Re: [boost] Mother of all Variants

Subject: Re: [boost] Mother of all Variants
From: Larry Evans (cppljevans_at_[hidden])
Date: 2019-04-16 12:41:25

On 4/16/19 4:28 AM, Jonathan Müller via Boost wrote:
> On 16.04.19 03:51, Robert Ramey via Boost wrote:
>> I'm way out of my depth in the variant discussion.  Seems to me it 
>> revolves around all the trade offs regarding design choices.  Could we 
>> perhaps decide not to decide.  Suppose we create a type which looks like:
>>
>> template<
>> Â Â  bool BasicGuaranteeSupported,
>> Â Â  bool StrongGuaranteeSupported,
>> Â Â  bool EmptyStateProhibited,
>> Â Â  bool AssignmentSupported,
>> Â Â  bool MoveSupported,
>> Â Â  class T ...
>> Â >
>> struct mother_of_variants{
>> Â Â  // some trivial mp11 code
>> };
>>
> 
> I did something similar back when I did type_safe::variant: You have a 
> `VariantPolicy` that controls whether the variant can be empty and a has 
> a `change_value()` function that switches the type in whatever way it 
> seems fit.
> 
> With that I have a `rarely_empty_variant` that is like `std::variant`,
> a `never_empty_variant` that assumes move is no-throw (and calls 
> terminate() if they throw) and an `optional_variant` that fully embraces 
> an empty state.
> 
> At the time I thought it was a good trade-off, but haven't re-evaluated 
> it since then.
> 

I like this; however, years ago there was some argument
about how providing such a policy for smart pointers
would be confusing to users.  However, I didn't really
agree with that argument and the alternative is, AFAICT,
not much better:
   std::variant, or boost::variant2 or libx::variant3 or ...