Implementing p0515 - spaceship operator

[David Brown]

On 11/01/18 11:13, Jonathan Wakely wrote:
> t all cleaOn 11 January 2018 at 10:05, David Brown wrote:
>> Maybe it is easier to say "gcc supports <=> in C++2a, and as an
>> extension also supports it in C and C++ of any standard" ?  I don't
>> believe there is any way for it to conflict with existing valid code, so
>> it would do no harm as a gcc extension like that - and C users can then
>> use it too.
> 
> It's not very useful in C because you need the comparison category
> types, which are classes defined in <compare> (see
> http://en.cppreference.com/w/cpp/header/compare)
> 
> C doesn't have those types, and can't define anything close.
> 

Surely you can get very close by simply returning an int, value -1, 0 or
1?  That is what other languages (like PHP) do for their <=> operator.
There are complications - such as for floating point when you have NaNs.
 But I think you could have a very successful operator if you defined "a
<=> b" to be the same as "(a > b) - (a < b)".

Whether it would be particularly /useful/ or not is another matter.  I
was thinking mainly in terms of saving effort when making C++2a support
- rather than having to making the new operator conditional on a
particular version of the standards, it could be accepted in any version.

> And it's non-conforming to declare those types in pre-C++2a, because
> the names could be used by user programs.
> 
> Potentially the types could be defined with reserved names like
> __strong_ordering, and then make std::strong_ordering a typedef for
> that, but there are also changes to the language spec that go with the
> new operator, and enabling those pre-C++2a could change the meaning of
> valid code.
> 
> So it's not ar it does no harm.
>