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digitalmars.D.learn - C++'s "defaulted comparison operators" proposal

reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
Can you come up with a D library solution to the following C++11 proposal:

   http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3950.html

The idea is to be able to have standard comparison behavior for structs 
without boilerplate. For example, if the ordering should consider all 
members of the following struct one after the other (first 'i', then 
'l', and finally 's'):

struct S
{
     int i;
     long l;
     string s;

     mixin DefaultComparisons!();    // something like this?
}

Or perhaps with an explicit comparison order? For example, to use 'l' as 
the primary key instead, and 'i' as the secondary key, and to ignore 
'buffer' altogether:

struct S
{
      Order(2)    int i;
      Order(1)    long l;
      OrderIgnore string buffer;

     mixin DefaultComparisons!();    // ?
}

Thank you,
Ali
Jun 17 2014
next sibling parent Artur Skawina via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On 06/18/14 07:49, Ali Çehreli via Digitalmars-d-learn wrote:


 The idea is to be able to have standard comparison behavior for structs
without boilerplate. For example, if the ordering should consider all members
of the following struct one after the other (first 'i', then 'l', and finally
's'):


Isn't this already implicitly done? (I thought it was just my old compiler
version that did not yet support this)

Anyway, something like this wrapped in an appropriate template
should do:

   int opCmp()(const typeof(this) b) const {
      foreach (I, _; typeof(this.tupleof))
         static if (is(typeof(this.tupleof[I].opCmp(b.tupleof[I])))) {
            if (auto d = this.tupleof[I].opCmp(b.tupleof[I]))
                return d;
         } else {
            if (auto d = this.tupleof[I] - b.tupleof[I])
                return d;
         }
      return 0;
   }

and

http://forum.dlang.org/post/mailman.1230.1333044904.4860.digitalmars-d puremagic.com

artur
Jun 18 2014
prev sibling parent "Ellery Newcomer" <ellery-newcomer utulsa.edu> writes:
On Wednesday, 18 June 2014 at 05:49:30 UTC, Ali Çehreli wrote:

 Can you come up with a D library solution to the following 
 C++11 proposal:

   
 http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3950.html


did this years ago (or something similar, at least):
https://bitbucket.org/ariovistus/multi_index/src/0993f8604c1438f6d9f26f580939b42fdd159a67/src/multi_index.d?at=default#cl-4654

example:

https://bitbucket.org/ariovistus/multi_index/src/0993f8604c1438f6d9f26f580939b42fdd159a67/unittests/multi_compare.d?at=default
Jun 18 2014