Re: [boost] [outcome] To variant, or not to variant?

Subject: Re: [boost] [outcome] To variant, or not to variant?
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-05-31 08:44:09

2017-05-31 10:26 GMT+02:00 Gavin Lambert via Boost <boost_at_[hidden]>:

>
>
> template<typename T>
> class outcome
> {
> public:
>     bool has_value() const { return !!m_value; }
>     T& value() { ensure(); return m_value.value(); }
>
>     //bool has_error() const { return !!m_error; }
>     const error_code& error() const { return m_error; }
>
>     //bool has_exception() const { return !!m_exception; }
>     const exception_ptr& exception() const { return m_exception; }
>
>     void ensure() const
>     {
>         if (m_exception) { rethrow_exception(m_exception); }
>         if (m_error) { throw system_error(m_error); }
>     }
>
>     void set(none_t)
>     {
>         m_value = none;
>         m_error = error_code();
>         m_exception = nullptr;
>     }
>     void set(const T& val)
>     {
>         m_value = val;
>         m_error = error_code();
>         m_exception = nullptr;
>     }
>     void set(error_code err)
>     {
>         m_value = none;
>         m_error = err;
>         m_exception = nullptr;
>     }
>     void set(exception_ptr ep)
>     {
>         m_value = none;
>         m_error = ep ? error_code(errc::has_exception) : error_code();
>         m_exception = ep;
>     }
>
> private:
>     optional<T> m_value;
>     error_code m_error;
>     exception_ptr m_exception;
> };
>
> (Don't get too hung up on the specifics.  This is a sketch, not a real
> implementation.  I've obviously omitted things that a real implementation
> would need such as more const methods and move support, and construction
> and assignment rather than using set methods.  The focus is on variant vs.
> non-variant.)
>
> Anyway, the point is that this could actually transport multiple things;
> in particular as above both an error_code and an exception_ptr; perhaps
> set(error_code) could construct an exception as well, although I've
> explained elsewhere why I don't like that option.
>

In your mental model what is the interpretation of the situation when you
have both an error_code and an exception_ptr?

Regards,
&rzej;