opCmp with and without const

[Basile B.]

On Friday, 6 December 2019 at 07:03:45 UTC, berni44 wrote:
> In std.typecons, in Tuple there are two opCmp functions, that 
> are almost identical; they only differ by one being const and 
> the other not:
>
>         int opCmp(R)(R rhs)
>         if (areCompatibleTuples!(typeof(this), R, "<"))
>         {
>             static foreach (i; 0 .. Types.length)
>             {
>                 if (field[i] != rhs.field[i])
>                 {
>                     return field[i] < rhs.field[i] ? -1 : 1;
>                 }
>             }
>             return 0;
>         }
>
>         int opCmp(R)(R rhs) const
>         if (areCompatibleTuples!(typeof(this), R, "<"))
>         {
>             static foreach (i; 0 .. Types.length)
>             {
>                 if (field[i] != rhs.field[i])
>                 {
>                     return field[i] < rhs.field[i] ? -1 : 1;
>                 }
>             }
>             return 0;
>         }
>
>
> What is the reason for having this? (I guess, that it's because 
> the function may indirectly call opCmp of other types which may 
> or may not be const.)
>
> My real question is: Can this code duplication be avoided 
> somehow?

Usually `inout` is used. I'm pretty sure this is not possible 
here otherwise it would be done to avoid the dup.

> (I ask, because I've got a PR running, which increases the size 
> of these functions and it doesn't feel good to have two long, 
> almost identical functions.)

Well the content of body could be mixed in