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

From: Vishnevetsky, Eugene \(IT\) (Eugene.Vishnevetsky_at_[hidden])
Date: 2005-07-05 21:06:27

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.

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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