I don't have any experience with using templates. Is it possible to create a RB tree containing structs, where the nodes are ordered by one struct member?

```
import std.stdio;
import std.container;

enum KeyID: uint
{
	KEY_A,
	KEY_S,
	KEY_D,
	KEY_W
}

struct KeyController
{
	KeyID ID;
	bool isDown;
}


void main()
{
	auto rbt = redBlackTree!KeyController;
}

```

When I run this I get a compile error:
```
Error: template instance `std.container.rbtree.RedBlackTree!(KeyController)` does not match template declaration RedBlackTree(T, alias less = "a < b", bool allowDuplicates = false) if (is(typeof(binaryFun!less(T.init, T.init))))
```

On Wednesday, 15 May 2019 at 13:08:18 UTC, Bogdan wrote:
> I don't have any experience with using templates. Is it possible to create a RB tree containing structs, where the nodes are ordered by one struct member?
>
> ```
> import std.stdio;
> import std.container;
>
> enum KeyID: uint
> {
> 	KEY_A,
> 	KEY_S,
> 	KEY_D,
> 	KEY_W
> }
>
> struct KeyController
> {
> 	KeyID ID;
> 	bool isDown;
> }
>
>
> void main()
> {
> 	auto rbt = redBlackTree!KeyController;
> }
>
> ```
>
> When I run this I get a compile error:
> ```
> Error: template instance `std.container.rbtree.RedBlackTree!(KeyController)` does not match template declaration RedBlackTree(T, alias less = "a < b", bool allowDuplicates = false) if (is(typeof(binaryFun!less(T.init, T.init))))
> ```

Key controller cannot be compared by less
which is why it fails, give it an opCmp and it'll work.

On 15.05.2019 16:08, Bogdan wrote:
> I don't have any experience with using templates. Is it possible to create a RB tree containing structs, where the nodes are ordered by one struct member?
> 
> ```
> import std.stdio;
> import std.container;
> 
> enum KeyID: uint
> {
>      KEY_A,
>      KEY_S,
>      KEY_D,
>      KEY_W
> }
> 
> struct KeyController
> {
>      KeyID ID;
>      bool isDown;
> }
> 
> 
> void main()
> {
>      auto rbt = redBlackTree!KeyController;
> }
> 
> ```
> 
> When I run this I get a compile error:
> ```
> Error: template instance `std.container.rbtree.RedBlackTree!(KeyController)` does not match template declaration RedBlackTree(T, alias less = "a < b", bool allowDuplicates = false) if (is(typeof(binaryFun!less(T.init, T.init))))
> ```
> 

You can use predicate for this purpose:
```
auto rbt = redBlackTree!((a, b) => a.ID < b.ID, KeyController);
```

https://run.dlang.io/is/CNRTQf

On Wednesday, 15 May 2019 at 13:15:50 UTC, Stefan Koch wrote:
> Key controller cannot be compared by less
> which is why it fails, give it an opCmp and it'll work.

Works fine, thank you! For some reason, I thought that this template uses references:
```
enum KeyID: uint
{
	KEY_A,
	KEY_S,
	KEY_D,
	KEY_W
}

struct KeyController
{
	KeyID ID;
	bool isDown;

	int opCmp(ref const KeyController other) const {return this.ID - other.ID;}
}

void main()
{
	KeyController[] monitoredKeys = [{KeyID.KEY_A}];

	auto rbt = redBlackTree!KeyController;

	rbt.insert(monitoredKeys);
	writeln(rbt);
	monitoredKeys[0].isDown = true;
	writeln(rbt);
}
```

Output:
```
RedBlackTree([const(KeyController)(KEY_A, false)])
RedBlackTree([const(KeyController)(KEY_A, false)])
```

On Wednesday, 15 May 2019 at 13:19:36 UTC, drug wrote:
> You can use predicate for this purpose:
> ```
> auto rbt = redBlackTree!((a, b) => a.ID < b.ID, KeyController);
> ```
>
> https://run.dlang.io/is/CNRTQf

Even better, thank you!