Can anyone please explain to me what's going on here?


import std.container;
import std.socket;
import std.typecons;

void main()
{
    /* Doesn't work
    alias Rec_type = Tuple!(Address, "x", int, "y", int, "z");
    RedBlackTree!Rec_type[] records;
    */

    // Works
    alias Rec_type = Tuple!(int, "y", int, "z");
    RedBlackTree!Rec_type[] records;
}


Why am I not allowed to put Address types in tuples?

On Wed, 23 Jan 2019 15:56:15 +0000, Steven O wrote:
> Why am I not allowed to put Address types in tuples?

As always, it helps a lot to post the error message the compiler gave you. The message is:

/usr/include/dmd/phobos/std/functional.d-mixin-215(215): Error: template
std.typecons.Tuple!(Address, int).Tuple.opCmp cannot deduce function from
argument types !()(Tuple!(Address, int)) inout, candidates are:
/usr/include/dmd/phobos/std/typecons.d(806):        std.typecons.Tuple!
(Address, int).Tuple.opCmp(R)(R rhs) if (areCompatibleTuples!
(typeof(this), R, "<"))
/usr/include/dmd/phobos/std/typecons.d(820):        std.typecons.Tuple!
(Address, int).Tuple.opCmp(R)(R rhs) if (areCompatibleTuples!
(typeof(this), R, "<"))
/usr/include/dmd/phobos/std/container/rbtree.d(866): Error: template
instance `std.functional.binaryFun!("a < b", "a", "b").binaryFun!
(inout(Tuple!(Address, int)), Tuple!(Address, int))` error instantiating
scratch.d(13):        instantiated from here: RedBlackTree!(Tuple!
(Address, int), "a < b", false)

The last line says that the error came when trying to instantiate the RedBlackTree template. At that point, Tuple was instantiated. There is no issue putting an address in a Tuple.

The issue is that Address doesn't have a comparison operator defined, so the resulting tuple type can't be compared with the standard operators. You need to define your own function for comparing the tuples in question and pass that to RedBlackTree.

On Wednesday, 23 January 2019 at 16:30:33 UTC, Neia Neutuladh wrote:
> As always, it helps a lot to post the error message the compiler gave you.

Sorry about that.

> The issue is that Address doesn't have a comparison operator defined, so the resulting tuple type can't be compared with the standard operators. You need to define your own function for comparing the tuples in question and pass that to RedBlackTree.

Is there any documentation or examples of how to do that? The RedBlackTree documentation gives trivial examples like

auto maxTree = redBlackTree!"a > b"(iota(5));

but, that doesn't really help me figure out how to do something more complex.

On Thu, 24 Jan 2019 01:35:57 +0000, Steven O wrote:
> Is there any documentation or examples of how to do that? The RedBlackTree documentation gives trivial examples like

You define a function implementing the "less" operation for a pair of Tuple!(Address, int) (or whatever you have).

---
alias V = Tuple!(Address, int);
bool less(V a, V b)
{
  if (a[0].classinfo != b[0].classinfo)
  {
    return a[0].classinfo < b[0].classinfo;
  }
  // do something for InternetAddress, Internet6Address, etc
  return 0;
}
---

Then pass that to RedBlackTree:

---
alias VTree = RedBlackTree!(V, less);
VTree tree = new VTree();
---