Example one

enum class A
{
	none, one
};

struct A_parser
{
    static A parse(std::string const & option)
    {
        if (option == "none") {
            return A::none;
        } 
        else if (option == "one") {
            return A::one;
        } else {
            throw 0;
        }
    }
};

template <class ValueType, class Parser>
class value_option
{
    
    void parse_option(std::string const & opt)
    {
        m_value = Parser::parse(opt);
    }
    
    ValueType value() { return m_value; }
    
private:
    ValueType m_value;
};