Re: minElement on array of const objects

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On Monday, 13 November 2017 at 10:20:51 UTC, Aurelien Fredouelle 
wrote:
Hi all,

It seems that it is not possible to use minElement on an array 
of const objects:

class A
{
  int val;
}

const(A) doStuff(const(A)[] v)
{
  import std.algorithm.searching : minElement;
  return v.minElement!"a.val";
}

This gets the following compiler error:

std/algorithm/searching.d(1256,28): Error: cannot implicitly 
convert expression (front(r)) of type const(A) to app.A
std/algorithm/searching.d(1286,35): Error: cannot implicitly 
convert expression (r[i]) of type const(A) to app.A
std/algorithm/searching.d(1258,36): Error: template instance 
std.algorithm.searching.extremum!("a.val", "a < b", const(A)[], 
A) error instantiating
std/algorithm/searching.d(3345,24):        instantiated from 
here: extremum!("a.val", "a < b", const(A)[])
source/app.d(11,11):        instantiated from here: 
minElement!("a.val", const(A)[])

Is there a reason why this is not allowed? Shouldn't minElement 
be able to return a const(A) in this situation?

Thanks,
Aurelien

It should not until it uses recursion to find min element.

const(Class) res = ...;
...
return res;

Algorithm tries to assign to res variable and fails because in D 
const(Class) means both const object and const reference to it. 
So we cannot assign another reference to const(Class) variable.