Skipper does not work in boost::spirit

Question

I use boost spirit to parse a color. That worked quite well, but after I changed the the iterator type, the skipper stopped working.

"rgb(1.0,1.0,0.5)"  // this works
" rgb(0.2,0.2,0.2)" // this fails

Here is the header:

struct ColorGrammar : public qi::grammar<StringIterator, Color(), chs::space_type>
{
//! Iterator type for this grammar
typedef StringIterator ItType;
//! Skipper type used in this grammar
typedef chs::space_type Skipper;



//! Rule to parse a number with up to 3 digits
qi::uint_parser<uint8, 10, 1, 3> number;
//! Rule to parse a hex digit
qi::uint_parser<uint8, 16, 1, 1> hexdigit;

ColorGrammar();

//! Rule for rgb(...)
qi::rule<ItType, Color(), qi::locals<float, float>, Skipper> rule_rgb;
//! Rule for rgba(...)
qi::rule<ItType, Color(), qi::locals<float, float, float>, Skipper> rule_rgba;
//! Mainrule
qi::rule<ItType, Color(), Skipper> rule_color;
};

Here is the cpp

ColorGrammar::ColorGrammar()
: ColorGrammar::base_type(rule_color, "color-grammar")
{
using namespace qi::labels;
using boost::phoenix::construct;
auto& _1 = qi::_1;


rule_rgb = '(' >> qi::float_[_a = _1] >> ',' >> qi::float_[_b = _1] >> ',' >> qi::float_[_val = phx::construct<Color>(_a, _b, _1)] >> ')';
rule_rgba = '(' >> qi::float_[_a = _1] >> ',' >> qi::float_[_b = _1] >> ',' >> qi::float_[_c = _1] >> ',' >> qi::float_[_val = phx::construct<Color>(_a, _b, _c, _1)] >> ')';
rule_color = (qi::lit("rgb") >> rule_rgb)
    | (qi::lit("rgba") >> rule_rgba);
}

And the call:

Color out;
StringIterator begin = str.cbegin();
StringIterator end = str.cend();

bool result = qi::phrase_parse(begin, end, color_, chs::space, out);

I'm sure, it is only a little misstake, but I am not able to see it. Maybe i watched too long at the source... can you see a misstake?

You're hurting your potential to get quick & easy answers by leaving just enough code out to make this a puzzle. Instead of SSCCE. Also "the Skipper stopped working" (Is this some secret code to signal that the bank robbery is underway?). What happens? What _does it say_? What is your input? — sehe, Feb 11 '13 at 12:47
Applied my turd polish. My answer shows how you'd probably want to write this rule instead. (But I couldn't reproduce the original complaint) — sehe, Feb 11 '13 at 13:25

sehe · Accepted Answer · 2013-02-11T13:53:14.477

I can't see what's wrong: I've taken the effort to reconstruct your SSCCE.

http://liveworkspace.org/code/1pDtmn$1

In the process, it seems I must have removed the problem. I suggest you do the same.

Oh, and this is how I'd write this:

no more phoenix
no more constructors
no more qi::locals
no more needless copying
using expectation points

In short: no more fuss.

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/adapted/struct.hpp>
#include <cstdint>

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

typedef std::string::const_iterator StringIterator;

struct Color
{
    float r,g,b,a; 
};

BOOST_FUSION_ADAPT_STRUCT(Color, (float, r)(float, g)(float, b)(float, a))

template <typename ItType, typename Skipper>
struct ColorGrammar : public qi::grammar<StringIterator, Color(), Skipper>
{
    ColorGrammar()
        : ColorGrammar::base_type(rule_color, "color-grammar")
    {
        using namespace qi;
        rule_rgb   = lit("rgb")  >> '(' > float_ > ',' > float_ > ',' > float_ >       attr(1.0f) > ')';
        rule_rgba  = lit("rgba") >> '(' > float_ > ',' > float_ > ',' > float_ > ',' > float_     > ')';
        rule_color = rule_rgb | rule_rgba;
    }

  private:
    qi::uint_parser<uint8_t, 10, 1, 3> number;   // unused
    qi::uint_parser<uint8_t, 16, 1, 1> hexdigit; // unused

    qi::rule<ItType, Color(), Skipper> rule_rgb, rule_rgba, rule_color;
};

int main()
{
    Color out;
    std::string str = " rgb ( 0.3 , .4 , 0.5 )";
    StringIterator begin = str.cbegin();
    StringIterator end   = str.cend();

    ColorGrammar<StringIterator, chs::space_type> color_;

    bool result = qi::phrase_parse(begin, end, color_, chs::space, out);
    std::cout << std::boolalpha << result << '\n';
    std::cout << "remains: '" << std::string(begin, end) << "'\n";
}

Live on http://liveworkspace.org/code/35htD$3

Much better, I always forget about `attr` to specify default values. Though I believe that alpha should be the last value (for the case when the input is rgba). — , Feb 11 '13 at 13:28
@llonesmiz I completely agree, but I wanted to keep the grammar itself unaltered — sehe, Feb 11 '13 at 13:38
Without seeing the asker's constructors it's hard to say, but I would bet that in the original grammar _a->red, _b->green, _c->blue, _1->alpha. — , Feb 11 '13 at 13:42
@llonesmiz I see it now. Fixed (also spotted a missing `','` spearator) — sehe, Feb 11 '13 at 13:50
Thank you for your effort. I argree, that your code is much better. Meanwhile i found the misstake and the reason why it was not possible to reproduce it. I forgot to include qi.hpp :( chs::space was declared but not defined... I do not know why I was able to compile it but now it works. thank you again. — Databyte, Feb 11 '13 at 18:16

Skipper does not work in boost::spirit

1 Answers1

Oh, and this is how I'd write this: