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.