Proper error handling with subclass inherits std::runtime_error

[vxcall]

by inheriting std::runtime_error, you can use message when you throw one.
Additionally you also gain polymorphic advantage which you can pass your custom error class to the function which takes param as std::runtime_error or std::exception.
Moreover, it has what function at the first state and it's guaranteed to not throw additional error from it.

class kur_error : public std::runtime_error {
public:
    kur_error(const std::string& message)
        : std::runtime_error(message) {
    }
};