Re: [boost] [outcome] High level summary of review feedback accepted so far

Le 27/05/2017 à 16:24, Peter Dimov via Boost a écrit :

Niall Douglas wrote:

...

I would assume an expected could only return a std::variant from its .error(). I can't think what else it could do.

As the foremost authority on the nonexistent expected, I can tell you what it does:

// F shall be in E... template<class F> bool has_error() const; template<class F> F error() const;

// if sizeof...(E) == 1 bool has_error() const; E1 error() const;

It returns the equivalent of variant not from error(), but from

unexpected unexpected() const;

which allows you to

expected function() { expected e1 = function1(); if( !e1 ) return e1.unexpected();

expected e2 = function2(); if( !e2 ) return e2.unexpected();

return e1.value() + e2.value(); } Yes this is the reason d'être fro the unexpected function.

I see that the implementation you have done don't allows to return it by reference, and I was wondering if the variant2::subset function (which could for usability purposes be a non-member function) couldn't return by reference. I know that this is something related to changes to the standard. There is currently a thread on C++-Core about the whether it is UB to cast a type to another with a common layout in order to access the common members. It seems that this is valid in C, but this is not anymore valid in C++. If at the end this kind of casts were valid as well in C++, I believe that we could return subset/unexpected by reference without UB. Note that a variant with less alternatives than another one has the "same shared representation". Best, Vicente