Re: [Boost-users] boost::lambda::bind question

From: Jaakko Järvi (jarvi_at_[hidden])
Date: 2005-07-06 04:51:24

On Jul 5, 2005, at 9:06 PM, Vishnevetsky, Eugene ((IT)) wrote:

> I came across this problem in the real project where I don't control  
> the
> class that corresponds to class Y of the example, so I can't change the
> signature of the member function. I could write a wrapper function that
> takes Y*, but after some point the code with a simple 'for' loop  
> becomes
> cleaner and more readable than the one using lambda.
> If this problem is not a bug, but an inherent limitation of the lambda
> library or its current implementation, it would be nice if it is
> mentioned in the documentation.
>
It's a bug, but requires quite a bit of changes to fix. LL/Phx fusion  
will (it will happen
eventually :) resolve it.

   Best, Jaakko

> Thanks,
> Eugene Vishnevetsky
>
>
>       Roman Krylov wrote:
> 	
> 	Perhaps the only solution here is to use Y* instead of Y& with
> corresponding modifications; this way there is no need for var() or  
> smth
> similar. Why not?
>
> 	Vishnevetsky, Eugene (IT) wrote:
> 	>Is it possible to use boost::ref() or boost::lambda::var() of
> an
> 	>abstract class as a parameter to boost::lambda::bind()?
> 	>It seems that compiler attempts to instantiate an object of
> that
> 	>abstract class.
> 	>Example:
> 	>
> 	>-----------------
> 	>using namespace boost;
> 	>using namespace boost::lambda;
> 	>class Y
> 	>{
> 	>public:
> 	> virtual void foo() = 0;
> 	> void foo2 (){ }
> 	>};
> 	>class Z : public Y
> 	>{
> 	>public:
> 	> void foo(){}
> 	>};
> 	>
> 	>class TT
> 	>{
> 	>public:
> 	> void test( Y& y ) { y.foo2(); }
> 	>};
> 	>
> 	>int main()
> 	>{
> 	> std::vector<TT> xt;
> 	> Z z;
> 	> Y &y = z;
> 	> std::for_each (xt.begin(), xt.end(), bind(&TT::test,_1,
> 	>ref(y)));
> 	> return 0;
> 	>}
> 	>----------------------
> 	>
> 	>And Error message from g++ (GCC) 3.2.3:
> 	>
> 	>---------------------------------
> 	>/boost/tuple/detail/tuple_basic.hpp: In instantiation of
> 	>`boost::tuples::cons<Y, boost::tuples::null_type>':
> 	>/boost/tuple/detail/tuple_basic.hpp:326: instantiated from
> 	>`boost::tuples::cons<boost::tuples::null_type,
> boost::tuples::cons<Y,
> 	>boost::tuples::null_type> >'
> 	>/boost/tuple/detail/tuple_basic.hpp:326: instantiated from
> 	>`boost::tuples::cons<void (TT::*const)(Y&),
> 	>boost::tuples::cons<boost::tuples::null_type,
> boost::tuples::cons<Y,
> 	>boost::tuples::null_type> > >'
> 	>/boost/lambda/detail/lambda_functor_base.hpp:151: instantiated
> from
> 	>`boost::lambda::detail::has_null_type<boost::tuples::cons<void
> 	>(TT::*const)(Y&), boost::tuples::cons<boost::tuples::null_type,
> 	>boost::tuples::cons<Y, boost::tuples::null_type> > > >'
> 	>/boost/lambda/detail/lambda_functor_base.hpp:229: instantiated
> from
> 	
>> `boost::lambda::detail::deduce_non_ref_argument_types<boost::tuples:: 
>> tu
> 	>p
> 	>le<void (TT::*const)(Y&), const
> 	>boost::lambda::lambda_functor<boost::lambda::placeholder<1> >,
> Y&,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type>, boost::tuples::null_type>'
> 	>/boost/lambda/detail/lambda_functor_base.hpp:412: instantiated
> from
> 	>`boost::lambda::lambda_functor_base<boost::lambda::action<3,
> 	>boost::lambda::function_action<3,
> boost::lambda::detail::unspecified>
> 	>>, boost::tuples::tuple<void (TT::*const)(Y&), const
> 	>boost::lambda::lambda_functor<boost::lambda::placeholder<1> >,
> Y&,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type> >::sig<boost::tuples::null_type>'
> 	>/boost/lambda/detail/lambda_functors.hpp:135: instantiated from
> 	
>> `boost::lambda::lambda_functor<boost::lambda:: 
>> lambda_functor_base<boost
> :
> 	>:lambda::action<3, boost::lambda::function_action<3,
> 	>boost::lambda::detail::unspecified> >,
> boost::tuples::tuple<void
> 	>(TT::*const)(Y&), const
> 	>boost::lambda::lambda_functor<boost::lambda::placeholder<1> >,
> Y&,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type, boost::tuples::null_type,
> 	>boost::tuples::null_type> > >'
> 	>lamtest.cpp:35: instantiated from here
> 	>/boost/tuple/detail/tuple_basic.hpp:320: cannot declare field
> 	>`boost::tuples::cons<Y, boost::tuples::null_type>::head'
> 	>to be
> 	>of type `Y'
> 	>/boost/tuple/detail/tuple_basic.hpp:320:
> 	>because the following virtual functions are abstract:
> 	>lamtest.cpp:12: virtual void Y::foo()
> 	>----------------------------------------
> 	>
> 	>Thanks,
> 	>
> 	>Eugene Vishnevetsky
> 	>--------------------------------------------------------
> 	>
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-- Jaakko Järvi, jarvi_at_[hidden]