http://stackoverflow.com/questions/8358975/cross-platform-way-to-get-line-number-of-an-ini-file-where-given-option-was-foun

.gitignore

*.swp
*.o
inireader.so
test

example.ini

[Cat1]
name1=100 #skipped



name2=200 \#not \\skipped
name3=   dhfj dhjgfd/* skipped

*/



[Cat_2]
UsagePage=9
Usage=19
Offset=0x1204

/*
[Cat_2_bak]
UsagePage=9
Usage=19
Offset=0x1204
*/

[Cat_3]
UsagePage=12
Usage=39
#Usage4=39
Offset=0x12304

inireader.h

#pragma once

#define POSITIONINFO 0

#include <map>
#include <string>
#include <iterator>
#include <boost/tuple/tuple_comparison.hpp>

template <typename S=std::string, typename Cmp=std::less<S> >
    class IniFile
{
    public:
    IniFile(Cmp cmp=Cmp()) : _cmp(cmp) 
        {}

    IniFile(const std::string& filename, Cmp cmp=Cmp()) : _cmp(cmp)
        { open(filename); }

    void open(const std::string& filename);

    typedef S string_t;
#if POSITIONINFO
    struct textnode_t
    {
        int sline, eline,
            scol, ecol;
        string_t text;

        operator const string_t&() const { return text; }
        friend std::ostream& operator<<(std::ostream& os, const textnode_t& t)
        { 
            os << "[L:" << t.sline << ",C" << t.scol << " .. L" << t.eline << ",C" << t.ecol << ":";
            for (typename string_t::const_iterator it=t.text.begin(); it!=t.text.end(); ++it)
            switch (*it)
            {
                case '\r' : os << "\\r"; break;
                case '\n' : os << "\\n"; break;
                case '\t' : os << "\\t"; break;
                case '\0' : os << "\\0"; break;
                default:    os << *it  ; break;
            }
            return os << "]"; 
        }

        bool operator<(const textnode_t& o) const 
            { return boost::tie(text/*, sline, eline, scol, ecol*/) <
                     boost::tie(o.text/*, o.sline, o.eline, o.scol, o.ecol*/); }

        textnode_t() : sline(0), eline(0), scol(0), ecol(0) { }
    };
#else
    typedef string_t textnode_t;
#endif

    typedef std::pair<textnode_t, textnode_t>   keyvalue_t;
    typedef std::map<textnode_t, textnode_t>    section_t;
    typedef std::map<textnode_t, section_t> sections_t;

  private:
    Cmp _cmp;
};

///////////////////////////////////////
// template implementation
//#define BOOST_SPIRIT_DEBUG

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/support_istream_iterator.hpp>
#include <boost/spirit/include/support_line_pos_iterator.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/fusion/adapted/std_pair.hpp>
#include <fstream>

namespace qi = boost::spirit::qi;
namespace phx= boost::phoenix;

namespace inireader
{
    struct printer
    {
        printer(std::ostream& os) : _os(os) {}
        std::ostream& _os;

        typedef boost::spirit::utf8_string string;

        void element(string const& tag, string const& value, int depth) const
        {
            for (int i = 0; i < (depth*4); ++i) // indent to depth
                _os << ' ';

            _os << "tag: " << tag;
            if (value != "")
                _os << ", value: " << value;
            _os << std::endl;
        }
    };

    void print_info(std::ostream& os, boost::spirit::info const& what)
    {
        using boost::spirit::basic_info_walker;

        printer pr(os);
        basic_info_walker<printer> walker(pr, what.tag, 0);
        boost::apply_visitor(walker, what.value);
    }

    template <typename It, typename Skipper, typename Ini>
        struct Grammar : qi::grammar<It, typename Ini::sections_t(), Skipper>
    {
        typedef typename Ini::string_t string_t;
        typedef typename Ini::textnode_t textnode_t;

        struct textbuilder
        {
            template <typename> struct result { typedef textnode_t type; };

            textbuilder(It begin) : _begin(begin) { }

            textnode_t operator()(const boost::iterator_range<It>& iters) const
            {
#if !POSITIONINFO
                return textnode_t(std::begin(iters), std::end(iters));
#else
                using boost::spirit::get_line;
                using boost::spirit::get_line_start;
                using boost::spirit::get_column;

                textnode_t element;
                element.text  = string_t       (std::begin(iters)  , std::end(iters));
                element.sline = get_line       (std::begin(iters));
                element.eline = get_line       (std::end(iters));
                It sol        = get_line_start (_begin             , std::begin(iters));
                element.scol  = get_column     (sol                , std::begin(iters));
                element.ecol  = get_column     (sol                , std::end(iters));

                return element;
#endif
            }

          private: 
            const It _begin;
        } makenode;

        Grammar(It begin) : Grammar::base_type(inifile), makenode(begin)
        {
            using namespace qi;
            txt_ch = (lit('\\') > char_) | (char_ - (eol | '#' | "/*"));

            key     = raw [ lexeme [ +(txt_ch - char_("=")) ] ] [ _val = phx::bind(makenode, _1) ];
            value   = raw [ lexeme [ +txt_ch ] ]                [ _val = phx::bind(makenode, _1) ];
            pair   %= key > '=' > value;

            heading  = ('[' > raw [ +~char_(']') ] > ']') [ _val = phx::bind(makenode, _1) ];
            section %= heading >> +eol >> -((pair-heading) % +eol);
            inifile %= -(section % +eol) >> *eol > eoi;

            comment = 
                  ('#' >> *(char_ - eol))
                | ("/*" > *(char_ - "*/") > "*/");

            //BOOST_SPIRIT_DEBUG_NODE(comment);
            //BOOST_SPIRIT_DEBUG_NODE(txt_ch);
            BOOST_SPIRIT_DEBUG_NODE(heading);
            BOOST_SPIRIT_DEBUG_NODE(section);
            BOOST_SPIRIT_DEBUG_NODE(key);
            BOOST_SPIRIT_DEBUG_NODE(value);
            BOOST_SPIRIT_DEBUG_NODE(pair);
            BOOST_SPIRIT_DEBUG_NODE(inifile);
        }

        typedef typename Ini::keyvalue_t keyvalue_t;
        typedef typename Ini::section_t  section_t;
        typedef typename Ini::sections_t sections_t;
        typedef typename string_t::value_type Char;
        qi::rule<It>                        comment;
        qi::rule<It, Char()>                txt_ch;
        qi::rule<It, textnode_t(), Skipper> key, value, heading;
        qi::rule<It, keyvalue_t(), Skipper> pair;
        qi::rule<It, std::pair<textnode_t, section_t>(), Skipper> section;
        qi::rule<It, sections_t(), Skipper> inifile;
    };

    template <typename It, typename Builder>
        typename Builder::template result<void>::type
            fragment(const It& first, const It& last, const Builder& builder)
            {
                size_t len = std::distance(first, last);
                It frag_end = first;
                std::advance(frag_end, std::min(10ul, len));
                return builder(boost::iterator_range<It>(first, frag_end));
            }
}

template <typename S, typename Cmp>
void IniFile<S, Cmp>::open(const std::string& filename)
{
    using namespace qi;

    std::ifstream ifs(filename.c_str());
    ifs.unsetf(std::ios::skipws);

#if POSITIONINFO
    typedef std::string::const_iterator RawIt;
    typedef boost::spirit::line_pos_iterator<RawIt> It;

    typedef rule<It> Skipper;

    std::string buffer(std::istreambuf_iterator<char>(ifs), (std::istreambuf_iterator<char>()));
    It f(buffer.begin()), l(buffer.end());
#else
    typedef boost::spirit::istream_iterator It;
    typedef rule<It> Skipper;

    It f(ifs), l;
#endif

    inireader::Grammar<It, Skipper, IniFile<S, Cmp> > grammar(f);
    Skipper skip = char_(" \t") | grammar.comment;

    try
    {
        sections_t data;
        bool ok = phrase_parse(f, l, grammar, skip, data);
        if (ok)
        {
            std::cout << "Parse success!" << std::endl;

            for (auto& section : data)
            {
                std::cout << "[" << section.first << "]" << std::endl;
                for (auto& pair : section.second)
                    std::cout << pair.first << " = " << pair.second << std::endl;
            }
        } else
        {
            std::cerr << "Parse failed" << std::endl;
        }
    } catch (const qi::expectation_failure<It>& e)
    {
        std::cerr << "Exception: " << e.what() << 
              " " << inireader::fragment(e.first, e.last, grammar.makenode) << "... ";
        inireader::print_info(std::cerr, e.what_);
    }
    if (f!=l)
    {
        std::cerr << "Stopped at: '" << inireader::fragment(f, l, grammar.makenode) << "'" << std::endl;
    }
}

Makefile

all: test | .depends

# BOOSTDIR=~/custom/boost_1_47_0
# CPPFLAGS+=-I $(BOOSTDIR)

CPPFLAGS+=-g -O2
CPPFLAGS+=-std=c++0x
CPPFLAGS+=-Wall

%.o: %.cpp
	g++ $(CPPFLAGS) -o $@ -c $<

%: %.o
	g++ $(CPPFLAGS) -o $@ $<

.depends: *.cpp
	g++ $(CPPFLAGS) -M $^ -o $@

clean:
	rm -rf test .depends

.PHONY: all clean
-include .depends

test.cpp

#include "inireader.h"
#include <fstream>
#include <iostream>

int main(int argc, const char *argv[])
{
    IniFile<std::string> ini("example.ini");
    
    return 0;
}

Does this code happen to be public domain/BSD? (Would be nice to have it for http://www.libelektra.org, see ElektraInitiative/libelektra#736)