Re: Adding optional<T&> find like method to associative containers in boost.

From: Neil Groves (neilgroves_at_[hidden])
Date: 2024-12-19 18:01:08

On Thu, 19 Dec 2024 at 17:50, Ivan Matek <libbooze_at_[hidden]> wrote:

>
>
> On Thu, Dec 19, 2024 at 6:26 PM Neil Groves via Boost <
> boost_at_[hidden]> wrote:
>
>> One aspect that strikes me about all of our alternatives is that none of
>> them work particularly well when we are using multi_ associative
>> containers.
>>
>> If I understand your comment correctly:
> "correct" return type is a std::ranges::subrange, equal_range returns
> std::pair so you need some helper to "rangify" that pair.
> (this example uses biset so you can ignore .get<1>).
>
> EmployeeSet ems{{1, "Alice"}, {2, "Bob"}, {3, "Chad"}, {4, "Alice"}};
>
> for (const Employee& employee : ems.get<1>().equal_range("Alice") | rangify)
> {
>
>    std::print("{}  {}\n", employee.id, employee.name);
>
> }
>
>
>
>
>
Yes exactly that. It's not exactly equal_range, in the same way that it
wasn't quite "find" either as they both return the mapped_type not the
dereferenced iterator value.

I intend to build upon this idea with a wrapper type around the
equal_mapped_range (better name required!) function with a result type that
can perform the range approaches nicely e.g. like the optional "map" member
function. However there are cases where the optional type has some nice
things that the typical range does not. I'm looking into this because I
think there may be powerful idioms for empty_range -> default_value , for
illegal iterator dereference converted to policy-based return. While it
would be easy to just fix find, what about std::ranges::find_if etc? I
appreciate this wouldn't have the mapped_value angle, but the default if
the value doesn't exist is the same problem. I believe reusable
abstractions can be built that will help build the specific use-case
solution, but also improve our set of tools available more generally.

I need to do more keyboard bashing and less flapping of my gums (keys). I
may be the only person excited, but I am genuinely enthused about this
direction.

Neil