CODEWITHSUNDEEP
c++boostlambdaboost-mpl
Marcin
Marcin

Reputation: 917

Passing a mpl lambda expression as a template argument

I'm trying to write a metafunction similar to boost::mpl::find_if but with the difference that it will traverse the sequence starting from the end. I am getting compilation errors which I guess come from computation of mpl::lambda passed as an argument of my metafunction. I would be very grateful on any pointers on what I am doing wrong.

Right now I'm trying a lazy solution (decorating the original find_if):

#include <boost/mpl/reverse.hpp>
#include <boost/mpl/find_if.hpp>
#include <boost/mpl/distance.hpp>
#include <boost/mpl/begin_end.hpp>
#include <boost/mpl/advance.hpp>
#include <boost/mpl/next_prior.hpp>
#include <boost/mpl/lambda.hpp>


using boost::mpl::reverse;
using boost::mpl::find_if;
using boost::mpl::distance;
using boost::mpl::end;
using boost::mpl::advance;
using boost::mpl::prior;
using boost::mpl::lambda;

template<typename SEQ, typename pred>
struct rfind_if {
private:
  // find the element in the reversed container    
  typedef typename reverse<SEQ>::type   rev_SEQ;
  typedef typename lambda<pred>::type   expanded_pred;    
  typedef typename find_if<rev_SEQ, expanded_pred>::type   rev_iter;
  // compute the distance of the iterator
  typedef typename distance<rev_iter, typename end<rev_SEQ>::type >::type  dist;
public:
  //compute the iterator
  typedef typename advance<typename begin<SEQ>::type, typename prior<dist>::type>::type   type;
};

The problem is that When trying to use this function:

typedef vector_c<int, 1, 2, 3, 6, 5, 4>::type  test_vect;
typedef find<test_vect, int_<6>::type>::type  it_cur;
typedef rfind_if<test_vect, lambda<less<deref<it_cur>::type, _1> >::type >::type  it_swap;
std::cout << "it_swap=" << deref<it_swap>::type::value << "\n\n";

I get cryptic errors which, I guess, come from lambda computations:

 /usr/include/boost/mpl/aux_/preprocessed/gcc/less.hpp:60: error: no type named ‘tag’ in ‘struct mpl_::void_’ (some more template noise)
 /usr/include/boost/mpl/not.hpp:43: error: ‘value’ is not a member of ‘boost::mpl::aux::nested_type_wknd<boost::mpl::aux::iter_apply1 (some more template noise)
 /usr/include/boost/mpl/aux_/preprocessed/gcc/iter_fold_if_impl.hpp:62: error: no type named ‘type’ in ‘struct boost::mpl::apply2<boost::mpl::protect<boost::mpl::aux::iter_fold_if_pred (some more template noise)
 ...and much more...

I've tested the internals of rfind_if (without passing the lambda as a template argument) and it worked, naming:

typedef vector_c<int, 1, 2, 3, 6, 5, 4>::type               test_vect;
typedef boost::mpl::reverse<test_vect>::type                rev_SEQ;
typedef find_if<rev_SEQ, less<int_<5>, _1> >::type          rev_iter;
typedef distance<rev_iter, end<rev_SEQ>::type >::type       dist;
typedef advance<begin<test_vect>::type, prior<dist>::type>::type    it_begin;

boost::mpl::for_each< rev_SEQ >( value_printer() );

produced correct results

I know that my function is far from efficient, but right now I want to understand the problem. I will write a proper implementation afterwards.

Best Regards

Upvotes: 0

Views: 785

Answers (1)

Ise Wisteria
Ise Wisteria

Reputation: 11669

As far as I see, rfind_if isn't the cause of the error, but the code in the question seems to dereference the end of test_vect.

1) The type of the elements in vector_c<int> seems integral_c<int>, not int_. So find<test_vect, int_<6>::type>::type is the end of test_vect. Therefore dereferencing the it_cur in deref<it_cur>::type is invalid.

2) If you mean less<int_<6>, _1> by less<deref<it_cur>::type, _1>, since test_vect doesn't have such an element, rfind_if<...>::type is again the end of test_vect. So dereferencing it in deref<it_swap>::type::value is invalid.

After fixing the above issues, the code could be compiled on ideone.

Upvotes: 1

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