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digitalmars.D.learn - auto ref function parameters in a free function

reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
This doesn't work:

bool less_than (T)(auto ref T a, auto ref T b)
{
	return a < b;
}

Error: auto can only be used for template function parameters

What am I doing wrong? Is this not a template function?
Aug 03 2014
next sibling parent reply "Martijn Pot" <martijnpot52 gmail.com> writes:
On Sunday, 3 August 2014 at 19:07:32 UTC, Vlad Levenfeld wrote:

 This doesn't work:

 bool less_than (T)(auto ref T a, auto ref T b)
 {
 	return a < b;
 }

 Error: auto can only be used for template function parameters

 What am I doing wrong? Is this not a template function?


I think you can just skip 'auto'.

Have a look at http://dlang.org/function.html , especially the 
function parameters section.
Aug 03 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
This would make the function always take its argument by 
reference. I'm trying to use the feature here:

http://dlang.org/template.html

from the section Function Templates with Auto Ref Parameters
Aug 03 2014
parent reply "Martijn Pot" <martijnpot52 gmail.com> writes:
On Sunday, 3 August 2014 at 19:25:32 UTC, Vlad Levenfeld wrote:

 This would make the function always take its argument by 
 reference. I'm trying to use the feature here:

 http://dlang.org/template.html

 from the section Function Templates with Auto Ref Parameters


I thought I finally saw a question I could (help) answer...

What is the benefit of 'auto ref' over 'in' as you are changing a 
nor b?
Aug 03 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
On Sunday, 3 August 2014 at 20:10:39 UTC, Martijn Pot wrote:

 What is the benefit of 'auto ref' over 'in' as you are changing 
 a nor b?


Because

   less_than (T)(T a, T b)

(or in, or const T) will copy a and b on their way into the 
function. Usually this is ok but certain data structures I use 
are not intended to be copied, or they are just very large or 
have very elaborate constructors that take up time and resources, 
or something.

I don't know if inlining would solve this or not but as far as I 
know there is no way to rely on inlining currently.
Aug 03 2014
parent "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
On Sunday, 3 August 2014 at 21:24:03 UTC, Vlad Levenfeld wrote:

 certain data structures I use are not intended to be copied, 
 .....


although these cases are probably better off being compared by 
some kind of key rather than directly... so, auto ref isn't 
necessary here, it was just something I had tried and was 
surprised at the error message I got.
Aug 03 2014
prev sibling parent reply "anonymous" <anonymous example.com> writes:
On Sunday, 3 August 2014 at 19:07:32 UTC, Vlad Levenfeld wrote:

 bool less_than (T)(auto ref T a, auto ref T b)
 {
 	return a < b;
 }

 Error: auto can only be used for template function parameters


Works for me with dmd versions back to 2.060. What compiler are
you using?
Aug 03 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
On Sunday, 3 August 2014 at 19:26:28 UTC, anonymous wrote:

 Works for me with dmd versions back to 2.060. What compiler are
 you using?


dmd 2.065
Aug 03 2014
parent reply "anonymous" <anonymous example.com> writes:
On Sunday, 3 August 2014 at 19:30:38 UTC, Vlad Levenfeld wrote:

 On Sunday, 3 August 2014 at 19:26:28 UTC, anonymous wrote:

 Works for me with dmd versions back to 2.060. What compiler are
 you using?


 dmd 2.065


Here's how I'm testing this:

$ dmd | head -n 1
DMD64 D Compiler v2.065
$ cat less_than.d
bool less_than (T)(auto ref T a, auto ref T b)
{
          return a < b;
}
void main()
{
          int a = 1, b = 2;
          assert(less_than(a, b));
          assert(less_than(a, 2));
          assert(less_than(1, b));
          assert(less_than(1, 2));
}
$ dmd less_than.d && echo ok
ok

If this exact code errors for you, it ... uhh ... could be
platform specific? Are you on Windows?
Aug 03 2014
parent reply "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
On Sunday, 3 August 2014 at 19:43:53 UTC, anonymous wrote:

 If this exact code errors for you, it ... uhh ... could be
 platform specific? Are you on Windows?


Debian, and your code works for me. I figured out the problem - I 
made less_than to serve as a default sorting predicate, so in a 
few places there is something like this:

void sort (R, T = ElementType!R, alias compare = less_than!T)(R 
range, T item)
{...}

And since auto ref parameters need actual parameters to deduce 
whether they should be ref or not, less_than!T isn't an 
instantiated template - its still an alias, because it still must 
choose between having ref and value parameters.

So, it looks like I can't use auto ref with less-than. Which is 
fine, if I need to quickly compare large structures I will just 
pass in a ref_less_than predicate or something.
Aug 03 2014
parent reply Artur Skawina via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
On 08/03/14 23:19, Vlad Levenfeld via Digitalmars-d-learn wrote:

 
 I made less_than to serve as a default sorting predicate, so in a few places
there is something like this:
 
 void sort (R, T = ElementType!R, alias compare = less_than!T)(R range, T item)
 {...}


 void sort (R, T = ElementType!R, alias compare = less_than)(R range, T item)

[should work iff the sort implementation only calls the predicate]

artur
Aug 03 2014
parent "Vlad Levenfeld" <vlevenfeld gmail.com> writes:
On Sunday, 3 August 2014 at 21:47:06 UTC, Artur Skawina via 
Digitalmars-d-learn wrote:

  void sort (R, T = ElementType!R, alias compare = less_than)(R 
 range, T item)

 [should work iff the sort implementation only calls the 
 predicate]

 artur


This works! Thanks!
Aug 03 2014