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Re: Further C++ operators for position

From: Hans Åberg
Subject: Re: Further C++ operators for position
Date: Mon, 4 Nov 2019 18:46:00 +0100

> On 4 Nov 2019, at 07:52, Akim Demaille <address@hidden> wrote:
>> Le 4 nov. 2019 à 05:27, Matthew Fernandez <address@hidden> a écrit :
>> I recently had a use case for comparing source positions coming out of a C++ 
>> Bison-produced parser. While operator== and operator!= are implemented on 
>> the position class [0], the ordering operators (<, <=, >, >=) are not. It 
>> was relatively straightforward to implement these myself, but I was 
>> wondering if these were of wider use and should live upstream in Bison’s 
>> position implementation. Perhaps there is some history behind this or some 
>> deliberate omission of these operators? Just wanted to ask if there’s a 
>> reason these don’t already exist before thinking about posting a patch. I’m 
>> not subscribed to the list, so please CC me in replies.
> The semantics for line and columns are quite clear, so comparing Positions in 
> the same file is quite well defined.
> But what should you do when the files are different?  (And Locations are 
> intervals, so there's no way to compare them totally in a natural order.)
> What we can do, though, is offer implementations for std::less, that would 
> blindly apply the lexicographic order in both cases.
> But the case of file names remains troublesome: should we compare the pointer 
> addresses (super fast, but non deterministic) or the pointees (super slow, 
> but deterministic)?

As it is not semantically well defined, but that one might want a total order 
for use in types like std::map, a pointer comparison might be used. Also 
containers like std::unordered_set have a total order through the iterators, so 
it fits with C++ paradigms, I would think.

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