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digitalmars.D.learn - std.socket.Address not allowed in tuples

reply Steven O <oliver.steven gmail.com> writes:
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?
Jan 23 2019
parent reply Neia Neutuladh <neia ikeran.org> writes:
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.
Jan 23 2019
parent reply Steven O <oliver.steven gmail.com> writes:
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.
Jan 23 2019
parent Neia Neutuladh <neia ikeran.org> writes:
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();
---
Jan 23 2019