#include <stdexcept>
#include <string>

class MyException : public std::runtime_error
{
    public:
        explicit MyException(const std::string& what_arg) :
                MyException(what_arg.data())
        {
        }

        explicit MyException(const char* what_arg) :
                runtime_error(""),
                what_("MyException:  ")
        {
            append(what_arg);
        }

        inline const char* what() const noexcept
        {
            return what_.data();
        }

        inline void append(const char* msg)
        {
            what_.append(msg);
        }

        inline void append(const std::string& msg)
        {
            what_.append(msg);
        }

    private:
        // As near as I can tell, there is no way to modify the what arg
        // of runtime_error, so I don't use it and make make my own what
        // arg here.
        //
        std::string what_;
};