I need to user Red-Black trees with non-default sorting.
Here is a minimal example
---- >8
import std.container;
struct CC {
    int a;
    int b; }

bool less(const ref CC lhs, const ref CC rhs) {
    if (lhs.a != rhs.a)
        return lhs.a < rhs.a;
    else
        return lhs.b < rhs.b; }

void main() {
    auto t = new RedBlackTree!(CC, "a.a != b.a ? a.a < b.a : a.b < b.b"); }
8< ----

It works, but I would like to pass the function "less" as comparison operator since in my real problem the comparison is a more complex.

I tried this ways:
    auto t = new RedBlackTree!(CC, less);
    auto t = new RedBlackTree!(CC, CC);
    auto t = new RedBlackTree!(CC, &less);
    auto t = new RedBlackTree!(CC, "less(a,b)");
    auto t = new RedBlackTree!(CC, "what the hell!? just call less! Is it too much to ask?");

None of them works, I am stuck. The documentation states:
(quote from: http://dlang.org/phobos/std_container.html#.RedBlackTree )
"""
To use a different comparison than "a < b", pass a different operator string that can be used by std.functional.binaryFun, or pass in a function, delegate, functor, or any type where less(a, b) results in a bool value.
"""

What is wrong? How I am supposed to pass less address to do the comparison?

Thanks

On 05/12/2013 02:09 PM, Paolo Bolzoni wrote:

> I need to user Red-Black trees with non-default sorting.
> Here is a minimal example
> ---- >8
> import std.container;
> struct CC {
>      int a;
>      int b; }
>
> bool less(const ref CC lhs, const ref CC rhs) {
>      if (lhs.a != rhs.a)
>          return lhs.a < rhs.a;
>      else
>          return lhs.b < rhs.b; }
>
> void main() {
>      auto t = new RedBlackTree!(CC, "a.a != b.a ? a.a < b.a : a.b <
> b.b"); }
> 8< ----
>
> It works, but I would like to pass the function "less" as comparison
> operator since in my real problem the comparison is a more complex.

I think this is an issue with the template constraint of RedBlackTree. It passes the alias template parameter through binaryFun and I think that fails the check:

final class RedBlackTree(T, alias less = "a < b", bool allowDuplicates = false)
    if(is(typeof(binaryFun!less(T.init, T.init))))

A workaround:

    auto t = new RedBlackTree!(CC, (a, b) => less(a, b));

Ali

On Sun, 12 May 2013 17:09:56 -0400, Paolo Bolzoni <paolo.bolzoni@g.invalid> wrote:

> What is wrong? How I am supposed to pass less address to do the comparison?

I think it is this issue:

http://d.puremagic.com/issues/show_bug.cgi?id=9513

I am going to fix this, soon.

-Steve

On Monday, 13 May 2013 at 15:26:12 UTC, Steven Schveighoffer wrote:
> On Sun, 12 May 2013 17:09:56 -0400, Paolo Bolzoni <paolo.bolzoni@g.invalid> wrote:
>
>> What is wrong? How I am supposed to pass less address to do the comparison?
>
> I think it is this issue:
>
> http://d.puremagic.com/issues/show_bug.cgi?id=9513
>
> I am going to fix this, soon.

Seems so.  For now, you can cheat the too strict "binaryFun" condition in this way:

bool less (T) (auto ref T lhs, auto ref T rhs)

As I understand it, this creates one instance (without refs) for the "binaryFun" condition, while another (with refs) is actually used.