Re: [boost] [variant2] Need rationale for never-empty guarantee

Subject: Re: [boost] [variant2] Need rationale for never-empty guarantee
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2019-03-02 22:56:04

AMDG

On 3/2/19 3:31 PM, Niall Douglas via Boost wrote:
>>> You see, I would consider any union-based storage as in the same
>>> category. By definition union-based storage must contain a "pointer" to
>>> the correct way to interpret that union-based storage.
>>
>> I don't understand this.  Andrzej says pointers and
>> means it literally: T*.  You say "pointers" and
>> I have no idea what you mean by it and what relation
>> it has to exception safety.
> 
> If an object *selects* another object, then in my opinion it needs to
> propagate the strongest possible exception guarantees it can for
> assignment, swap and emplace.
> 
> If an object *aggregates* other objects, then in my opinion it is
> permitted to have partial operations. So swap can get half way into
> swapping the individual members in the aggregate, and bail out.
> 
> This might help:
> 
> T*: Selects a T[1...N]
> unique_ptr<T[x]>: Selects a T[x]
> vector<T>: Selects a T[0...N]
> optional<T>: Selects a T[0], or a T[1]
> variant<A, B, C>: Selects one of a A[1], B[1], or C[1], with all others [0]
> 
> ---
> 
> struct: Aggregate of heterogeneous types
> T[x]: Aggregate of x homogeneous T's
> array<T, x>: Aggregate of x homogeneous T's
> 
> 
> Can you see the difference yet? The first group selects an aggregate
> *elsewhere*. The second grouo IS an aggregate.
> 

Nope.  The second group makes sense to me.  The
first is just a grab bag of everything that isn't
the second.

In Christ,
Steven Watanabe