It's just that closure-types don't have nested typedefs that boost::mpl
apparently requires. It works if you convert the lambda expression to std::function
:
#include <iostream>
#include <iterator>
#include <vector>
#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>
template<class R>
auto delta_beg(R const& rng1, R const& rng2)
{
using Elem = typename R::value_type;
std::function<Elem(Elem const&)> f =
[first2 = begin(rng2)](Elem const& e) { return e - *first2; };
return rng1 | boost::adaptors::transformed(f);
}
template<class R>
auto delta_rng(R const& rng1, R const& rng2)
{
using Elem = typename R::value_type;
std::function<Elem(Elem const&)> f =
[first2 = begin(rng2)](Elem const& e) mutable { return e - *first2++; };
return rng1 | boost::adaptors::transformed(f);
}
int main()
{
auto r1 = std::vector<int>{ 8, 10, 12, 15 };
auto r2 = std::vector<int>{ 1, 2, 9, 13 };
// prints 7, 9, 11, 14
boost::copy(delta_beg(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
// ERROR, should print 7, 8, 3, 2
boost::copy(delta_rng(r1, r2), std::ostream_iterator<int>(std::cout, ",")); std::cout << "\n";
}