Supporting foreach (k, v; T.init) for a user-defined (container) type

[H. S. Teoh]

On Mon, Oct 24, 2022 at 09:26:14PM +0000, Per Nordlöw via Digitalmars-d-learn wrote:
> What property of a container (type) `T` enables iteration as
> 
> ```d
> foreach (k, v; T.init)
> {
>     ...
> }
> ```
> 
> ? I thought it sufficed to define `T.byKeyValue` but its presence seem
> to have no effect.

You want opApply.

Full working example:

------
struct A {
	static int[string] impl;
	static this() {
		impl = [
			"a": 1,
			"b": 2,
		];
	}

	int opApply(scope int delegate(string a, int b) dg) {
		foreach (k, v; impl) {
			auto r = dg(k, v);
			if (r) return r;
		}
		return 0;
	}
}

void main() {
	import std;
	A a;
	foreach (k, v; a) {
		writefln("%s -> %s", k, v);
	}
}
------


T

-- 
Computers shouldn't beep through the keyhole.