making a vector of shared pointers from Spirit Qi

Question

This is a followup question from a previous question.

I can parse into vectors of strings from my grammar, but I cannot seem to parse into a vector of shared pointers to strings; i.e. std::vector<std::shared_ptr<std::string> >, and need a bit of help.

My compiling header:

#define BOOST_SPIRIT_USE_PHOENIX_V3 1


#include <boost/spirit/include/qi_core.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <iostream>
#include <string>


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

#include <boost/phoenix/bind/bind_member_function.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>



// this solution for lazy make shared comes from the SO forum, user sehe.
// https://stackoverflow.com/questions/21516201/how-to-create-boost-phoenix-make-shared
//    post found using google search terms `phoenix construct shared_ptr`
// changed from boost::shared_ptr to std::shared_ptr
namespace {
    template <typename T>
    struct make_shared_f
    {
        template <typename... A> struct result
        { typedef std::shared_ptr<T> type; };

        template <typename... A>
        typename result<A...>::type operator()(A&&... a) const {
            return std::make_shared<T>(std::forward<A>(a)...);
        }
    };

    template <typename T>
    using make_shared_ = boost::phoenix::function<make_shared_f<T> >;
}




namespace qi = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;


template<typename Iterator, typename Skipper = ascii::space_type>
struct SystemParser : qi::grammar<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper>
{


    SystemParser() : SystemParser::base_type(variable_group_)
    {
        namespace phx = boost::phoenix;
        using qi::_1;
        using qi::_val;
        using qi::eps;
        using qi::lit;


        var_counter = 0;

        declarative_symbols.add("variable_group",0);

        variable_group_ = "variable_group" > genericvargp_ > ';';
        genericvargp_ = new_variable_ % ','; //
        new_variable_ = unencountered_symbol_ [_val = make_shared_<std::string>() (_1)];
        unencountered_symbol_ = valid_variable_name_ - ( encountered_variables | declarative_symbols );
        valid_variable_name_ = +qi::alpha >> *(qi::alnum | qi::char_("[]_") );



//      debug(variable_group_); debug(unencountered_symbol_); debug(new_variable_); debug(genericvargp_);
//      BOOST_SPIRIT_DEBUG_NODES((variable_group_) (valid_variable_name_) (unencountered_symbol_) (new_variable_) (genericvargp_))
    }


    // rule declarations.  these are member variables for the parser.
    qi::rule<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper > variable_group_;
    qi::rule<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper > genericvargp_;
    qi::rule<Iterator, std::shared_ptr<std::string()> >  new_variable_;
    qi::rule<Iterator, std::string()> unencountered_symbol_;
    qi::rule<Iterator, std::string()> valid_variable_name_;


    unsigned var_counter;
    qi::symbols<char,int> encountered_variables;
    qi::symbols<char,int> declarative_symbols;
};

with driver code:

int main(int argc, char** argv)
{

    std::vector<std::shared_ptr<std::string> > V;
    std::string str = "variable_group x, y, z; ";


    std::string::const_iterator iter = str.begin();
    std::string::const_iterator end = str.end();


    SystemParser<std::string::const_iterator> S;


    bool s = phrase_parse(iter, end, S, boost::spirit::ascii::space, V);

    if (s)
    {
        std::cout << "Parse succeeded: " << V.size() << " variables\n";
        for (auto& s : V)
            std::cout << " - '" << s << "'\n";
    }
    else
        std::cout << "Parse failed\n";

    if (iter!=end)
        std::cout << "Remaining unparsed: '" << std::string(iter, end) << "'\n";


    return 0;
}

The text is parsed correctly, but the resulting vector is of length 0, while it should be of length 3. Somehow, the std::shared_ptr<string> is not pushed onto the back of the vector resulting from the rule genericvargp_.

I've tried many things, including reading all the debug information from a test parse, and placement of the %= signs for rule definitions, which should be used for rules for which there is a semantic action that does not assign _val unless I am mistaken. I've also played all night and day with using phx::bind to manually push onto the back of _val, but got nowhere. I've further verified that the make_shared_ provided by sehe in another answer is in fact lazy for std::shared_ptr.

As an aside, I have also struggled with getting the result of an unencountered_symbol_ to add to encountered_variables so as to enforce uniqueness of variable names...

The problem seems to be the propagation of the result of the new_variable_ rule onto the desired vector of shared pointers in the genericvargp_ rule.

Answer 1

This declaration

qi::rule<Iterator, std::shared_ptr<std::string()> > new_variable_;

Doesn't match the desired type:

qi::rule<Iterator, std::shared_ptr<std::string>() > new_variable_;

Sadly, in old SpiritV2 this attribute is silently ignored and no attribute propagation is done. This also explains why it didn't error out on compile time.

Live On Coliru

#define BOOST_SPIRIT_USE_PHOENIX_V3 1
#define BOOST_SPIRIT_DEBUG 1

#include <boost/spirit/include/qi_core.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <iostream>
#include <string>

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

#include <boost/phoenix/bind/bind_member_function.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>

// this solution for lazy make shared comes from the SO forum, user sehe.
// https://stackoverflow.com/questions/21516201/how-to-create-boost-phoenix-make-shared
//    post found using google search terms `phoenix construct shared_ptr`
// changed from boost::shared_ptr to std::shared_ptr
namespace {
    template <typename T> struct make_shared_f {
        template <typename... A> struct result { typedef std::shared_ptr<T> type; };

        template <typename... A> typename result<A...>::type operator()(A &&... a) const {
            return std::make_shared<T>(std::forward<A>(a)...);
        }
    };

    template <typename T> using make_shared_ = boost::phoenix::function<make_shared_f<T> >;
}

namespace qi    = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;

template <typename Iterator, typename Skipper = ascii::space_type>
struct SystemParser : qi::grammar<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper> {

    SystemParser() : SystemParser::base_type(variable_group_) {
        namespace phx = boost::phoenix;
        using qi::_1;
        using qi::_val;
        using qi::eps;
        using qi::lit;

        var_counter = 0;

        declarative_symbols.add("variable_group", 0);

        variable_group_       = "variable_group" > genericvargp_ > ';';
        genericvargp_         = new_variable_ % ',';                                                                                      //
        new_variable_         = unencountered_symbol_ [_val = make_shared_<std::string>()(_1)];
        unencountered_symbol_ = valid_variable_name_ - (encountered_variables | declarative_symbols);
        valid_variable_name_  = +qi::alpha >> *(qi::alnum | qi::char_("[]_"));

        BOOST_SPIRIT_DEBUG_NODES((variable_group_) (valid_variable_name_) (unencountered_symbol_) (new_variable_) (genericvargp_))
    }

    // rule declarations.  these are member variables for the parser.
    qi::rule<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper> variable_group_;
    qi::rule<Iterator, std::vector<std::shared_ptr<std::string> >(), Skipper> genericvargp_;
    qi::rule<Iterator, std::shared_ptr<std::string>() > new_variable_;
    qi::rule<Iterator, std::string()> unencountered_symbol_;
    qi::rule<Iterator, std::string()> valid_variable_name_;

    unsigned var_counter;
    qi::symbols<char, qi::unused_type> encountered_variables;
    qi::symbols<char, qi::unused_type> declarative_symbols;
};

int main()
{
    std::vector<std::shared_ptr<std::string> > V;
    std::string str = "variable_group x, y, z; ";

    std::string::const_iterator iter = str.begin();
    std::string::const_iterator end = str.end();

    SystemParser<std::string::const_iterator> S;

    bool s = phrase_parse(iter, end, S, boost::spirit::ascii::space, V);

    if (s)
    {
        std::cout << "Parse succeeded: " << V.size() << " variables\n";
        for (auto& s : V)
            std::cout << " - '" << *s << "'\n";
    }
    else
        std::cout << "Parse failed\n";

    if (iter!=end)
        std::cout << "Remaining unparsed: '" << std::string(iter, end) << "'\n";
}

Prints

Parse succeeded: 3 variables
 - 'x'
 - 'y'
 - 'z'

As well as a lot of debug information