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This code is not my actual code, but just illustrates the issue. I have a rule that matches 0 or more digits, and an action that is supposed to count them and return that count as the synthesized attribute.
The attribute of *qi::ascii::digit should be a std::vector of the matched digits. So, I wrote the action so that it simply calls .size() and assigns the result to the synthesized attribute. To call .size(), I use boost.phoenix to bind a lambda that calls .size().
#include <boost/phoenix/bind/bind_function_object.hpp>
#include <boost/phoenix/core.hpp>
#include <boost/phoenix/operator.hpp>
#include <boost/spirit/include/qi.hpp>
#include <cstdint>
#include <iostream>
#include <string>
namespace phx = boost::phoenix;
namespace qi = boost::spirit::qi;
using It = std::string::iterator;
struct Parser : qi::grammar<std::string::iterator, size_t()> {
Parser() : Parser::base_type(r) {
auto count = [](auto&& v) { return v.size(); };
r %= (*qi::ascii::digit)[qi::_val = phx::bind(count, qi::_1)];
}
qi::rule<It, size_t()> r;
};
int main() {
using namespace std;
std::string s = "123";
int c;
Parser p;
qi::parse(s.begin(), s.end(), p, c);
std::cout << c << '\n';
}
But it doesn't work. I expect this code to compile and print 3. Instead, I get a very long compile error. The error is roughly what I would expect if the action were simply qi::_val = qi::_1, which perplexes me.
In file included from /usr/local/include/boost/type_traits/is_convertible.hpp:20,
from /usr/local/include/boost/type_traits/is_empty.hpp:12,
from /usr/local/include/boost/mpl/empty_base.hpp:23,
from /usr/local/include/boost/fusion/sequence/intrinsic/at.hpp:15,
from /usr/local/include/boost/phoenix/core/expression.hpp:11,
from /usr/local/include/boost/phoenix/bind/bind_function_object.hpp:17,
from spirit2.cpp:1:
/usr/local/include/boost/spirit/home/qi/parse.hpp: In instantiation of ‘bool boost::spirit::qi::parse(Iterator&, Iterator, const Expr&, Attr&) [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Expr = int; Attr = Parser]’:
/usr/local/include/boost/spirit/home/qi/parse.hpp:100:25: required from ‘bool boost::spirit::qi::parse(const Iterator&, Iterator, const Expr&, Attr&) [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Expr = int; Attr = Parser]’
spirit2.cpp:27:39: required from here
/usr/local/include/boost/spirit/home/qi/parse.hpp:85:9: error: static assertion failed: error_invalid_expression
BOOST_SPIRIT_ASSERT_MATCH(qi::domain, Expr);
^~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/local/include/boost/spirit/home/qi/detail/parse_auto.hpp:14,
from /usr/local/include/boost/spirit/home/qi/auto.hpp:16,
from /usr/local/include/boost/spirit/home/qi.hpp:15,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from spirit2.cpp:4:
/usr/local/include/boost/spirit/home/qi/parse.hpp:88:42: error: request for member ‘parse’ in ‘boost::spirit::compile<boost::spirit::qi::domain, int>((* & expr))’, which is of non-class type ‘boost::spirit::result_of::compile<boost::spirit::qi::domain, boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<int>, 0>, boost::spirit::unused_type, void>::type’ {aka ‘int’}
return compile<qi::domain>(expr).parse(first, last, context, unused, attr);
~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
In file included from /usr/local/include/boost/spirit/home/qi/auxiliary/attr.hpp:18,
from /usr/local/include/boost/spirit/home/qi/auxiliary.hpp:19,
from /usr/local/include/boost/spirit/home/qi.hpp:16,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from spirit2.cpp:4:
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp: In instantiation of ‘static void boost::spirit::traits::assign_to_attribute_from_value<Attribute, T, Enable>::call(const T_&, Attribute&, mpl_::false_) [with T_ = std::vector<char, std::allocator<char> >; Attribute = long unsigned int; T = std::vector<char, std::allocator<char> >; Enable = void; mpl_::false_ = mpl_::bool_<false>]’:
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:171:17: required from ‘static void boost::spirit::traits::assign_to_attribute_from_value<Attribute, T, Enable>::call(const T&, Attribute&) [with Attribute = long unsigned int; T = std::vector<char, std::allocator<char> >; Enable = void]’
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:370:63: required from ‘void boost::spirit::traits::detail::assign_to(const T&, Attribute&, mpl_::false_) [with T = std::vector<char, std::allocator<char> >; Attribute = long unsigned int; mpl_::false_ = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:393:26: required from ‘void boost::spirit::traits::assign_to(const T&, Attribute&) [with T = std::vector<char, std::allocator<char> >; Attribute = long unsigned int]’
/usr/local/include/boost/spirit/home/qi/detail/attributes.hpp:27:30: required from ‘static void boost::spirit::qi::default_transform_attribute<Exposed, Transformed>::post(Exposed&, const Transformed&) [with Exposed = long unsigned int; Transformed = std::vector<char, std::allocator<char> >]’
/usr/local/include/boost/spirit/home/qi/action/action.hpp:71:36: required from ‘bool boost::spirit::qi::action<Subject, Action>::parse(Iterator&, const Iterator&, Context&, const Skipper&, Attribute&) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Context = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >; Skipper = boost::spirit::unused_type; Attribute = long unsigned int; Subject = boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >; Action = boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:73:54: [ skipping 3 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/local/include/boost/function/function_template.hpp:720:7: required from ‘boost::function4<R, T1, T2, T3, T4>::function4(Functor, typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1068:16: required from ‘boost::function<R(T0, T1, T2, T3)>::function(Functor, typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1121:5: required from ‘typename boost::enable_if_<(! boost::is_integral<Functor>::value), boost::function<R(T0, T1, T2, T3)>&>::type boost::function<R(T0, T1, T2, T3)>::operator=(Functor) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), boost::function<R(T0, T1, T2, T3)>&>::type = boost::function<bool(__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&, const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&, boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&, const boost::spirit::unused_type&)>&]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:185:19: required from ‘static void boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::define(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, const Expr&, mpl_::true_) [with Auto = mpl_::bool_<true>; Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript, boost::proto::argsns_::list2<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::dereference, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> >, 0>&>, 1>&, const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >&>, 2>; Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; T1 = long unsigned int(); T2 = boost::spirit::unused_type; T3 = boost::spirit::unused_type; T4 = boost::spirit::unused_type; mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:249:31: required from ‘boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>& boost::spirit::qi::operator%=(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, Expr&&) [with Expr = const boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript, boost::proto::argsns_::list2<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::dereference, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> >, 0>&>, 1>&, const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >&>, 2>; Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; T1 = long unsigned int(); T2 = boost::spirit::unused_type; T3 = boost::spirit::unused_type; T4 = boost::spirit::unused_type]’
spirit2.cpp:16:69: required from here
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:153:20: error: invalid static_cast from type ‘const std::vector<char, std::allocator<char> >’ to type ‘long unsigned int’
attr = static_cast<Attribute>(val);
^~~~~~~~~~~~~~~~~~~~~~~~~~~
The solution is to drop %:
r = (*qi::ascii::digit)[qi::_val = phx::bind(count, qi::_1)];
The important error message is in the last line: error: invalid static_cast from type ‘const std::vector<char, std::allocator<char> >’ to type ‘long unsigned int’
For why it produces vector<char> see here.
There is use of semantic action and:
r %= p and r = p are equivalent if there are no semantic actions
associated with p.
Read more about it here.
The doqtor did an excellent job of introducing the relevant concepts. I'd just like to show some simplification if you have a recent compiler (which it looks like you do):
phx::function count { [](auto&& v) { return v.size(); } };
r = (*digit)[_val = count(_1)];
Live On Coliru
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace phx = boost::phoenix;
namespace qi = boost::spirit::qi;
using It = std::string::const_iterator;
struct Parser : qi::grammar<It, size_t()> {
Parser() : Parser::base_type(r) {
using namespace qi;
phx::function count { [](auto&& v) { return v.size(); } };
r = (*digit)[_val = count(_1)];
}
private:
qi::rule<It, size_t()> r;
};
int main() {
std::string const s = "123";
int c;
Parser p;
if (qi::parse(s.begin(), s.end(), p, c)) {
std::cout << "Number of digits: " << c << "\n";
}
}
Prints
Number of digits: 3
boost::make_recursive_variant is intended to make variants which are recursive, without requiring the creation of an intermediate type by using the tag type boost::recursive_variant_. For std::vector<recursive_variant_> this appears to work fine, however for a std::tuple<int, recursive_variant_> it fails.
Example:
#include <type_traits>
#include <tuple>
#include <vector>
#include <boost/variant.hpp>
using std::cout;
using std::vector;
using std::tuple;
using boost::variant;
using boost::static_visitor;
using boost::apply_visitor;
using boost::make_recursive_variant;
using boost::recursive_variant_;
using A = typename make_recursive_variant<
int,
vector<recursive_variant_>
>::type;
using B = vector<A>;
using C = typename make_recursive_variant<
int,
tuple<int, recursive_variant_>
>::type;
using D = tuple<int, C>;
int main(int, char**)
{
A a = 0;
B b{1, 2};
a = b;
C c = 0;
D d{0, 1};
//c = d;
}
If the last line is uncommented, the program no longer compiles. Is this a usage error (and if yes, how so?), or is it bug or limitation in boost::tuple?
Error message (on mingw-w64 with gcc 4.8.1):
In file included from D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant.hpp:17:0,
from D:\projects\test\main.cpp:4:
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp: In instantiation of 'void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >; T0_ = boost::detail::variant::recursive_flag<int>; T1 = std::tuple<int, boost::recursive_variant_>; 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_; mpl_::false_ = mpl_::bool_<false>]':
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:1703:38: required from 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(const T&) [with T = std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >; T0_ = boost::detail::variant::recursive_flag<int>; T1 = std::tuple<int, boost::recursive_variant_>; 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_]'
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:2129:29: required from 'void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::assign(const T&) [with T = std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >; T0_ = boost::detail::variant::recursive_flag<int>; T1 = std::tuple<int, boost::recursive_variant_>; 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_]'
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:2168:19: required from 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>& boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::operator=(const T&) [with T = std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >; T0_ = boost::detail::variant::recursive_flag<int>; T1 = std::tuple<int, boost::recursive_variant_>; 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_]'
D:\projects\test\main.cpp:37:4: required from here
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:1557:17: error: no matching function for call to 'boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_>::initializer::initialize(void*, const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >&)'
)
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:1557:17: note: candidates are:
In file included from D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/variant.hpp:31:0,
from D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant.hpp:17,
from D:\projects\test\main.cpp:4:
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:104:24: note: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param_T) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node, mpl_::int_<1> >; Iterator = boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> >; boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param_T = const std::tuple<int, boost::recursive_variant_>&]
static int initialize(void* dest, param_T operand)
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:104:24: note: no known conversion for argument 2 from 'const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >' to 'boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node, mpl_::int_<1> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > >::initializer_node::param_T {aka const std::tuple<int, boost::recursive_variant_>&}'
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:115:24: note: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param2_T) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node, mpl_::int_<1> >; Iterator = boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> >; boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param2_T = std::tuple<int, boost::recursive_variant_>&&]
static int initialize(void* dest, param2_T operand)
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:115:24: note: no known conversion for argument 2 from 'const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >' to 'boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node, mpl_::int_<1> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > >::initializer_node::param2_T {aka std::tuple<int, boost::recursive_variant_>&&}'
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:149:17: note: static void boost::detail::variant::initializer_root::initialize()
static void initialize();
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:149:17: note: candidate expects 0 arguments, 2 provided
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:115:24: note: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param2_T) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >; Iterator = boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > >; boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param2_T = int&&]
static int initialize(void* dest, param2_T operand)
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:115:24: note: no known conversion for argument 2 from 'const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >' to 'boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node::param2_T {aka int&&}'
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:104:24: note: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param_T) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >; Iterator = boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > >; boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::param_T = const int&]
static int initialize(void* dest, param_T operand)
^
D:/libs/3rdparty-prebuild-vc11-win64/vc11/win64/boost-1_54_0-vc11-win64/include/boost-1_54/boost/variant/detail/initializer.hpp:104:24: note: no known conversion for argument 2 from 'const std::tuple<int, boost::variant<boost::detail::variant::recursive_flag<int>, std::tuple<int, boost::recursive_variant_>, 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_> >' to 'boost::detail::variant::make_initializer_node::apply<boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::l_item<mpl_::long_<2l>, int, boost::mpl::l_item<mpl_::long_<1l>, std::tuple<int, boost::recursive_variant_>, boost::mpl::l_end> > > >::initializer_node::param_T {aka const int&}'
The problem is that make_recursive_variant isn't aware of variadic templates, so it doesn't know to expand recursive_variant_ within std::tuple<..., recursive_variant_>.
You can test this by changing std::tuple to std::pair (a fixed-adic template) and observing that that works fine.
As a workaround, you can teach Boost.Variant how to expand within variadic templates with the following snippet (place at the top level of your source file):
namespace boost { namespace detail { namespace variant {
template<template<typename...> class F, typename... Ts, typename Dest, typename Source>
struct substitute<
F<Ts...>
, Dest
, Source
BOOST_MPL_AUX_LAMBDA_ARITY_PARAM(mpl::int_<-1>)
>
{
typedef F<typename substitute<
Ts, Dest, Source
>::type...> type;
};
}}} // namespace boost::detail::variant
Edit: I've put in a pull request: https://github.com/boostorg/variant/pull/2
This question already has an answer here:
Spirit Qi attribute propagation issue with single-member struct
(1 answer)
Closed 8 years ago.
#include <iostream>
#include <vector>
#include <string>
#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/io.hpp>
namespace qi = boost::spirit::qi;
struct VectorWrapper
{
std::vector<std::string> data;
};
BOOST_FUSION_ADAPT_STRUCT(
VectorWrapper,
(std::vector<std::string>, data)
)
int main ()
{
std::string input("aa aa aa");
std::string::iterator strbegin = input.begin();
qi::rule<std::string::iterator, VectorWrapper(), qi::space_type > testRule =
+(qi::string("aa"));
VectorWrapper result;
bool ok = qi::phrase_parse(
strbegin,
input.end(),
testRule,
qi::space,
result);
if (ok && strbegin == input.end()) {
std::cout << result.data.size();
for (int i = 0; i < result.data.size(); ++i) {
std::cout << result.data[i] << " ";
}
} else {
std::cout << "fail" << std::endl;
std::cout << std::string(strbegin, input.end()) << std::endl;
}
}
I need help to find out problem in this code. Compiler complains about type in VectorWrapper struct but shouldn't BOOST_FUSION_ADAPT_STRUCT add it?
After adding dummy field (from this answer)I am getting the same error:
struct VectorWrapper
{
std::vector<std::string> data;
qi::unused_type dummy;
};
BOOST_FUSION_ADAPT_STRUCT(
VectorWrapper,
(std::vector<std::string>, data)
(qi::unused_type, dummy)
)
The error message:
In file included from /usr/local/include/boost/spirit/home/support/string_traits.hpp:16:0,
from /usr/local/include/boost/spirit/home/support/nonterminal/expand_arg.hpp:20,
from /usr/local/include/boost/spirit/home/support/context.hpp:18,
from /usr/local/include/boost/spirit/home/qi/domain.hpp:18,
from /usr/local/include/boost/spirit/home/qi/meta_compiler.hpp:15,
from /usr/local/include/boost/spirit/home/qi/action/action.hpp:14,
from /usr/local/include/boost/spirit/home/qi/action.hpp:14,
from /usr/local/include/boost/spirit/home/qi.hpp:14,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from main.cpp:4:
/usr/local/include/boost/spirit/home/support/container.hpp: In instantiation of ‘boost::spirit::traits::container_value<VectorWrapper, void>’:
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp:296:66: instantiated from ‘bool boost::spirit::qi::detail::pass_container<F, Attr, Sequence>::dispatch_attribute(const Component&, mpl_::true_) const [with Component = boost::spirit::qi::literal_string<const char (&)[3], false>, F = boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, Attr = VectorWrapper, Sequence = mpl_::bool_<false>, mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp:331:61: instantiated from ‘bool boost::spirit::qi::detail::pass_container<F, Attr, Sequence>::operator()(const Component&) const [with Component = boost::spirit::qi::literal_string<const char (&)[3], false>, F = boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, Attr = VectorWrapper, Sequence = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/operator/plus.hpp:63:13: instantiated from ‘bool boost::spirit::qi::plus<Subject>::parse_container(F) const [with F = boost::spirit::qi::detail::pass_container<boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, VectorWrapper, mpl_::bool_<false> >, Subject = boost::spirit::qi::literal_string<const char (&)[3], false>]’
/usr/local/include/boost/spirit/home/qi/operator/plus.hpp:85:13: instantiated from ‘bool boost::spirit::qi::plus<Subject>::parse(Iterator&, const Iterator&, Context&, const Skipper&, Attribute&) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, Context = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, Skipper = boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, Attribute = VectorWrapper, Subject = boost::spirit::qi::literal_string<const char (&)[3], false>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:43:54: instantiated from ‘bool boost::spirit::qi::detail::parser_binder<Parser, Auto>::call(Iterator&, const Iterator&, Context&, const Skipper&, mpl_::false_) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, Skipper = boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, Context = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, Parser = boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, Auto = mpl_::bool_<false>, mpl_::false_ = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:53:67: [ skipping 3 instantiation contexts ]
/usr/local/include/boost/function/function_template.hpp:722:7: instantiated from ‘boost::function4<R, T1, T2, T3, T4>::function4(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1069:16: instantiated from ‘boost::function<R(T0, T1, T2, T3)>::function(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1124:5: instantiated from ‘typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, boost::function<R(T0, T1, T2, T3)>&>::type boost::function<R(T0, T1, T2, T3)>::operator=(Functor) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, boost::function<R(T0, T1, T2, T3)>&>::type = boost::function<bool(__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&)>&]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:182:13: instantiated from ‘static void boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::define(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, const Expr&, mpl_::true_) [with Auto = mpl_::bool_<false>, Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::unary_plus, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::char_code<boost::spirit::tag::string, boost::spirit::char_encoding::standard>, boost::fusion::vector1<const char (&)[3]> > >, 0l>&>, 1l>, Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, T1 = VectorWrapper(), T2 = boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l>, T3 = boost::spirit::unused_type, T4 = boost::spirit::unused_type, boost::spirit::qi::rule<Iterator, T1, T2, T3, T4> = boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, VectorWrapper(), boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l> >, mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:191:13: instantiated from ‘boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::rule(const Expr&, const string&) [with Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::unary_plus, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::char_code<boost::spirit::tag::string, boost::spirit::char_encoding::standard>, boost::fusion::vector1<const char (&)[3]> > >, 0l>&>, 1l>, Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, T1 = VectorWrapper(), T2 = boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l>, T3 = boost::spirit::unused_type, T4 = boost::spirit::unused_type, std::string = std::basic_string<char>]’
main.cpp:27:27: instantiated from here
/usr/local/include/boost/spirit/home/support/container.hpp:116:12: error: no type named ‘value_type’ in ‘struct VectorWrapper’
In file included from /usr/local/include/boost/spirit/home/qi/operator/kleene.hpp:20:0,
from /usr/local/include/boost/spirit/home/qi/directive/repeat.hpp:18,
from /usr/local/include/boost/spirit/home/qi/directive.hpp:23,
from /usr/local/include/boost/spirit/home/qi.hpp:19,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from main.cpp:4:
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp: In member function ‘bool boost::spirit::qi::detail::pass_container<F, Attr, Sequence>::dispatch_attribute(const Component&, mpl_::true_) const [with Component = boost::spirit::qi::literal_string<const char (&)[3], false>, F = boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, Attr = VectorWrapper, Sequence = mpl_::bool_<false>, mpl_::true_ = mpl_::bool_<true>]’:
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp:331:61: instantiated from ‘bool boost::spirit::qi::detail::pass_container<F, Attr, Sequence>::operator()(const Component&) const [with Component = boost::spirit::qi::literal_string<const char (&)[3], false>, F = boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, Attr = VectorWrapper, Sequence = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/operator/plus.hpp:63:13: instantiated from ‘bool boost::spirit::qi::plus<Subject>::parse_container(F) const [with F = boost::spirit::qi::detail::pass_container<boost::spirit::qi::detail::fail_function<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> > >, VectorWrapper, mpl_::bool_<false> >, Subject = boost::spirit::qi::literal_string<const char (&)[3], false>]’
/usr/local/include/boost/spirit/home/qi/operator/plus.hpp:85:13: instantiated from ‘bool boost::spirit::qi::plus<Subject>::parse(Iterator&, const Iterator&, Context&, const Skipper&, Attribute&) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, Context = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, Skipper = boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, Attribute = VectorWrapper, Subject = boost::spirit::qi::literal_string<const char (&)[3], false>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:43:54: instantiated from ‘bool boost::spirit::qi::detail::parser_binder<Parser, Auto>::call(Iterator&, const Iterator&, Context&, const Skipper&, mpl_::false_) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, Skipper = boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, Context = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, Parser = boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, Auto = mpl_::bool_<false>, mpl_::false_ = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:53:67: instantiated from ‘bool boost::spirit::qi::detail::parser_binder<Parser, Auto>::operator()(Iterator&, const Iterator&, Context&, const Skipper&) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, Skipper = boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, Context = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >, Parser = boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, Auto = mpl_::bool_<false>]’
/usr/local/include/boost/function/function_template.hpp:132:42: [ skipping 2 instantiation contexts ]
/usr/local/include/boost/function/function_template.hpp:722:7: instantiated from ‘boost::function4<R, T1, T2, T3, T4>::function4(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1069:16: instantiated from ‘boost::function<R(T0, T1, T2, T3)>::function(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1124:5: instantiated from ‘typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, boost::function<R(T0, T1, T2, T3)>&>::type boost::function<R(T0, T1, T2, T3)>::operator=(Functor) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::plus<boost::spirit::qi::literal_string<const char (&)[3], false> >, mpl_::bool_<false> >, R = bool, T0 = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T1 = const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, T2 = boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, T3 = const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, boost::function<R(T0, T1, T2, T3)>&>::type = boost::function<bool(__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, const __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >&, boost::spirit::context<boost::fusion::cons<VectorWrapper&, boost::fusion::nil>, boost::fusion::vector0<> >&, const boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >&)>&]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:182:13: instantiated from ‘static void boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::define(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, const Expr&, mpl_::true_) [with Auto = mpl_::bool_<false>, Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::unary_plus, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::char_code<boost::spirit::tag::string, boost::spirit::char_encoding::standard>, boost::fusion::vector1<const char (&)[3]> > >, 0l>&>, 1l>, Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, T1 = VectorWrapper(), T2 = boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l>, T3 = boost::spirit::unused_type, T4 = boost::spirit::unused_type, boost::spirit::qi::rule<Iterator, T1, T2, T3, T4> = boost::spirit::qi::rule<__gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, VectorWrapper(), boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l> >, mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:191:13: instantiated from ‘boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::rule(const Expr&, const string&) [with Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::unary_plus, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::char_code<boost::spirit::tag::string, boost::spirit::char_encoding::standard>, boost::fusion::vector1<const char (&)[3]> > >, 0l>&>, 1l>, Iterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, T1 = VectorWrapper(), T2 = boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::space, boost::spirit::char_encoding::standard> >, 0l>, T3 = boost::spirit::unused_type, T4 = boost::spirit::unused_type, std::string = std::basic_string<char>]’
main.cpp:27:27: instantiated from here
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp:296:66: error: no type named ‘type’ in ‘struct boost::spirit::traits::container_value<VectorWrapper, void>’
/usr/local/include/boost/spirit/home/qi/detail/pass_container.hpp:309:15: error: no type named ‘type’ in ‘struct boost::spirit::traits::container_value<VectorWrapper, void>’
make: *** [all] Error 1
You need to emit an unused type in your rule:
qi::rule<std::string::iterator, VectorWrapper(), qi::space_type > testRule =
+(qi::string("aa")) >> qi::attr(false);
The linked answer should be enough. Since you report trouble still: here's something that works on my compilers:
qi::rule<std::string::iterator, VectorWrapper(), qi::space_type > testRule =
qi::eps >> +(qi::string("aa"));
See it Live On Coliru
I have tried for hours to code a class deriving from boost::variant. But I do not understand what is the problem (I do not understand what the compilation error means).
What are the rules to implement a clean boost::variant derived-class?
#include <boost/variant.hpp>
class MyVariant : public boost::variant<char,bool>
{
public:
MyVariant () : boost::variant<char,bool>( ) {}
template <typename T>
MyVariant( T& v) : boost::variant<char,bool>(v) {}
template <typename T>
MyVariant(const T& v) : boost::variant<char,bool>(v) {}
};
int main ()
{
MyVariant a;
MyVariant b = a; //compilation error
// MyVariant c = MyVariant();
// MyVariant d (true);
// MyVariant e ('E');
}
Why do I want to use inheritance? (EDIT to give more details to #zaufi)
I want an empty state
I want to accept const char* as string
I want to accept int as long
I want to give enum types
For instance, in pseudo C++ code, my hopes:
class MyVariant : public boost::variant<char,bool,long,std::string>
{
typedef boost::variant<char,bool,long,std::string> super;
public:
// I know here I should specialize templeted constructors
// but I is more clear like that, isn't it?
MyVariant() : super('e') {} //empty -> char
MyVariant(char c) : super(std::string(1,c)){} //char -> string
MyVariant(const char* s) : super(std::string(s) ) {} //char* -> string
MyVariant(int v) : super(long (v) ) {} //TODO boundaries
/* other constructors ... */
enum Type
{
NONE, //my empty state = char type
BOOL,
LONG,
STRING
};
Type type() const { return (Type) which(); }
};
The basic snippet code (on top of the question) has been tested on different platforms
boost v1.33 + GCC 4.1 (Linux)
boost v1.52 + GCC 4.7 (MinGW)
boost v1.52 + Visual C++ 2010 (v10)
Below my errors for the two versions of GCC
(I can remove one of the both if it bothers someone...)
$ g++ --version
g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ g++ myVariant.cpp
/usr/include/boost/variant/variant.hpp: In constructor 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(T&) [with T = MyVariant, T0_ = char, T1 = bool, 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_]':
myVariant.cpp:10: instantiated from 'MyVariant::MyVariant(T&) [with T = MyVariant]'
myVariant.cpp:19: instantiated from here
/usr/include/boost/variant/variant.hpp:1348: error: call of overloaded 'convert_construct(MyVariant&, long int)' is ambiguous
/usr/include/boost/variant/variant.hpp:1262: note: candidates are: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = MyVariant, T0_ = char, T1 = bool, 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_]
/usr/include/boost/variant/variant.hpp:1321: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char, U1 = bool, U2 = boost::detail::variant::void_, U3 = boost::detail::variant::void_, U4 = boost::detail::variant::void_, U5 = boost::detail::variant::void_, U6 = boost::detail::variant::void_, U7 = boost::detail::variant::void_, U8 = boost::detail::variant::void_, U9 = boost::detail::variant::void_, U10 = boost::detail::variant::void_, U11 = boost::detail::variant::void_, U12 = boost::detail::variant::void_, U13 = boost::detail::variant::void_, U14 = boost::detail::variant::void_, U15 = boost::detail::variant::void_, U16 = boost::detail::variant::void_, U17 = boost::detail::variant::void_, U18 = boost::detail::variant::void_, U19 = boost::detail::variant::void_, T0_ = char, T1 = bool, 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_]
/usr/include/boost/variant/variant.hpp:1330: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char, U1 = bool, U2 = boost::detail::variant::void_, U3 = boost::detail::variant::void_, U4 = boost::detail::variant::void_, U5 = boost::detail::variant::void_, U6 = boost::detail::variant::void_, U7 = boost::detail::variant::void_, U8 = boost::detail::variant::void_, U9 = boost::detail::variant::void_, U10 = boost::detail::variant::void_, U11 = boost::detail::variant::void_, U12 = boost::detail::variant::void_, U13 = boost::detail::variant::void_, U14 = boost::detail::variant::void_, U15 = boost::detail::variant::void_, U16 = boost::detail::variant::void_, U17 = boost::detail::variant::void_, U18 = boost::detail::variant::void_, U19 = boost::detail::variant::void_, T0_ = char, T1 = bool, 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_]
$ g++ --version
g++.exe (GCC) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ g++ myVariant.cpp -I /c/.../include/
In file included from c:/.../include/boost/variant.hpp:17:0,
from myVariant.cpp:1:
c:/.../include/boost/variant/variant.hpp: In instantiation of 'boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::variant(T&) [with T = MyVariant; T0_ = char; T1 = bool; 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_]':
myVariant.cpp:9:66: required from 'MyVariant::MyVariant(T&) [with T = MyVariant]'
myVariant.cpp:18:25: required from here
c:/.../include/boost/variant/variant.hpp:1406:9: error: call of overloaded convert_construct(MyVariant&, long int)' is ambiguous
c:/.../include/boost/variant/variant.hpp:1406:9: note: candidates are:
c:/.../include/boost/variant/variant.hpp:1316:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(T&, int, mpl_::false_) [with T = MyVariant; T0_ = char; T1 = bool; 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_; mpl_::false_ = mpl_::bool_<false>]
c:/.../include/boost/variant/variant.hpp:1376:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char; U1 = bool; U2 = boost::detail::variant::void_; U3 = boost::detail::variant::void_; U4 = boost::detail::variant::void_; U5 = boost::detail::variant::void_; U6 = boost::detail::variant::void_; U7 = boost::detail::variant::void_; U8 = boost::detail::variant::void_; U9 = boost::detail::variant::void_; U10 = boost::detail::variant::void_; U11 = boost::detail::variant::void_; U12 = boost::detail::variant::void_; U13 = boost::detail::variant::void_; U14 = boost::detail::variant::void_; U15 = boost::detail::variant::void_; U16 = boost::detail::variant::void_; U17 = boost::detail::variant::void_; U18 = boost::detail::variant::void_; U19 = boost::detail::variant::void_; T0_ = char; T1 = bool; 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_]
c:/.../include/boost/variant/variant.hpp:1385:10: note: void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::convert_construct(const boost::variant<U0, U1, U2, U3, U4, U5, U6, U7, U8, U9, U10, U11, U12, U13, U14, U15, U16, U17, U18, U19>&, long int) [with U0 = char; U1 = bool; U2 = boost::detail::variant::void_; U3 = boost::detail::variant::void_; U4 = boost::detail::variant::void_; U5 = boost::detail::variant::void_; U6 = boost::detail::variant::void_; U7 = boost::detail::variant::void_; U8 = boost::detail::variant::void_; U9 = boost::detail::variant::void_; U10 = boost::detail::variant::void_; U11 = boost::detail::variant::void_; U12 = boost::detail::variant::void_; U13 = boost::detail::variant::void_; U14 = boost::detail::variant::void_; U15 = boost::detail::variant::void_; U16 = boost::detail::variant::void_; U17 = boost::detail::variant::void_; U18 = boost::detail::variant::void_; U19 = boost::detail::variant::void_; T0_ = char; T1 = bool; 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_]
The accepted answer and the other answers do not answer the question. There are good reasons to derive from boost::variant.
you want to add methods into the variant, so that it behaves more polymorphic
you want to keep using boost::apply_visitor(visitor(), variant) without jumping through hoops
Here is a working example of a type MyVariant inheriting from boost::variant. The code requires C++11 to inherit the (complex) constructors of boost::variant.
Edit: Operators such as equal comparison implemented in boost::variant do not automatically work for the derived class. This is a (unintended) consequence of how boost::variant is implemented, it explicitly prevents comparisons to "foreign" types using templates. Operators can be enabled with explicit overloads, as shown in the example.
#include<iostream>
#include<boost/variant.hpp>
struct type_size_visitor : public boost::static_visitor<unsigned> {
template <typename T>
unsigned operator()(const T&) const { return sizeof(T); }
};
template <typename... Types>
class MyVariant : public boost::variant<Types...>
{
using base_type = boost::variant<Types...>;
public:
// inherit constructors
using base_type::base_type;
// add a new method
unsigned type_size() { return boost::apply_visitor(type_size_visitor(), *this); }
/* Inheriting operators from boost::variant does not
work as it normally would. We have to add explicit
overloads for the operators we want to use.
*/
bool operator==(const MyVariant& rhs) const {
// cast rhs to base and delegate to operator in base
return base_type::operator==(static_cast<const base_type&>(rhs));
}
};
int main() {
MyVariant<std::string, char> v;
v = 1;
std::cout << v.type_size() << " " << sizeof(char) << std::endl;
v = std::string("foo");
std::cout << v.type_size() << " " << sizeof(std::string) << std::endl;
// comparison operators need workaround shown above
MyVariant<std::string, char> a, b;
a = 1; b = 1;
assert(a == b);
}
I want an empty state
boost::variant<boost::blank, bool, long, std::string>
There. That was much easier. No need for messy inheritance.
I want to give enum types
enum Type
{
NONE,
BOOL,
LONG,
STRING
};
struct GetType : public boost::static_visitor<Type>
{
Type operator()(boost::blank) {return NONE;}
Type operator()(bool) {return BOOL;}
Type operator()(long) {return LONG;}
Type operator()(const std::string&) {return STRING;}
};
//Get the type
Type t = boost::apply_visitor(GetType(), theData);
That was easy too. Plus, if you add a new type to the variant, your code will break if you don't update GetType to match.
The other two criteria require you to use a class, but you don't need inheritance. You need containment.
typedef boost::variant<boost::blank, std::string, long> VarType;
class MyVariant
{
public:
//Default construction will initialize with boost::blank.
MyVariant(char c) : m_var(std::string(1,c)) {}
MyVariant(const char* s) : m_var(std::string(s)) {}
MyVariant(int v) : m_var(long(v)) {}
MyVariant(long v) : m_var(long(v)) {}
VarType &operator *() {return m_var;}
const VarType &operator *() const {return m_var;}
private:
VarType m_var;
};
...
Type t = boost::apply_visitor(GetType(), *theData);
Finally my implementation based on Nicol Bolas's answer:
Variant.h
class Variant
{
public:
Variant() : v_(boost::blank()) {}
Variant(bool v) : v_(v) {}
Variant(long v) : v_(v) {}
Variant(int v) : v_(long(v)) {}
Variant(double v) : v_(v) {}
Variant(const std::string& s) : v_(s) {}
Variant(const char* s) : v_(std::string(s)) {}
typedef boost::variant <boost::blank, bool,
long, double, std::string> bstvar;
enum Type {
NONE, BOOL, // above underlying types
LONG, DOUBLE, STRING // and enum must be consistent:
}; // (order must be same)
operator bstvar& () { return v_; }
operator const bstvar& () const { return v_; }
bstvar & operator*() { return v_; }
bstvar const& operator*() const { return v_; }
bool empty () const { return type() == NONE; }
bool toBool () const { return boost::get<bool > (v_); }
long toLong () const { return boost::get<long > (v_); }
double toDouble () const { return boost::get<double> (v_); }
std::string const& toStr () const { return boost::get<std::string> (v_); }
std::string & toStr () { return boost::get<std::string> (v_); }
inline Type type () const { return (Type) v_.which(); }
static Type type (const std::string&);
static std::string type (Type t);
private: bstvar v_; // Data
};
Variant.cpp
#include "Variant.h"
#include <sstream>
#include <algorithm> //std::transform
Variant::Type Variant::type (const std::string& str)
{
std::string lower = str;
std::transform (lower.begin(), lower.end(), lower.begin(), ::tolower);
if (lower == "bool") return BOOL;
if (lower == "long") return LONG;
if (lower == "double") return DOUBLE;
if (lower == "string") return STRING;
else return NONE;
}
std::string Variant::type (Type t)
{
switch (t)
{
case NONE: return "none";
case BOOL: return "bool";
case LONG: return "long";
case DOUBLE: return "double";
case STRING: return "string";
default:
;//see below
}
std::ostringstream oss;
oss <<"Unexpected type="<< t;
return oss.str();
}
I want to inline visitation of variant types with lambdas. At the moment i have the following code:
struct Foo {
boost::variant< boost::blank , int , string , vector< int > > var;
template <typename T, typename IL , typename SL , typename VL>
void ApplyOptionals( T& ref, IL&& intOption , SL&& stringOption , VL&& vectorOption ) {
if (var.which() == 1) {
intOption( ref , boost::get< int >(var) );
} else if (var.which() ==2) {
stringOption( ref , boost::get< string>(var) );
} else if (var.which() == 3) {
vectorOption( ref , boost::get< vector< int > >(var) );
}
};
};
// ...
myFooV.ApplyOptionals(
obj,
[](Obj& o, int v) -> void { cout << "int: " << v << endl; o.value = v; },
[](Obj& o, string v) -> void{ cout << "string: " << v << endl; o.name = v; },
[](Obj& o, vector< int >& v) -> void{ v.push_back(257); cout << " vector.. has elements: " << v.size() << endl; o.ids = v; }
);
However the main drawback of this approach is that it depends on the order of variant type parameters and doesn't detect at compile-time unhandled types like boost::static_visitor would do
Can i get the best of both approaches?
Working on the excellent answer from RMartinho, i'm trying to work out this error, it seems that variant thinks the operator() calls are amibiguous (i'm using g++ 4.5.1, it's like it couldn't see the lambda operators.
looking at this question request for member `...' is ambiguous in g++, it seems that c++ doesn't like multiple inheritance as a way to provide multiple overloads (even if the calls are completely non-ambiguous because of different signature)
#include <iostream>
#include <string>
#include <vector>
#include <boost/variant.hpp>
using namespace std;
typedef boost::variant< boost::blank , int , string , vector< int > > var_t;
template <typename ReturnType, typename... Lambdas>
struct lambda_visitor : public boost::static_visitor<ReturnType>, public Lambdas... {
lambda_visitor(Lambdas... lambdas) : Lambdas(lambdas)... { }
};
template <typename ReturnType>
struct lambda_visitor<ReturnType> : public boost::static_visitor<ReturnType> {
lambda_visitor() {}
};
template <typename ReturnType, typename... Lambdas>
lambda_visitor<ReturnType, Lambdas...> make_lambda_visitor(Lambdas... lambdas) {
return { lambdas... };
// you can use the following instead if your compiler doesn't
// support list-initialization yet
// return lambda_visitor<ReturnType, Lambdas...>(lambdas...);
}
int main() {
vector< int > vit;
vit.push_back(7);
var_t myFooV = vit;
auto visitor = make_lambda_visitor<void>(
[](int v) -> void { cout << "int: " << v << endl; },
[](string& v) -> void{ cout << "string: " << v << endl; },
[](vector< int >& v) -> void{ v.push_back(27); boost::get< vector< int > >(myFooV).push_back(34); cout << " vector.. has elements: " << v.size() << endl; }
);
cout << " and for the grand finale.. " << endl;
boost::apply_visitor( visitor , myFooV );
};
This, gives me roughly a bunch of template boost errors, but the distinct part is:
boost_1_46_0/boost/variant/variant.hpp:832:32: error: request for member ‘operator()’ is ambiguous
test2.cpp:44:54: error: candidates are: main()::<lambda(std::vector<int>&)>
test2.cpp:43:47: error: main()::<lambda(std::string&)>
test2.cpp:42:55: error: main()::<lambda(int)>
boost_1_46_0/boost/variant/variant.hpp:832:32: error: return-statement with a value, in function returning 'void'
This is the whole error, just in case i'm missing some other relevant info:
boost_1_46_0/boost/variant/variant.hpp: In member function ‘boost::detail::variant::invoke_visitor<Visitor>::result_type boost::detail::variant::invoke_visitor<Visitor>::internal_visit(T&, int) [with T = std::vector<int>, Visitor = lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> >, boost::detail::variant::invoke_visitor<Visitor>::result_type = void]’:
boost_1_46_0/boost/variant/detail/visitation_impl.hpp:130:9: instantiated from ‘typename Visitor::result_type boost::detail::variant::visitation_impl_invoke_impl(int, Visitor&, VoidPtrCV, T*, mpl_::true_) [with Visitor = boost::detail::variant::invoke_visitor<lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> > >, VoidPtrCV = void*, T = std::vector<int>, typename Visitor::result_type = void, mpl_::true_ = mpl_::bool_<true>]’
boost_1_46_0/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<lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> > >, VoidPtrCV = void*, T = std::vector<int>, NoBackupFlag = boost::variant<boost::blank, int, std::basic_string<char>, std::vector<int> >::has_fallback_type_, typename Visitor::result_type = void]’
boost_1_46_0/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_<4l>, boost::blank, boost::mpl::l_item<mpl_::long_<3l>, int, boost::mpl::l_item<mpl_::long_<2l>, std::basic_string<char>, boost::mpl::l_item<mpl_::long_<1l>, std::vector<int>, boost::mpl::l_end> > > > >, boost::mpl::l_iter<boost::mpl::l_end> >, Visitor = boost::detail::variant::invoke_visitor<lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> > >, VoidPtrCV = void*, NoBackupFlag = boost::variant<boost::blank, int, std::basic_string<char>, std::vector<int> >::has_fallback_type_, typename Visitor::result_type = void, mpl_::false_ = mpl_::bool_<false>]’
boost_1_46_0/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<lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> > >, VoidPtrCV = void*, T0_ = boost::blank, T1 = int, T2 = std::basic_string<char>, T3 = std::vector<int>, 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]’
boost_1_46_0/boost/variant/variant.hpp:1787:13: 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&) [with Visitor = boost::detail::variant::invoke_visitor<lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> > >, T0_ = boost::blank, T1 = int, T2 = std::basic_string<char>, T3 = std::vector<int>, 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]’
boost_1_46_0/boost/variant/variant.hpp:1810:52: 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&) [with Visitor = lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> >, T0_ = boost::blank, T1 = int, T2 = std::basic_string<char>, T3 = std::vector<int>, 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]’
boost_1_46_0/boost/variant/detail/apply_visitor_unary.hpp:60:43: instantiated from ‘typename Visitor::result_type boost::apply_visitor(Visitor&, Visitable&) [with Visitor = lambda_visitor<void, main()::<lambda(int)>, main()::<lambda(std::string&)>, main()::<lambda(std::vector<int>&)> >, Visitable = boost::variant<boost::blank, int, std::basic_string<char>, std::vector<int> >, typename Visitor::result_type = void]’
test2.cpp:49:40: instantiated from here
boost_1_46_0/boost/variant/variant.hpp:832:32: error: request for member ‘operator()’ is ambiguous
test2.cpp:44:54: error: candidates are: main()::<lambda(std::vector<int>&)>
test2.cpp:43:47: error: main()::<lambda(std::string&)>
test2.cpp:42:55: error: main()::<lambda(int)>
boost_1_46_0/boost/variant/variant.hpp:832:32: error: return-statement with a value, in function returning 'void'
conclusion:
I want to add the final version of this utility, including tests:
lambda_visitor.h
#include <boost/variant.hpp>
namespace Variant {
template <typename ReturnType, typename... Lambdas>
struct lambda_visitor;
template <typename ReturnType, typename Lambda1, typename... Lambdas>
struct lambda_visitor< ReturnType, Lambda1, Lambdas...>
: public lambda_visitor<ReturnType, Lambdas...>, public Lambda1 {
using Lambda1::operator();
using lambda_visitor< ReturnType, Lambdas...>::operator();
typedef ReturnType ReturnType_t;
lambda_visitor(Lambda1 l1, Lambdas... lambdas) : Lambda1(l1), lambda_visitor< ReturnType, Lambdas...> (lambdas...) {
}
lambda_visitor(Lambda1 && l1, Lambdas && ... lambdas) : Lambda1(l1), lambda_visitor< ReturnType, Lambdas...> (lambdas...) {
}
};
template <typename ReturnType, typename Lambda1>
struct lambda_visitor<ReturnType, Lambda1>
: public boost::static_visitor<ReturnType>, public Lambda1 {
using Lambda1::operator();
typedef ReturnType ReturnType_t;
lambda_visitor(Lambda1 l1) : boost::static_visitor<ReturnType > (), Lambda1(l1) {
}
lambda_visitor(Lambda1 && l1) : boost::static_visitor<ReturnType > (), Lambda1(l1) {
}
};
template <typename ReturnType>
struct lambda_visitor<ReturnType> : public boost::static_visitor<ReturnType> {
typedef ReturnType ReturnType_t;
lambda_visitor() : boost::static_visitor<ReturnType > () {
}
};
template <typename ReturnType>
struct default_blank_visitor {
typedef ReturnType ReturnType_t;
inline ReturnType operator() (const boost::blank&) const {
return (ReturnType) 0;
};
};
template<>
struct default_blank_visitor<void> {
typedef void ReturnType_t;
inline void operator() (const boost::blank&) const {};
};
template <typename ReturnType, typename... Lambdas>
lambda_visitor<ReturnType, default_blank_visitor< ReturnType >, Lambdas...> make_lambda_visitor(Lambdas... lambdas) {
return
{
default_blank_visitor<ReturnType > (), lambdas...
};
// you can use the following instead if your compiler doesn't
// support list-initialization yet
//return lambda_visitor<ReturnType, default_blank_visitor<ReturnType> , Lambdas...>( default_blank_visitor<ReturnType>(), lambdas...);
};
/*
template <typename ReturnType, typename... Lambdas>
lambda_visitor<ReturnType, default_blank_visitor< ReturnType >, Lambdas...> make_lambda_visitor(Lambdas && ... lambdas) {
return
{
default_blank_visitor<ReturnType > (), lambdas...
};
// you can use the following instead if your compiler doesn't
// support list-initialization yet
//return lambda_visitor<ReturnType, default_blank_visitor<ReturnType> , Lambdas...>( default_blank_visitor<ReturnType>(), lambdas...);
};*/
template <typename ReturnType, typename... Lambdas>
lambda_visitor<ReturnType, Lambdas...> make_lambda_visitor_override_blank(Lambdas... lambdas) {
return
{
lambdas...
};
// you can use the following instead if your compiler doesn't
// support list-initialization yet
//return lambda_visitor<ReturnType, Lambdas...>(lambdas...);
}
namespace basic_usage
{
struct Test
{
typedef boost::variant< boost::blank , int , double > variant_t;
void run()
{
variant_t a, b, c;
a = 42;
b = 3.14159265;
auto visitor = Variant::make_lambda_visitor<int>( [](int v) -> int { return v+1; } , [](double v) -> int { return (int)v*2; } );
int result = boost::apply_visitor(visitor, a);
HAssertMsg( result == (42 + 1) , "unexpected");
result = boost::apply_visitor( visitor , b);
HAssertMsg( result == 6 , "unexpected");
auto blankVisitor = Variant::make_lambda_visitor_override_blank<int>(
[](int v) -> int { return -1; }
, [](double v) -> int { return -1; }
, [](boost::blank ) -> int { return 0;} );
result = boost::apply_visitor( blankVisitor , c);
HAssertMsg( result == 0 , "unexpected");
//same as previous case, but using lambda coalescing :-)
auto blankVisitor2 = Variant::make_lambda_visitor_override_blank<int>(
[](boost::variant< int , double >& v) -> int { return -1; }
, [](boost::blank ) -> int { return 0;} );
result = boost::apply_visitor( blankVisitor2 , c);
HAssertMsg( result == 0 , "unexpected");
result = boost::apply_visitor( blankVisitor2 , a);
HAssertMsg( result == -1 , "unexpected");
result = boost::apply_visitor( blankVisitor2 , b);
HAssertMsg( result == -1 , "unexpected");
}
};
}
};
You could use variadic templates to take the lambdas, and build a variant visitor using inheritance. That would retain the compile time checks.
template <typename ReturnType, typename... Lambdas>
struct lambda_visitor : public static_visitor<ReturnType>, public Lambdas... {
lambda_visitor(Lambdas... lambdas) : Lambdas(lambdas)... {}
};
And a little helper function to use argument type deduction (required for lambdas):
template <typename ReturnType, typename... Lambdas>
lambda_visitor<ReturnType, Lambdas...> make_lambda_visitor(Lambdas... lambdas) {
return { lambdas... };
// you can use the following instead if your compiler doesn't
// support list-initialization yet
// return lambda_visitor<ReturnType, Lambdas...>(lambdas...);
}
Now you can make visitors like this:
auto visitor = make_lambda_visitor<int>([](int) { return 42; },
[](std::string) { return 17; },
[](std::vector<int>) { return 23; });
Note: due to a detail of the overload resolution process that I wasn't aware of, this elegant solution causes weird ambiguity errors :(
See the follow-up question for the fix.