Mathias Gaunard-2 wrote:
On 03/05/2011 21:44, lcaminiti wrote:
Is there a way to do this or something similar with Boost.Phoenix (or Boost.Lambda)?
std::vector<double> v(3); std::for_each(v.begin(), v.end(), ref(sum) += factor * _1);
std::vector<int> w(3); std::for_each(w.begin(), w.end(), ref(sum) += factor * _1);
or if you want to avoid repetition,
auto local_add = ref(sum) += factor * _1;
std::vector<double> v(3); std::for_each(v.begin(), v.end(), local_add);
std::vector<int> w(3); std::for_each(w.begin(), w.end(), local_add);
Note you need auto, since using type erasure would force the function object to be monomorphic.
I think I can do this also without auto so it compiles on ISO C++. The following works (even if the type expression that replaces auto is very ugly -- perfect example of why C++0x's auto is useful): #include <boost/spirit/include/phoenix.hpp> #include <iostream> #include <vector> int main() { using boost::phoenix::arg_names::_1; boost::phoenix::actor<boost::phoenix::composite<boost::phoenix::shift_left_eval, boost::fusion::vector<boost::phoenix::composite<boost::phoenix::shift_left_eval, boost::fusion::vector<boost::phoenix::reference<std::basic_ostream<char, std::char_traits<char> > >, boost::phoenix::argument<0>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> >, boost::phoenix::value<const char*>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_> > > f = std::cout << _1 << "\n"; std::vector<double> v(3); v[0] = -1.0; v[1] = -2.0; v[2] = -3.0; std::for_each(v.begin(), v.end(), f); std::vector<std::string> s(3); s[0] = "aa"; s[1] = "bb"; s[2] = "cc"; std::for_each(s.begin(), s.end(), f); return 0; } This does what I was looking for: 1) Compiles on ISO C++. 2) Declares f locally (information hiding). 3) Does not repeat f declaration multiple times (avoid code duplication). 4) f is polymorphic in its argument type _1 (it accepts both doubles and std::strings). I am thinking that I will add the monomorphic (Boost.Local, Local Functors, and C++0x lambdas*) vs. polymorphic (Boost.Phoenix, Boost.Local, and Global Functors) comparison to the Alternatives section. (*) BTW, as I understand it, C++0x lambdas are monomorphic in their argument types -- correct? Thanks. --Lorenzo -- View this message in context: http://boost.2283326.n4.nabble.com/local-Help-for-the-Alternatives-section-t... Sent from the Boost - Dev mailing list archive at Nabble.com.