Skipping expected characters like scanf() with cin

Question

How to achieve scanf(\"%d # %d\",&a,&b);sort of effect with cin in C++ ?

Answer 1

Here's another way. You can classify # as a whitespace character through the std::ctype facet imbued in the locale:

#include 
#include 
#include 

namespace detail
{
    enum options { add, remove };

    class ctype : public std::ctype
    {
    private:
        static mask* get_table(const std::string& ws, options opt)
        {
            static std::vector table(classic_table(),
                                           classic_table() + table_size);
            for (char c : ws)
            {
                if (opt == add)
                    table[c] |= space;
                else if (opt == remove)
                    table[c] &= ~space;
            }
            return &table[0];
        }
    public:
        ctype(const std::string& ws, options opt)
            : std::ctype(get_table(ws, opt)) { }
    };
}

class adjustws_impl
{
public:
    adjustws_impl(const std::string& ws, detail::options opt) :
        m_ws(ws),
        m_opt(opt)
    { }

    friend std::istream& operator>>(std::istream& is,
                                    const adjustws_impl& manip)
    {
        is.imbue(std::locale(is.getloc(),
                             new detail::ctype(manip.m_ws, manip.m_opt)));
        return is;
    }
private:
    std::string m_ws;
    detail::options m_opt;
};

adjustws_impl setws(const std::string& ws)
{
    return adjustws_impl(ws, detail::add);
}

adjustws_impl unsetws(const std::string& ws)
{
    return adjustws_impl(ws, detail::remove);
}

int main()
{
    std::istringstream iss("10 # 20");
    int a, b;

    iss >> setws("#");
    iss >> a >> b;

    iss >> unsetws("#");
    std::cout << a << ' ' << b; // 10 20
}