Getting key value pairs from a file.

gistfile1.cpp

#include <iostream>
#include <limits>
#include <string>
#include <fstream>

namespace utility
{
    class stream_ok
    {
    public:
        stream_ok(std::ios& str) : ios(str) { }
        
        bool okay()
        {
            err = ios.rdstate();
            // check the stream state
            try
            {
                // do exception handling
            } catch(...) { }
            return ios;
        }
        
    protected:
        std::basic_ios<char>& ios;
        std::ios_base::iostate err;
    };
    
    template <class Facet>
    struct erasable_facet : Facet
    {
        template<typename... Args>
        erasable_facet(Args&&... args) : Facet(args...) { }
        ~erasable_facet() { }
    };
    
    static int key()   { static int i = std::ios_base::xalloc(); return i; }
    static int value() { static int i = std::ios_base::xalloc(); return i; }
}

class read_parsed_line : public utility::stream_ok
{
public:
    read_parsed_line(std::istream& _is)
        : stream_ok(_is), is(_is)
    { }
        
    void scan_input();
private:
    std::istream& is;
    utility::erasable_facet<std::ctype<char>> ctype;
    utility::erasable_facet<std::num_get<char, std::string::const_iterator>> numget;
    
    bool valid(long l)
    {
        return std::numeric_limits<int>::min() <= l &&
               std::numeric_limits<int>::max() >= l;
    }
    
    void scan_is(std::string& result)
    {
        result = ctype.scan_is(std::ctype_base::digit,
                                   &result.front(), &result.back());
    }
};

void read_parsed_line::scan_input()
{
    std::string result;
    result.assign(std::istreambuf_iterator<char>(is),
                  std::istreambuf_iterator<char>());
    long l;
                  
    scan_is(result);
    
    numget.get(result.begin(), result.end(), ios, err, l);
    result.erase(result.begin());
    
    if (!(valid(l) && okay()))
        return;
    else
        is.iword(utility::key()) = l;
    
    scan_is(result);
    
    numget.get(result.begin(), result.end(), ios, err, l);
    
    if (!(valid(l) && okay()))
        return;
    else
        is.iword(utility::value()) = l;
}

struct S { };

std::ostream& operator <<(std::ostream& os, const S &)
{
    static std::ifstream is("in.txt");
    
    std::istream::sentry s(is);
    
    if (s)
    {
        read_parsed_line rpl(is);
        rpl.scan_input();
        int key   = is.iword(utility::key());
        int value = is.iword(utility::value());
        
        os << key << ", " << value << '\n';
    }
    return os;
}

int main()
{
    S s;
    out << s;
}