reflect: add MapIter.Reset

This allows callers to do (amortized) allocation-free iteration
over many maps.

Fixes #46293

Change-Id: I3aa6134dd00da35b508bd1e3b487332a871a3673
Reviewed-on: https://go-review.googlesource.com/c/go/+/321891
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 90f31bae0a..39c3c0f06e 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1700,6 +1700,18 @@ func (it *MapIter) Next() bool {
 	return mapiterkey(&it.hiter) != nil
 }
 
+// Reset modifies it to iterate over v.
+// It panics if v's Kind is not Map and v is not the zero Value.
+// Reset(Value{}) causes it to not to refer to any map,
+// which may allow the previously iterated-over map to be garbage collected.
+func (it *MapIter) Reset(v Value) {
+	if v.IsValid() {
+		v.mustBe(Map)
+	}
+	it.m = v
+	it.hiter = hiter{}
+}
+
 // MapRange returns a range iterator for a map.
 // It panics if v's Kind is not Map.
 //