Re: [boost] [review] Review of Outcome v2 (Fri-19-Jan to Sun-28-Jan, 2018)

Subject: Re: [boost] [review] Review of Outcome v2 (Fri-19-Jan to Sun-28-Jan, 2018)
From: Peter Dimov (lists_at_[hidden])
Date: 2018-02-01 15:01:28

Gavin Lambert wrote:
>    static std::unique_ptr<A> A::create(args...) noexcept

...

> If you don't like the heap usage, the same concept applies if it returns 
> an optional instead of a pointer (assuming it has a no-fail move).
>
> With Boost.Outcome, it can be implemented exactly as above (including not 
> letting an invalid instance escape), but *also* indicate a failure reason 
> to the caller, through a simple change to the return type.

Following this train of thought to its logical conclusion is interesting. 
Suppose we need to copy. That would be

unique_ptr<A> A::copy( A const & rhs, error_code & ec ) noexcept;

because if construction can fail, copying most likely can fail, too.

If we switch to optional, we'll have

optional<A> A::create( error_code& ec ) noexcept;
optional<A> A::copy( A const& rhs, error_code& ec ) noexcept;

and then to result:

result<A> A::create() noexcept;
result<A> A::copy( A const& rhs ) noexcept;

This implies that under this style of programming, a `result` is not copied 
with its copy constructor.