Re: Should boost have ofind(_if), that returns boost::optional<T&>?

From: Gavin Lambert (boost_at_[hidden])
Date: 2022-04-04 00:46:28

Mere moments ago, quoth I:
> On 4/04/2022 10:58, Marshall Clow wrote:
>> On Apr 3, 2022, at 3:09 PM, Viktor Sehr wrote:
>>>
>>> Why not return a pointer/nullptr instead? pfind_if
>>
>> Why not just check to see if the returned iterator == end () ?
> 
> The annoyance with the iterator == end() implementation is that it's a 
> double indirection -- if you want to access the value afterwards it has 
> to be in a separate statement, which means you can't do it with an 
> rvalue collection.

To be clearer, it's just the need to access end() again that requires 
putting the collection in a variable (assuming that you're using a 
member or range algorithm, or something that otherwise doesn't already 
require passing separate begin/end iterators).

> The optional return doesn't have that problem; because it copies the 
> value, the value remains valid when returned from an rvalue collection.

And this applies only for optional<T>, not optional<T&> as was being 
suggested.  So a different case that's also not going to be valid for 
rvalue collections.