r203293 - [C++11] Revert uses of lambdas with array_pod_sort.

[Arthur O'Dwyer]

On Sun, Mar 9, 2014 at 12:14 AM, Ahmed Charles <acharles at outlook.com> wrote:
> ________________________________
>> Date: Fri, 7 Mar 2014 14:15:32 -0800
>> Subject: Re: r203293 - [C++11] Revert uses of lambdas with array_pod_sort.
>> From: richard at metafoo.co.uk
>> To: arthur.j.odwyer at gmail.com
>> CC: cfe-commits at cs.uiuc.edu
>>
>> On Fri, Mar 7, 2014 at 2:10 PM, Arthur O'Dwyer <arthur.j.odwyer at gmail.com> wrote:
>>
>>> - The spaceship operator <=> is equivalent to (a < b) ? -1 : (a> b)
>>
>> The more usual idiom of
>>
>>    if (!=)
>>      return <
>>
>> naturally extends to lexicographical comparisons of multiple things. I
>> don't see how to do that with (a<b?-1:a>b).

Recognizing that this is off-topic and also completely "personal
style", I think your classification of (a != b) ? (a < b) ? -1 : +1 :
0 as "more usual" is subjective. In my subjective experience, (a < b)
? -1 : (a > b) is "more usual". ;)

If you have multiple fields in your struct, then (as Ahmed suggests)
the most stylish idiom is to 'tie' your fields together.

    template<typename T> int spaceship(const T& a, const T& b) {
return (a < b) ? -1 : (a > b); }  // trivial

    struct S {
        int x, y, z;
        decltype(auto) toTuple() const { return std::tie(x,y,z); }  //
unfortunately annoying to type pre-C++14, but still trivial
    };

    bool operator< (const S& a, const S& b) { return a.toTuple() <
b.toTuple(); }  // trivial
    int spaceship(const S& a, const S& b) { return
spaceship(a.toTuple(), b.toTuple()); }  // trivial

Unfortunately I don't think C++1y has added any special library
support for the spaceship function, which means all the solutions
named so far involve computing (a.x==b.x), (a.y==b.y), etc., up to the
point of the first difference, TWICE. You can work around the lack of
std::spaceship (or std::tuple::tail or whatever you choose as your
missing primitive) and make the stylish solution also efficient, but I
won't write it here because the guts are embarrassingly ugly. :P

–Arthur