From 5447a230701c156383fa6c6887d8f0babf0fd34d Mon Sep 17 00:00:00 2001
From: Markus Scherer <markus.icu@gmail.com>
Date: Thu, 12 Sep 2024 14:22:15 -0700
Subject: [PATCH] ICU-22876 add U_SHOW_CPLUSPLUS_HEADER_API for C++ header-only
 APIs

---
 icu4c/source/common/unicode/uset.h   |  4 ++--
 icu4c/source/common/unicode/utypes.h | 19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/icu4c/source/common/unicode/uset.h b/icu4c/source/common/unicode/uset.h
index 7cf0565f485f..cca2b0278a80 100644
--- a/icu4c/source/common/unicode/uset.h
+++ b/icu4c/source/common/unicode/uset.h
@@ -1325,7 +1325,7 @@ U_CAPI UBool U_EXPORT2
 uset_getSerializedRange(const USerializedSet* set, int32_t rangeIndex,
                         UChar32* pStart, UChar32* pEnd);
 
-#if U_SHOW_CPLUSPLUS_API
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
 #ifndef U_HIDE_DRAFT_API
 
 namespace U_HEADER_ONLY_NAMESPACE {
@@ -1893,6 +1893,6 @@ class USetElements {
 }  // namespace U_HEADER_ONLY_NAMESPACE
 
 #endif  // U_HIDE_DRAFT_API
-#endif  // U_SHOW_CPLUSPLUS_API
+#endif  // U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
 
 #endif  // __USET_H__
diff --git a/icu4c/source/common/unicode/utypes.h b/icu4c/source/common/unicode/utypes.h
index f1a89770f1f7..ba27be222349 100644
--- a/icu4c/source/common/unicode/utypes.h
+++ b/icu4c/source/common/unicode/utypes.h
@@ -54,22 +54,37 @@
  * integer and other types.
  */
 
+/** @{ API visibility control */
 
 /**
  * \def U_SHOW_CPLUSPLUS_API
+ * When defined to 1 (=default) and compiled with a C++ compiler, both C and C++ APIs are visible.
+ * Otherwise, only C APIs are visible; this is for C++ users who want to
+ * restrict their usage to binary stable C APIs exported by ICU DLLs.
+ * @internal
+ */
+/**
+ * \def U_SHOW_CPLUSPLUS_HEADER_API
+ * When defined to 1 (=default) and compiled with a C++ compiler, C++ header-only APIs are visible.
+ * This is for C++ users who restrict their usage to binary stable C APIs exported by ICU DLLs
+ * (U_SHOW_CPLUSPLUS_API=0)
+ * but who still want to use C++ header-only APIs which do not rely on ICU DLL exports.
  * @internal
  */
 #ifdef __cplusplus
 #   ifndef U_SHOW_CPLUSPLUS_API
 #       define U_SHOW_CPLUSPLUS_API 1
 #   endif
+#   ifndef U_SHOW_CPLUSPLUS_HEADER_API
+#       define U_SHOW_CPLUSPLUS_HEADER_API 1
+#   endif
 #else
 #   undef U_SHOW_CPLUSPLUS_API
 #   define U_SHOW_CPLUSPLUS_API 0
+#   undef U_SHOW_CPLUSPLUS_HEADER_API
+#   define U_SHOW_CPLUSPLUS_HEADER_API 0
 #endif
 
-/** @{ API visibility control */
-
 /**
  * \def U_HIDE_DRAFT_API
  * Define this to 1 to request that draft API be "hidden"