From 0a7d70851cbae5aee58d97a8b24fdbdddd5fb0b0 Mon Sep 17 00:00:00 2001
From: Sergey Zubkov <cubbi@cubbi.com>
Date: Mon, 8 Aug 2022 15:11:21 -0400
Subject: [PATCH] F.17: note about passing reference wrappers by value (issue
 #1948)

---
 CppCoreGuidelines.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index e7d75a7..9d14c56 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -2954,6 +2954,18 @@ This makes it clear to callers that the object is assumed to be modified.
 
 ##### Note
 
+Some user-defined and standard library types, such as `span<T>` or the iterators
+are [cheap to copy](#Rf-in) and may be passed by value, while doing so has
+mutable (in-out) reference semantics:
+
+    void increment_all(span<int> a)
+    {
+      for (auto&& e : a)
+        ++e;
+    }
+
+##### Note
+
 A `T&` argument can pass information into a function as well as out of it.
 Thus `T&` could be an in-out-parameter. That can in itself be a problem and a source of errors: