Adapt class containing a string member as synthesized attribute

Question

I’m trying to parse a character string into an attribute of a custom type symbol, which contains a std::string member. I thought I could use BOOST_FUSION_ADAPT_STRUCT here but that doesn’t work.

If I declare the rule as rule<It, std::string(), space_type> it works. If I define it as rule<It, symbol(), space_type> it fails with the error “no type name value_type in symbol”. I think Spirit is trying to append the value character-for-character to the attribute, which fails as expected. But isn’t there a way of making this work, without adding an additional intermediate rule which captures the std::string attribute?

Here’s the full MWE:

#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>

struct symbol 
{ 
    std::string repr; 
};

BOOST_FUSION_ADAPT_STRUCT(symbol, (std::string, repr))

namespace qi = boost::spirit::qi;

template <typename Iterator>
struct test_grammar : qi::grammar<Iterator, symbol(), qi::ascii::space_type> {
    test_grammar() : test_grammar::base_type{start} {
        start = qi::lexeme[+qi::char_("a-z")];
    }

    qi::rule<Iterator, symbol(), qi::ascii::space_type> start;
};

#include <iostream>

auto main() -> int {
    test_grammar<std::string::iterator> grammar{};
    auto input = std::string{"test"};
    auto output = symbol{};
    auto e = end(input);
    if (qi::phrase_parse(begin(input), e, grammar, qi::ascii::space, output))
        std::cout << output.repr;
}

Answer 1

As you have seen from the linked dupe, you can get around this with a well-placed qi::eps.

    start = qi::eps >> qi::lexeme[+qi::char_("a-z")];

See it Live On Coliru

However, there are cases where qi::eps doesn't save. (Will try to find links later). Therefore, I've come to favour the 'old-fashioned approach' for attribute compatibility:

#include <boost/spirit/include/qi.hpp>

struct symbol 
{ 
    std::string repr; 

    symbol(std::string repr = std::string()) : repr(std::move(repr)) {}
};

namespace qi = boost::spirit::qi;

template <typename Iterator>
struct test_grammar : qi::grammar<Iterator, symbol(), qi::ascii::space_type> {
    test_grammar() : test_grammar::base_type(start) {
        using namespace qi;

        start = as_string[ lexeme[ +char_("a-z") ] ];
    }

    qi::rule<Iterator, symbol(), qi::ascii::space_type> start;
};

#include <iostream>

auto main() -> int {
    test_grammar<std::string::iterator> grammar{};
    auto input = std::string{"test"};
    auto output = symbol{};
    auto e = end(input);
    if (qi::phrase_parse(begin(input), e, grammar, qi::ascii::space, output))
        std::cout << output.repr;
}

This is also likely a bit lighter on the compiler. See it Live on Coliru as well.

If all else fails you can have your cake and eat it too because attribute transformation/assignment are customization points in the library.

• Attribute Propagation and Attribute Compatibility
• Store a Parsed Attribute Value (Qi)
• Store Parsed Attribute Values into a Container (Qi)
• search for these traits in answers on SO for good examples