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

Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Peter Dimov (lists_at_[hidden])
Date: 2017-05-30 12:25:50

Niall Douglas wrote:

> > value() id wide operator*() is narrow.
>
> Why? Just because optional made that mistake?

The motivation here is that in

if( o )
{
    o->this_();
    o->that();
    o->this_other_thing();
    o->that_too();

    o->almost_forgot();
}

there is needless replication of the check. This, as I already mentioned, 
stems from a desire to operate on the value inside 'o' directly, which is 
not my preferred style in result's case, where I'd prefer to extract it 
instead.

There is a middle ground here, which I was pondering for a while, make 
has_value return T* instead of bool.

    T* has_value();
    T const* has_value() const;

    Returns: a pointer to the contained value if one is present, otherwise 
nullptr

Now the above sequence is spelled

if( auto* p = o.has_value() )
{
    p->this_();
    p->and_so_on();
}

Not sure I particularly like that, but for people who like "consistency with 
X" arguments, this is like variant's get_if, whereas value() is like 
variant's get().

This still allows the ordinary if( r.has_value() ) check to work as-is, so 
nothing is lost in principle.

But if this has_value signature strikes you as a bit too novel, the more 
traditional T* value_if() is also an option. Note how it strikes a balance 
where the function itself has a wide contract, but then if you use the 
returned pointer without a check you're on your own.