stringstream unsigned input validation

Question

I\'m writing part of program which parses and validates some user input in program console arguments. I choose to use stringstream for that purpose, but encounter a problem

Answer 1

A num_get facet to support the explicit check for signedness. Rejects any non-zero number beginning with a '-' (after white-spaces) for unsigned types and uses the default C locale's num_get to do the actual conversion.

#include 
#include 
#include 
#include 

template  >
class num_get_strictsignedness : public std::num_get 
{
public:
    typedef charT char_type;
    typedef InputIterator iter_type;

    explicit num_get_strictsignedness(std::size_t refs = 0)
        : std::num_get(refs)
    {}
    ~num_get_strictsignedness()
    {}

private:
    #define DEFINE_DO_GET(TYPE) \
        virtual iter_type do_get(iter_type in, iter_type end,      \
            std::ios_base& str, std::ios_base::iostate& err,       \
            TYPE& val) const override                              \
        {  return do_get_templ(in, end, str, err, val);  }         // MACRO END

    DEFINE_DO_GET(unsigned short)
    DEFINE_DO_GET(unsigned int)
    DEFINE_DO_GET(unsigned long)
    DEFINE_DO_GET(unsigned long long)

    // not sure if a static locale::id is required..

    template 
    iter_type do_get_templ(iter_type in, iter_type end, std::ios_base& str,
                           std::ios_base::iostate& err, T& val) const
    {
        using namespace std;

        if(in == end)
        {
            err |= ios_base::eofbit;
            return in;
        }

        // leading white spaces have already been discarded by the
        // formatted input function (via sentry's constructor)

        // (assuming that) the sign, if present, has to be the first character
        // for the formatting required by the locale used for conversion

        // use the "C" locale; could use any locale, e.g. as a data member

        // note: the signedness check isn't actually required
        //       (because we only overload the unsigned versions)
        bool do_check = false;
        if(std::is_unsigned{} && *in == '-')
        {
            ++in;  // not required
            do_check = true;
        }

        in = use_facet< num_get >(locale::classic())
                 .get(in, end, str, err, val);

        if(do_check && 0 != val)
        {
            err |= ios_base::failbit;
            val = 0;
        }

        return in;
    }
};

Usage example:

#include 
#include 
int main()
{
    std::locale loc( std::locale::classic(),
                     new num_get_strictsignedness() );
    std::stringstream ss("-10");
    ss.imbue(loc);
    unsigned int ui = 42;
    ss >> ui;
    std::cout << "ui = "<

Notes:

• the allocation on the free store is not required, you could use e.g. a (static) local variable where you initialize the ref counter with 1 in the ctor
• for every character type you want to support (like char, wchar_t, charXY_t), you need to add an own facet (can be different instantiations of the num_get_strictsignedness template)
• "-0" is accepted