explicitly defined then it fails to
compile on g++ 4.8.2 I think this is due to the fact that g++ 4.8.2
erroneously instantiates Bar's default copy constructor in the latter
case. Is this a Boost.Move bug or a gcc bug?
The copy constructor shall not be defined.
???
If you declare a class as BOOST_MOVABLE_BUT_NOT_COPYABLE(Foo) you must 
not declare the assignment operator. Follow the steps explained here:

http://www.boost.org/doc/libs/1_56_0/doc/html/move/movable_only_classes.html
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Here's the updated code with error messages. Note, that on g++ 4.8.2 if
(1) is commented out then it compiles fine but if uncommented then the
error messages that follow the code are produced. In the latter case,
the error messages are due to (2). Note also that (1) is the copy
constructor for the movable *and* copyable type Bar which has a single
member of type Foo that is movable but *not* copyable. Also note that
Bar is a template class.
If Foo is not copyable, then you have a logic error in your code:

     Bar(Bar const & rhs)
     : f(rhs.f) <---- Tries to COPY Foo
     {}

If this copy constructor is instantiated, and in many compilers

   Bar<Foo> b(( Bar<Foo>() ));

requires the copy constructor, then you get a compilation error. Just 
try this equivalent (but more inefficient) code:

   Bar<Foo> a;
   Bar<Foo> b(a);

Then the copy constructor of Bar<Foo> is called and tries to COPY Foo. 
As Foo is noncopyable, a compilation error is triggered.

Maybe you should design your code in C++11 and port it to Boost.Move, 
you might get better compiler help.

Best,

Ion