ICU-22876 add U_SHOW_CPLUSPLUS_HEADER_API for C++ header-only APIs

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__

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"