How do I use a class with only one attribute in a AST with Boost Spirit?

Question

I want to parse a file into an AST using Boost Spirit.

The root of my AST is a class with only one attribute :

typedef boost::variant<FunctionDeclaration, GlobalVariableDeclaration> FirstLevelBlock;

struct Program {
    std::vector<FirstLevelBlock> blocks;
};

BOOST_FUSION_ADAPT_STRUCT(
    ::Program,
    (std::vector<eddic::FirstLevelBlock>, blocks)
)

If I parse using a single rule :

program %= *(function | globalDeclaration);

it doesn't compiles, but if I add a single string name to Program, it works well. I could use the vector as the root, but I want to use the class, because I want to add some methods to the Program class.

EDIT :

If I surround my program with braces, it works well :

program %= lexer.left_brace >> *(function | globalDeclaration) >> lexer.right_brace;

compiles and works fine, but :

program %= *(function | globalDeclaration);

does not compile...

Is there something in Boost Spirit that prevent using such simple rules ?

Answer 1

Edited question version 2

If I surround my program with braces, it works well [...], but program %= *(function | globalDeclaration); does not compile...

Is there something in Boost Spirit that prevent using such simple rules ?

Firstly, we can't really tell without the defintion for function and globalDeclaration.

Secondly I tried, changing my PoC lines to

static const qi::rule<It, Program(), space_type> program  = *(function | global);
Program d = test("void test(); int abc; int xyz; void last();" , program); 

Lo and behold, I get your compiler error! Now I would certainly agree that this looks very much like an attribute conversion bug. Also, Here is a preliminary workaround:

program %= eps >> *(function | global);

As you can see, qi::eps to the rescue

---

Answer to original question version 1

Mmm. I think you need to post a minimal working sample. Here is a proof of concept starting from your question, and it all works rather nicely.

_{Note that I compiled with g++ -std=c++0x in order to get the default Attr parameter argument on the test function.}

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/karma.hpp>
#include <boost/fusion/adapted.hpp>
#include <boost/strong_typedef.hpp>

// added missing bits
namespace eddic 
{
    typedef std::string FunctionDeclaration;
    typedef std::string GlobalVariableDeclaration;

    typedef boost::variant<FunctionDeclaration, GlobalVariableDeclaration> FirstLevelBlock;
}

using namespace eddic;
// end missing bits

struct Program {
    std::vector<FirstLevelBlock> blocks;
};

BOOST_FUSION_ADAPT_STRUCT(
    ::Program,
    (std::vector<eddic::FirstLevelBlock>, blocks)
)

namespace /*anon*/
{    
    using namespace boost::spirit::karma;

    struct dumpvariant : boost::static_visitor<std::ostream&>
    {
        dumpvariant(std::ostream& os) : _os(os) {}
        template <typename T> std::ostream& operator ()(const T& t) const
            { return _os << format(stream, t); }

      private: std::ostream& _os;
    };

    std::ostream& operator<<(std::ostream& os, const FirstLevelBlock& block)
    { 
        os << "variant[" << block.which() << ", ";
        boost::apply_visitor(dumpvariant(os), block);
        return os << "]";
    }

    std::ostream& operator<<(std::ostream& os, const std::vector<FirstLevelBlock>& blocks)
    { return os << format(-(stream % eol), blocks); }

    std::ostream& operator<<(std::ostream& os, const Program& program)
    { return os << "BEGIN\n" << program.blocks << "\nEND"; }
}

namespace qi = boost::spirit::qi;

template <typename Rule, typename Attr = typename Rule::attr_type>
    Attr test(const std::string& input, const Rule& rule)
{
    typedef std::string::const_iterator It;
    It f(input.begin()), l(input.end());

    Attr result;
    try
    {
        bool ok = qi::phrase_parse(f, l, rule, qi::space, result);
        if (!ok)
            std::cerr << " -- ERR: parse failed" << std::endl;
    } catch(qi::expectation_failure<It>& e)
    {
        std::cerr << " -- ERR: expectation failure at '" << std::string(e.first, e.last) << "'" << std::endl;
    }
    if (f!=l)
        std::cerr << " -- WARN: remaing input '" << std::string(f,l) << "'" << std::endl;
    return result;
}

int main()
{
    typedef std::string::const_iterator It;
    static const qi::rule<It, FunctionDeclaration(), space_type>        function = "void " > +~qi::char_("()") > "();";
    static const qi::rule<It, GlobalVariableDeclaration(), space_type>  global   = "int "  > +~qi::char_(";")  > ";";
    static const qi::rule<It, FirstLevelBlock(), space_type>            block    = function | global;
    static const qi::rule<It, Program(), space_type>                    program  = '{' >> *(function | global) >> '}';

    FunctionDeclaration       a = test("void test();", function); 
    std::cout << "FunctionDeclaration a         : " << a << std::endl;

    GlobalVariableDeclaration b = test("int abc;", global); 
    std::cout << "GlobalVariableDeclaration b   : " << b << std::endl;

    FirstLevelBlock c = test("void more();", block); 
    std::cout << "FirstLevelBlock c             : " << c << std::endl;

    /*FirstLevelBlock*/ c = test("int bcd;", block); 
    std::cout << "FirstLevelBlock c             : " << c << std::endl;

    Program d = test("{"
            "void test();"
            "int abc"
            ";"
            "int xyz; void last();"
            "}", program); 
    std::cout << "Program d                     : " << d << std::endl;
}

Output:

FunctionDeclaration a         : test
GlobalVariableDeclaration b   : abc
FirstLevelBlock c             : variant[1, more]
FirstLevelBlock c             : variant[1, bcd]
Program d                     : BEGIN
test
abc
xyz
last
END