Transitioning Boost Spirit parser from boost::variant to std::variant 2

Question

I am trying to have an x3 parser work with std::variant instead of boost::variant, without success.

I followed the post on this subject: transitioning boost spirit parser from boost::variant to std::variant but the required boiler plate to have it work does not seem to work in my case. The (simplified) error message states:

std::variant<...> has no member named apply_visitor

which I confess, but this boiler plates being so an UFO to me, I won't be able to fix it.

Here is a minimal test code to reproduce my issue, available on coliru:

#include <iostream>
#include <variant>
#define BOOST_SPIRIT_X3_DEBUG
#include "boost/spirit/home/x3.hpp"
#include "boost/spirit/include/support_istream_iterator.hpp"

// boiler plate provided in the original post
namespace boost::spirit::x3::traits {
    template<typename... T>
    struct is_variant<std::variant<T...> >
        : mpl::true_ {};

    template <typename Attribute, typename... T>
    struct variant_has_substitute_impl<std::variant<T...>, Attribute>
    {
        typedef std::variant<T...> variant_type;
        typedef typename mpl::transform<
              mpl::list<T...>
            , unwrap_recursive<mpl::_1>
            >::type types;
        typedef typename mpl::end<types>::type end;

        typedef typename mpl::find<types, Attribute>::type iter_1;

        typedef typename
            mpl::eval_if<
                is_same<iter_1, end>,
                mpl::find_if<types, traits::is_substitute<mpl::_1, Attribute>>,
                mpl::identity<iter_1>
            >::type
        iter;

        typedef mpl::not_<is_same<iter, end>> type;
    };


    template <typename Attribute, typename... T>
    struct variant_find_substitute<std::variant<T...>, Attribute>
    {
        typedef std::variant<T...> variant_type;
        typedef typename mpl::transform<
              mpl::list<T...>
            , unwrap_recursive<mpl::_1>
            >::type types;

        typedef typename mpl::end<types>::type end;

        typedef typename mpl::find<types, Attribute>::type iter_1;

        typedef typename
            mpl::eval_if<
                is_same<iter_1, end>,
                mpl::find_if<types, traits::is_substitute<mpl::_1, Attribute> >,
                mpl::identity<iter_1>
            >::type
        iter;

        typedef typename
            mpl::eval_if<
                is_same<iter, end>,
                mpl::identity<Attribute>,
                mpl::deref<iter>
            >::type
        type;
    };
    
    template <typename... T>
    struct variant_find_substitute<std::variant<T...>, std::variant<T...> >
        : mpl::identity<std::variant<T...> > {};
}



namespace x3 = boost::spirit::x3;

typedef std::variant<unsigned int, std::string> type_t;

static_assert(x3::traits::is_variant<type_t>{}, "");

static std::ostream & operator<<(std::ostream& o, type_t const & v)
{
    if (std::holds_alternative<unsigned int>(v))
        o << std::get<unsigned int>(v) << "\n";
    else
        o << std::get<std::string>(v) << "\n";
    return o;
}

int main(void)
{
    static_assert(x3::traits::is_variant<type_t>::value);
    auto const parser
    = x3::rule<struct parser_tag, type_t >{ "parser" }
    = ('{' >> (x3::uint_ | x3::string("Hello")) >> '}');

    type_t var;
    std::string input("{12}");
    bool v = x3::phrase_parse(input.begin(), input.end(), parser ,x3::space,var);
    std::cout << "parsing : " << (v ? "OK" : "KO") << "\n";
    std::cout << "var = " << var << "\n";
    return 0;
}

Answer 1

The code is fine. The adaptation merely doesn't define a way to print the variant for the debug handlers. So, commenting

//#define BOOST_SPIRIT_X3_DEBUG

Makes it compile: http://coliru.stacked-crooked.com/a/304122ff888cef2b

parsing : OK
var = 12

Add Your Own Printing

Tracing the code shows that nonterminal/rule.hpp includes simple_trace.hpp, which uses print_attribute traits to print things.

namespace boost::spirit::x3::traits {
    template <typename Out, typename T, typename Enable = void>
    struct print_attribute_debug;
}

Okay, let's see whether we can specialize this trait for our purposes:

namespace boost::spirit::x3::traits {
    template <typename Out, typename... Ts>
    struct print_attribute_debug<Out, std::variant<Ts...>, void> {
        template <typename U>
        static void call(Out& out, U const& value) {
            std::visit([&out](auto const& v) {
                x3::traits::print_attribute(out, v);
            }, value);
        }
    };
}

Success

Live On Coliru

#include <iostream>

#include <variant>
#define BOOST_SPIRIT_X3_DEBUG
#include "boost/spirit/home/x3.hpp"
#include "boost/fusion/include/for_each.hpp"
#include "boost/spirit/include/support_istream_iterator.hpp"

namespace boost::spirit::x3::traits {
    template<typename... T>
    struct is_variant<std::variant<T...> >
        : mpl::true_ {};

    template <typename Attribute, typename... T>
    struct variant_has_substitute_impl<std::variant<T...>, Attribute>
    {
        typedef std::variant<T...> variant_type;
        typedef typename mpl::transform<
              mpl::list<T...>
            , unwrap_recursive<mpl::_1>
            >::type types;
        typedef typename mpl::end<types>::type end;

        typedef typename mpl::find<types, Attribute>::type iter_1;

        typedef typename
            mpl::eval_if<
                is_same<iter_1, end>,
                mpl::find_if<types, traits::is_substitute<mpl::_1, Attribute>>,
                mpl::identity<iter_1>
            >::type
        iter;

        typedef mpl::not_<is_same<iter, end>> type;
    };


    template <typename Attribute, typename... T>
    struct variant_find_substitute<std::variant<T...>, Attribute>
    {
        typedef std::variant<T...> variant_type;
        typedef typename mpl::transform<
              mpl::list<T...>
            , unwrap_recursive<mpl::_1>
            >::type types;

        typedef typename mpl::end<types>::type end;

        typedef typename mpl::find<types, Attribute>::type iter_1;

        typedef typename
            mpl::eval_if<
                is_same<iter_1, end>,
                mpl::find_if<types, traits::is_substitute<mpl::_1, Attribute> >,
                mpl::identity<iter_1>
            >::type
        iter;

        typedef typename
            mpl::eval_if<
                is_same<iter, end>,
                mpl::identity<Attribute>,
                mpl::deref<iter>
            >::type
        type;
    };
    
    template <typename... T>
    struct variant_find_substitute<std::variant<T...>, std::variant<T...> >
        : mpl::identity<std::variant<T...> > {};
}

namespace boost::spirit::x3::traits {
    template <typename Out, typename... Ts>
    struct print_attribute_debug<Out, std::variant<Ts...>, void> {
        template <typename U>
        static void call(Out& out, U const& value) {
            std::visit([&out](auto const& v) {
                x3::traits::print_attribute(out, v);
            }, value);
        }
    };
}

namespace x3 = boost::spirit::x3;

typedef std::variant<unsigned int, std::string> type_t;

static_assert(x3::traits::is_variant<type_t>{}, "");

static std::ostream & operator<<(std::ostream& o, type_t const & v)
{
    if (std::holds_alternative<unsigned int>(v))
        o << std::get<unsigned int>(v) << "\n";
    else
        o << std::get<std::string>(v) << "\n";
    return o;
}

int main(void)
{
    static_assert(x3::traits::is_variant<type_t>::value);
    auto const parser
    = x3::rule<struct parser_tag, type_t >{ "parser" }
    = ('{' >> (x3::uint_ | x3::string("Hello")) >> '}');

    type_t var;
    std::string input("{12}");
    bool v = x3::phrase_parse(input.begin(), input.end(), parser ,x3::space,var);
    std::cout << "parsing : " << (v ? "OK" : "KO") << "\n";
    std::cout << "var = " << var << "\n";
    return 0;
}

Prints

<parser>
  <try>{12}</try>
  <success></success>
  <attributes>12</attributes>
</parser>
parsing : OK
var = 12