local temp object warning

osx-clang-warning.txt

In file included from src/metawriter.cpp:25:
In file included from include/mapnik/metawriter_json.hpp:28:
In file included from include/mapnik/parse_path.hpp:28:
include/mapnik/attribute.hpp:42:16: warning: returning reference to local temporary object [-Wreturn-stack-address]
        return f.get(name_); 
               ^~~~~~~~~~~~
include/mapnik/parse_path.hpp:67:31: note: in instantiation of function template specialization 'mapnik::attribute::value<mapnik::value, mapnik::metawriter_property_map>' requested
      here
            filename_ += attr.value<mapnik::value,feature_type>(feature_).to_string();
                              ^
/opt/boost-48/include/boost/variant/variant.hpp:858:16: note: in instantiation of member function
      'mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_::operator()' requested here
        return visitor_(operand);
               ^
/opt/boost-48/include/boost/variant/detail/visitation_impl.hpp:143:24: note: in instantiation of function template specialization
      'boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>::internal_visit<const mapnik::attribute>' requested here
        return visitor.internal_visit(
                       ^
/opt/boost-48/include/boost/variant/detail/visitation_impl.hpp:170:12: note: in instantiation of function template specialization
      'boost::detail::variant::visitation_impl_invoke_impl<boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, const void
      *, mapnik::attribute>' requested here
    return (visitation_impl_invoke_impl)(
           ^
/opt/boost-48/include/boost/variant/detail/visitation_impl.hpp:258:11: note: in instantiation of function template specialization
      'boost::detail::variant::visitation_impl_invoke<boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, const void *,
      mapnik::attribute, boost::variant<std::basic_string<char>, mapnik::attribute, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_>::has_fallback_type_>' requested here
        , BOOST_VARIANT_AUX_APPLY_VISITOR_STEP_CASE
          ^
/opt/boost-48/include/boost/preprocessor/repetition/repeat.hpp:38:60: note: expanded from macro 'BOOST_PP_REPEAT_1'
# define BOOST_PP_REPEAT_1(c, m, d) BOOST_PP_REPEAT_1_I(c, m, d)
                                                           ^
/opt/boost-48/include/boost/preprocessor/repetition/repeat.hpp:43:63: note: expanded from macro 'BOOST_PP_REPEAT_1_I'
# define BOOST_PP_REPEAT_1_I(c, m, d) BOOST_PP_REPEAT_1_ ## c(m, d)
                                                              ^
/opt/boost-48/include/boost/preprocessor/repetition/repeat.hpp:71:58: note: expanded from macro 'BOOST_PP_REPEAT_1_20'
# define BOOST_PP_REPEAT_1_20(m, d) BOOST_PP_REPEAT_1_19(m, d) m(2, 19, d)
                                                         ^
note: (skipping 16 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
/opt/boost-48/include/boost/preprocessor/repetition/repeat.hpp:54:56: note: expanded from macro 'BOOST_PP_REPEAT_1_3'
# define BOOST_PP_REPEAT_1_3(m, d) BOOST_PP_REPEAT_1_2(m, d) m(2, 2, d)
                                                       ^
/opt/boost-48/include/boost/preprocessor/repetition/repeat.hpp:53:62: note: expanded from macro 'BOOST_PP_REPEAT_1_2'
# define BOOST_PP_REPEAT_1_2(m, d) BOOST_PP_REPEAT_1_1(m, d) m(2, 1, d)
                                                             ^
/opt/boost-48/include/boost/variant/detail/visitation_impl.hpp:249:16: note: expanded from macro 'BOOST_VARIANT_AUX_APPLY_VISITOR_STEP_CASE'
        return (visitation_impl_invoke)( \
               ^
/opt/boost-48/include/boost/variant/variant.hpp:1797:16: note: (skipping 1 context in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        return detail::variant::visitation_impl(
               ^
/opt/boost-48/include/boost/variant/variant.hpp:1822:16: note: in instantiation of function template specialization 'boost::variant<std::basic_string<char>, mapnik::attribute,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_>::internal_apply_visitor_impl<boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>,
      const void *>' requested here
        return internal_apply_visitor_impl(
               ^
/opt/boost-48/include/boost/variant/variant.hpp:1846:22: note: in instantiation of function template specialization 'boost::variant<std::basic_string<char>, mapnik::attribute,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_>::internal_apply_visitor<boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_> >'
      requested here
        return this->internal_apply_visitor(invoker);
                     ^
/opt/boost-48/include/boost/variant/detail/apply_visitor_unary.hpp:60:22: note: in instantiation of function template specialization 'boost::variant<std::basic_string<char>,
      mapnik::attribute, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_>::apply_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>' requested here
    return visitable.apply_visitor(visitor);
                     ^
include/mapnik/parse_path.hpp:118:13: note: in instantiation of function template specialization
      'boost::apply_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_, const boost::variant<std::basic_string<char>, mapnik::attribute,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_,
      boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_> >' requested here
            boost::apply_visitor(eval,token);
            ^
src/metawriter.cpp:332:58: note: in instantiation of member function 'mapnik::path_processor<mapnik::metawriter_property_map>::evaluate' requested here
    filename_ = path_processor<metawriter_property_map>::evaluate(*fn_, properties);
                                                         ^
1 warning generated.

ubuntu-g++-warning.txt

```
In file included from include/mapnik/parse_path.hpp:28:0,
                 from include/mapnik/metawriter_json.hpp:28,
                 from src/metawriter.cpp:25:
include/mapnik/attribute.hpp: In member function ‘const V& mapnik::attribute::value(const F&) const [with V = mapnik::value, F = mapnik::metawriter_property_map]’:
include/mapnik/parse_path.hpp:67:13:   instantiated from ‘void mapnik::path_processor<T>::path_visitor_::operator()(const mapnik::attribute&) const [with T = mapnik::metawriter_property_map]’
/usr/include/boost/variant/variant.hpp:832:32:   instantiated from ‘boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = const mapnik::attribute, Visitor = mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_, boost::detail::variant::invoke_visitor<Visitor>::result_type = void]’
/usr/include/boost/variant/detail/visitation_impl.hpp:145:13:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::false_) [with Visitor = boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, VoidPtrCV = const void*, T = mapnik::attribute, typename Visitor::result_type = void, mpl_::false_ = mpl_::bool_<false>]’
/usr/include/boost/variant/detail/visitation_impl.hpp:173:9:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke(int, Visitor&, VoidPtrCV, T*, NoBackupFlag, int) [with Visitor = boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, VoidPtrCV = const void*, T = mapnik::attribute, NoBackupFlag = boost::variant<std::basic_string<char>, mapnik::attribute>::has_fallback_type_, typename Visitor::result_type = void]’
/usr/include/boost/variant/detail/visitation_impl.hpp:260:1:   instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl(int, int, Visitor&, VoidPtrCV, mpl_::false_, NoBackupFlag, Which*, step0*) [with Which = mpl_::int_<0>, step0 = boost::detail::variant::visitation_impl_step<boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<1l>, mapnik::attribute, boost::mpl::l_end> > >, boost::mpl::l_iter<boost::mpl::l_end> >, Visitor = boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, VoidPtrCV = const void*, NoBackupFlag = boost::variant<std::basic_string<char>, mapnik::attribute>::has_fallback_type_, typename Visitor::result_type = void, mpl_::false_ = mpl_::bool_<false>]’
/usr/include/boost/variant/variant.hpp:1776:13:   instantiated from ‘static typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor_impl(int, int, Visitor&, VoidPtrCV) [with Visitor = boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, VoidPtrCV = const void*, T0_ = std::basic_string<char>, T1 = mapnik::attribute, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/include/boost/variant/variant.hpp:1798:13:   instantiated from ‘typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::internal_apply_visitor(Visitor&) const [with Visitor = boost::detail::variant::invoke_visitor<mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_>, T0_ = std::basic_string<char>, T1 = mapnik::attribute, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/include/boost/variant/variant.hpp:1820:52:   instantiated from ‘typename Visitor::result_type boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::apply_visitor(Visitor&) const [with Visitor = mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_, T0_ = std::basic_string<char>, T1 = mapnik::attribute, T2 = boost::detail::variant::void_, T3 = boost::detail::variant::void_, T4 = boost::detail::variant::void_, T5 = boost::detail::variant::void_, T6 = boost::detail::variant::void_, T7 = boost::detail::variant::void_, T8 = boost::detail::variant::void_, T9 = boost::detail::variant::void_, T10 = boost::detail::variant::void_, T11 = boost::detail::variant::void_, T12 = boost::detail::variant::void_, T13 = boost::detail::variant::void_, T14 = boost::detail::variant::void_, T15 = boost::detail::variant::void_, T16 = boost::detail::variant::void_, T17 = boost::detail::variant::void_, T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_, typename Visitor::result_type = void]’
/usr/include/boost/variant/detail/apply_visitor_unary.hpp:60:43:   instantiated from ‘typename Visitor::result_type boost::apply_visitor(Visitor&, Visitable&) [with Visitor = mapnik::path_processor<mapnik::metawriter_property_map>::path_visitor_, Visitable = const boost::variant<std::basic_string<char>, mapnik::attribute>, typename Visitor::result_type = void]’
include/mapnik/parse_path.hpp:118:13:   instantiated from ‘static std::string mapnik::path_processor<T>::evaluate(const path_expression&, const feature_type&) [with T = mapnik::metawriter_property_map, std::string = std::basic_string<char>, mapnik::path_expression = std::vector<boost::variant<std::basic_string<char>, mapnik::attribute> >, mapnik::path_processor<T>::feature_type = mapnik::metawriter_property_map]’
src/metawriter.cpp:332:58:   instantiated from here
include/mapnik/attribute.hpp:42:27: warning: returning reference to temporary [enabled by default]
```