diff --git a/icu4c/source/common/characterproperties.cpp b/icu4c/source/common/characterproperties.cpp
index 963ac8342151..db42fa4eea70 100644
--- a/icu4c/source/common/characterproperties.cpp
+++ b/icu4c/source/common/characterproperties.cpp
@@ -24,7 +24,7 @@
 #include "umutex.h"
 #include "uprops.h"
 
-using icu::LocalPointer;
+using U_ICU_NAMESPACE_OR_INTERNAL::LocalPointer;
 #if !UCONFIG_NO_NORMALIZATION
 using icu::Normalizer2Factory;
 using icu::Normalizer2Impl;
@@ -340,7 +340,7 @@ UnicodeSet *makeSet(UProperty property, UErrorCode &errorCode) {
 UCPMap *makeMap(UProperty property, UErrorCode &errorCode) {
     if (U_FAILURE(errorCode)) { return nullptr; }
     uint32_t nullValue = property == UCHAR_SCRIPT ? USCRIPT_UNKNOWN : 0;
-    icu::LocalUMutableCPTriePointer mutableTrie(
+    U_ICU_NAMESPACE_OR_INTERNAL::LocalUMutableCPTriePointer mutableTrie(
         umutablecptrie_open(nullValue, nullValue, &errorCode));
     const UnicodeSet *inclusions =
         icu::CharacterProperties::getInclusionsForProperty(property, errorCode);
diff --git a/icu4c/source/common/locavailable.cpp b/icu4c/source/common/locavailable.cpp
index 4a2600e88c59..e02ad88a17e9 100644
--- a/icu4c/source/common/locavailable.cpp
+++ b/icu4c/source/common/locavailable.cpp
@@ -208,7 +208,8 @@ UBool U_CALLCONV uloc_cleanup() {
 void U_CALLCONV loadInstalledLocales(UErrorCode& status) {
     ucln_common_registerCleanup(UCLN_COMMON_ULOC, uloc_cleanup);
 
-    icu::LocalUResourceBundlePointer rb(ures_openDirect(nullptr, "res_index", &status));
+    U_ICU_NAMESPACE_OR_INTERNAL::LocalUResourceBundlePointer rb(
+        ures_openDirect(nullptr, "res_index", &status));
     AvailableLocalesSink sink;
     ures_getAllItemsWithFallback(rb.getAlias(), "", sink, status);
 }
diff --git a/icu4c/source/common/locdispnames.cpp b/icu4c/source/common/locdispnames.cpp
index d3521e879b60..294d8edb8c10 100644
--- a/icu4c/source/common/locdispnames.cpp
+++ b/icu4c/source/common/locdispnames.cpp
@@ -37,6 +37,9 @@
 #include "ureslocs.h"
 #include "ustr_imp.h"
 
+using U_ICU_NAMESPACE_OR_INTERNAL::LocalUEnumerationPointer;
+using U_ICU_NAMESPACE_OR_INTERNAL::LocalUResourceBundlePointer;
+
 // C++ API ----------------------------------------------------------------- ***
 
 U_NAMESPACE_BEGIN
@@ -313,7 +316,7 @@ _getStringOrCopyKey(const char *path, const char *locale,
 
     if(itemKey==nullptr) {
         /* top-level item: normal resource bundle access */
-        icu::LocalUResourceBundlePointer rb(ures_open(path, locale, &errorCode));
+        LocalUResourceBundlePointer rb(ures_open(path, locale, &errorCode));
 
         if(U_SUCCESS(errorCode)) {
             s=ures_getStringByKey(rb.getAlias(), tableKey, &length, &errorCode);
@@ -539,9 +542,9 @@ uloc_getDisplayName(const char *locale,
     {
         UErrorCode status = U_ZERO_ERROR;
 
-        icu::LocalUResourceBundlePointer locbundle(
+        LocalUResourceBundlePointer locbundle(
                 ures_open(U_ICUDATA_LANG, displayLocale, &status));
-        icu::LocalUResourceBundlePointer dspbundle(
+        LocalUResourceBundlePointer dspbundle(
                 ures_getByKeyWithFallback(locbundle.getAlias(), _kLocaleDisplayPattern, nullptr, &status));
 
         separator=ures_getStringByKeyWithFallback(dspbundle.getAlias(), _kSeparator, &sepLen, &status);
@@ -613,7 +616,7 @@ uloc_getDisplayName(const char *locale,
         int32_t langPos=0; /* position in output of language substitution */
         int32_t restLen=0; /* length of 'everything else' substitution */
         int32_t restPos=0; /* position in output of 'everything else' substitution */
-        icu::LocalUEnumerationPointer kenum; /* keyword enumeration */
+        LocalUEnumerationPointer kenum; /* keyword enumeration */
 
         /* prefix of pattern, extremely likely to be empty */
         if(sub0Pos) {
@@ -855,11 +858,11 @@ uloc_getDisplayKeywordValue(   const char* locale,
         int32_t dispNameLen = 0;
         const char16_t *dispName = nullptr;
 
-        icu::LocalUResourceBundlePointer bundle(
+        LocalUResourceBundlePointer bundle(
                 ures_open(U_ICUDATA_CURR, displayLocale, status));
-        icu::LocalUResourceBundlePointer currencies(
+        LocalUResourceBundlePointer currencies(
                 ures_getByKey(bundle.getAlias(), _kCurrencies, nullptr, status));
-        icu::LocalUResourceBundlePointer currency(
+        LocalUResourceBundlePointer currency(
                 ures_getByKeyWithFallback(currencies.getAlias(), keywordValue.data(), nullptr, status));
 
         dispName = ures_getStringByIndex(currency.getAlias(), UCURRENCY_DISPLAY_NAME_INDEX, &dispNameLen, status);
diff --git a/icu4c/source/common/locresdata.cpp b/icu4c/source/common/locresdata.cpp
index 725e66091598..35b7ff3f1e59 100644
--- a/icu4c/source/common/locresdata.cpp
+++ b/icu4c/source/common/locresdata.cpp
@@ -59,7 +59,8 @@ uloc_getTableStringWithFallback(const char *path, const char *locale,
      * this falls back through the locale's chain to root
      */
     errorCode=U_ZERO_ERROR;
-    icu::LocalUResourceBundlePointer rb(ures_open(path, locale, &errorCode));
+    U_ICU_NAMESPACE_OR_INTERNAL::LocalUResourceBundlePointer rb(
+        ures_open(path, locale, &errorCode));
 
     if(U_FAILURE(errorCode)) {
         /* total failure, not even root could be opened */
diff --git a/icu4c/source/common/locutil.cpp b/icu4c/source/common/locutil.cpp
index a257ec593a80..f5623f2fe4ae 100644
--- a/icu4c/source/common/locutil.cpp
+++ b/icu4c/source/common/locutil.cpp
@@ -233,7 +233,8 @@ LocaleUtility::getAvailableLocaleNames(const UnicodeString& bundleID)
             CharString cbundleID;
             cbundleID.appendInvariantChars(bundleID, status);
             const char* path = cbundleID.isEmpty() ? nullptr : cbundleID.data();
-            icu::LocalUEnumerationPointer uenum(ures_openAvailableLocales(path, &status));
+            U_ICU_NAMESPACE_OR_INTERNAL::LocalUEnumerationPointer uenum(
+                ures_openAvailableLocales(path, &status));
             for (;;) {
                 const char16_t* id = uenum_unext(uenum.getAlias(), nullptr, &status);
                 if (id == nullptr) {
diff --git a/icu4c/source/common/uloc_tag.cpp b/icu4c/source/common/uloc_tag.cpp
index 4a5a83a6c548..0678c15c50e5 100644
--- a/icu4c/source/common/uloc_tag.cpp
+++ b/icu4c/source/common/uloc_tag.cpp
@@ -27,6 +27,9 @@
 #include "ulocimp.h"
 #include "uassert.h"
 
+using U_ICU_NAMESPACE_OR_INTERNAL::LocalPointer;
+using U_ICU_NAMESPACE_OR_INTERNAL::LocalUEnumerationPointer;
+
 namespace {
 
 /* struct holding a single variant */
@@ -354,10 +357,6 @@ const char*
 ultag_getLegacy(const ULanguageTag* langtag);
 #endif
 
-}  // namespace
-
-U_NAMESPACE_BEGIN
-
 /**
  * \class LocalULanguageTagPointer
  * "Smart pointer" class, closes a ULanguageTag via ultag_close().
@@ -369,7 +368,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalULanguageTagPointer, ULanguageTag, ultag_close);
 
-U_NAMESPACE_END
+}  // namespace
 
 /*
 * -------------------------------------------------
@@ -869,7 +868,7 @@ namespace {
 */
 
 bool
-_addVariantToList(VariantListEntry **first, icu::LocalPointer<VariantListEntry> var) {
+_addVariantToList(VariantListEntry **first, LocalPointer<VariantListEntry> var) {
     if (*first == nullptr) {
         var->next = nullptr;
         *first = var.orphan();
@@ -1212,7 +1211,7 @@ _appendVariantsToLanguageTag(std::string_view localeID, icu::ByteSink& sink, boo
                     if (_isVariantSubtag(pVar, -1)) {
                         if (uprv_strcmp(pVar, POSIX_VALUE) || buf.length() != static_cast<int32_t>(uprv_strlen(POSIX_VALUE))) {
                             /* emit the variant to the list */
-                            icu::LocalPointer<VariantListEntry> var(new VariantListEntry, status);
+                            LocalPointer<VariantListEntry> var(new VariantListEntry, status);
                             if (U_FAILURE(status)) {
                                 break;
                             }
@@ -1284,7 +1283,7 @@ _appendKeywordsToLanguageTag(const char* localeID, icu::ByteSink& sink, bool str
     icu::MemoryPool<ExtensionListEntry> extPool;
     icu::MemoryPool<icu::CharString> strPool;
 
-    icu::LocalUEnumerationPointer keywordEnum(uloc_openKeywords(localeID, &status));
+    LocalUEnumerationPointer keywordEnum(uloc_openKeywords(localeID, &status));
     if (U_FAILURE(status) && !hadPosix) {
         return;
     }
@@ -1984,7 +1983,7 @@ ultag_parse(const char* tag, int32_t tagLen, int32_t* parsedLen, UErrorCode& sta
     char *pSubtag, *pNext, *pLastGoodPosition;
     int32_t subtagLen;
     int32_t extlangIdx;
-    icu::LocalPointer<ExtensionListEntry> pExtension;
+    LocalPointer<ExtensionListEntry> pExtension;
     char *pExtValueSubtag, *pExtValueSubtagEnd;
     int32_t i;
     bool privateuseVar = false;
@@ -2011,7 +2010,7 @@ ultag_parse(const char* tag, int32_t tagLen, int32_t* parsedLen, UErrorCode& sta
     *(tagBuf + tagLen) = 0;
 
     /* create a ULanguageTag */
-    icu::LocalULanguageTagPointer t(
+    LocalULanguageTagPointer t(
             static_cast<ULanguageTag*>(uprv_malloc(sizeof(ULanguageTag))));
     if (t.isNull()) {
         uprv_free(tagBuf);
@@ -2197,7 +2196,7 @@ ultag_parse(const char* tag, int32_t tagLen, int32_t* parsedLen, UErrorCode& sta
         if (next & VART) {
             if (_isVariantSubtag(pSubtag, subtagLen) ||
                (privateuseVar && _isPrivateuseVariantSubtag(pSubtag, subtagLen))) {
-                icu::LocalPointer<VariantListEntry> var(new VariantListEntry, status);
+                LocalPointer<VariantListEntry> var(new VariantListEntry, status);
                 if (U_FAILURE(status)) {
                     return nullptr;
                 }
@@ -2609,7 +2608,7 @@ ulocimp_toLanguageTag(const char* localeID,
         int kwdCnt = 0;
         bool done = false;
 
-        icu::LocalUEnumerationPointer kwdEnum(uloc_openKeywords(canonical.data(), &tmpStatus));
+        LocalUEnumerationPointer kwdEnum(uloc_openKeywords(canonical.data(), &tmpStatus));
         if (U_SUCCESS(tmpStatus)) {
             kwdCnt = uenum_count(kwdEnum.getAlias(), &tmpStatus);
             if (kwdCnt == 1) {
@@ -2691,7 +2690,7 @@ ulocimp_forLanguageTag(const char* langtag,
     int32_t i, n;
     bool noRegion = true;
 
-    icu::LocalULanguageTagPointer lt(ultag_parse(langtag, tagLen, parsedLength, status));
+    LocalULanguageTagPointer lt(ultag_parse(langtag, tagLen, parsedLength, status));
     if (U_FAILURE(status)) {
         return;
     }
diff --git a/icu4c/source/common/unicode/localpointer.h b/icu4c/source/common/unicode/localpointer.h
index 487ddb48b780..1bc4ea6cda6f 100644
--- a/icu4c/source/common/unicode/localpointer.h
+++ b/icu4c/source/common/unicode/localpointer.h
@@ -548,7 +548,8 @@ class LocalArray : public LocalPointerBase<T> {
  * @stable ICU 4.4
  */
 #define U_DEFINE_LOCAL_OPEN_POINTER(LocalPointerClassName, Type, closeFunction) \
-    using LocalPointerClassName = internal::LocalOpenPointer<Type, closeFunction>
+    using LocalPointerClassName = \
+        U_ICU_NAMESPACE_OR_INTERNAL::internal::LocalOpenPointer<Type, closeFunction>
 
 #ifndef U_IN_DOXYGEN
 namespace internal {
diff --git a/icu4c/source/common/unicode/ubidi.h b/icu4c/source/common/unicode/ubidi.h
index 536f4172bc20..93b2612c8c86 100644
--- a/icu4c/source/common/unicode/ubidi.h
+++ b/icu4c/source/common/unicode/ubidi.h
@@ -563,9 +563,8 @@ ubidi_openSized(int32_t maxLength, int32_t maxRunCount, UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 ubidi_close(UBiDi *pBiDi);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUBiDiPointer
@@ -578,8 +577,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUBiDiPointer, UBiDi, ubidi_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ubiditransform.h b/icu4c/source/common/unicode/ubiditransform.h
index 24433aa8aca0..77d64899313a 100644
--- a/icu4c/source/common/unicode/ubiditransform.h
+++ b/icu4c/source/common/unicode/ubiditransform.h
@@ -304,9 +304,8 @@ ubiditransform_open(UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 ubiditransform_close(UBiDiTransform *pBidiTransform);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUBiDiTransformPointer
@@ -319,8 +318,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUBiDiTransformPointer, UBiDiTransform, ubiditransform_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 #endif
diff --git a/icu4c/source/common/unicode/ubrk.h b/icu4c/source/common/unicode/ubrk.h
index 2b3dc7aa5768..ba6597a0e14d 100644
--- a/icu4c/source/common/unicode/ubrk.h
+++ b/icu4c/source/common/unicode/ubrk.h
@@ -356,9 +356,8 @@ ubrk_clone(const UBreakIterator *bi,
 U_CAPI void U_EXPORT2
 ubrk_close(UBreakIterator *bi);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUBreakIteratorPointer
@@ -371,8 +370,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUBreakIteratorPointer, UBreakIterator, ubrk_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ucasemap.h b/icu4c/source/common/unicode/ucasemap.h
index 4b623e691071..c84401611b14 100644
--- a/icu4c/source/common/unicode/ucasemap.h
+++ b/icu4c/source/common/unicode/ucasemap.h
@@ -83,9 +83,8 @@ ucasemap_open(const char *locale, uint32_t options, UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 ucasemap_close(UCaseMap *csm);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUCaseMapPointer
@@ -98,8 +97,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUCaseMapPointer, UCaseMap, ucasemap_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ucnv.h b/icu4c/source/common/unicode/ucnv.h
index 4a7972c95b75..b8d04d8e62e4 100644
--- a/icu4c/source/common/unicode/ucnv.h
+++ b/icu4c/source/common/unicode/ucnv.h
@@ -581,9 +581,8 @@ ucnv_safeClone(const UConverter *cnv,
 U_CAPI void  U_EXPORT2
 ucnv_close(UConverter * converter);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUConverterPointer
@@ -596,8 +595,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUConverterPointer, UConverter, ucnv_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ucnvsel.h b/icu4c/source/common/unicode/ucnvsel.h
index 9373ec951bf8..2921a6da6b68 100644
--- a/icu4c/source/common/unicode/ucnvsel.h
+++ b/icu4c/source/common/unicode/ucnvsel.h
@@ -97,9 +97,8 @@ ucnvsel_open(const char* const*  converterList, int32_t converterListSize,
 U_CAPI void U_EXPORT2
 ucnvsel_close(UConverterSelector *sel);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUConverterSelectorPointer
@@ -112,8 +111,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUConverterSelectorPointer, UConverterSelector, ucnvsel_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ucptrie.h b/icu4c/source/common/unicode/ucptrie.h
index dadef79c5120..710b393dfcd6 100644
--- a/icu4c/source/common/unicode/ucptrie.h
+++ b/icu4c/source/common/unicode/ucptrie.h
@@ -623,9 +623,8 @@ U_CDECL_END
 
 #endif  // U_IN_DOXYGEN
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUCPTriePointer
@@ -638,8 +637,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUCPTriePointer, UCPTrie, ucptrie_close);
 
-U_NAMESPACE_END
-
-#endif  // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 #endif
diff --git a/icu4c/source/common/unicode/udata.h b/icu4c/source/common/unicode/udata.h
index 4cda255010ad..e969f6aca4b3 100644
--- a/icu4c/source/common/unicode/udata.h
+++ b/icu4c/source/common/unicode/udata.h
@@ -418,9 +418,8 @@ udata_setFileAccess(UDataFileAccess access, UErrorCode *status);
 
 U_CDECL_END
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUDataMemoryPointer
@@ -433,8 +432,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUDataMemoryPointer, UDataMemory, udata_close);
 
-U_NAMESPACE_END
-
-#endif  // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 #endif
diff --git a/icu4c/source/common/unicode/uenum.h b/icu4c/source/common/unicode/uenum.h
index d9c893e06d92..89997bd32dc7 100644
--- a/icu4c/source/common/unicode/uenum.h
+++ b/icu4c/source/common/unicode/uenum.h
@@ -53,9 +53,8 @@ typedef struct UEnumeration UEnumeration;
 U_CAPI void U_EXPORT2
 uenum_close(UEnumeration* en);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUEnumerationPointer
@@ -68,8 +67,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUEnumerationPointer, UEnumeration, uenum_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/uidna.h b/icu4c/source/common/unicode/uidna.h
index 362a2dcbe65a..f6647ab48e28 100644
--- a/icu4c/source/common/unicode/uidna.h
+++ b/icu4c/source/common/unicode/uidna.h
@@ -171,9 +171,8 @@ uidna_openUTS46(uint32_t options, UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 uidna_close(UIDNA *idna);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUIDNAPointer
@@ -186,8 +185,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUIDNAPointer, UIDNA, uidna_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/uldnames.h b/icu4c/source/common/unicode/uldnames.h
index 47b047ece97b..174f14666e24 100644
--- a/icu4c/source/common/unicode/uldnames.h
+++ b/icu4c/source/common/unicode/uldnames.h
@@ -81,9 +81,8 @@ uldn_open(const char * locale,
 U_CAPI void U_EXPORT2
 uldn_close(ULocaleDisplayNames *ldn);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalULocaleDisplayNamesPointer
@@ -96,8 +95,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalULocaleDisplayNamesPointer, ULocaleDisplayNames, uldn_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /* getters for state */
diff --git a/icu4c/source/common/unicode/ulocale.h b/icu4c/source/common/unicode/ulocale.h
index 1b3af3a5f26d..9bf91a426572 100644
--- a/icu4c/source/common/unicode/ulocale.h
+++ b/icu4c/source/common/unicode/ulocale.h
@@ -204,9 +204,8 @@ ulocale_getUnicodeKeywordValue(
     const ULocale* locale, const char* keyword, int32_t keywordLength,
     char* valueBuffer, int32_t valueBufferCapacity, UErrorCode *err);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalULocalePointer
@@ -219,8 +218,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalULocalePointer, ULocale, ulocale_close);
 
-U_NAMESPACE_END
-
-#endif  /* U_SHOW_CPLUSPLUS_API */
+}
+#endif
 
 #endif /*_ULOCALE */
diff --git a/icu4c/source/common/unicode/ulocbuilder.h b/icu4c/source/common/unicode/ulocbuilder.h
index ce61995bde62..18f8c7ea9244 100644
--- a/icu4c/source/common/unicode/ulocbuilder.h
+++ b/icu4c/source/common/unicode/ulocbuilder.h
@@ -415,9 +415,8 @@ ulocbld_buildLanguageTag(ULocaleBuilder* builder, char* language,
 U_CAPI UBool U_EXPORT2
 ulocbld_copyErrorTo(const ULocaleBuilder* builder, UErrorCode *outErrorCode);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalULocaleBuilderPointer
@@ -430,8 +429,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalULocaleBuilderPointer, ULocaleBuilder, ulocbld_close);
 
-U_NAMESPACE_END
-
-#endif  /* U_SHOW_CPLUSPLUS_API */
+}
+#endif
 
 #endif  // __ULOCBUILDER_H__
diff --git a/icu4c/source/common/unicode/umutablecptrie.h b/icu4c/source/common/unicode/umutablecptrie.h
index d60fd618191c..4740e04d7ac1 100644
--- a/icu4c/source/common/unicode/umutablecptrie.h
+++ b/icu4c/source/common/unicode/umutablecptrie.h
@@ -218,9 +218,8 @@ umutablecptrie_buildImmutable(UMutableCPTrie *trie, UCPTrieType type, UCPTrieVal
 
 U_CDECL_END
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUMutableCPTriePointer
@@ -233,8 +232,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUMutableCPTriePointer, UMutableCPTrie, umutablecptrie_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 #endif
diff --git a/icu4c/source/common/unicode/unorm2.h b/icu4c/source/common/unicode/unorm2.h
index 48f614d74fbf..3fc4fd14bf2d 100644
--- a/icu4c/source/common/unicode/unorm2.h
+++ b/icu4c/source/common/unicode/unorm2.h
@@ -268,9 +268,8 @@ unorm2_openFiltered(const UNormalizer2 *norm2, const USet *filterSet, UErrorCode
 U_CAPI void U_EXPORT2
 unorm2_close(UNormalizer2 *norm2);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUNormalizer2Pointer
@@ -283,8 +282,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUNormalizer2Pointer, UNormalizer2, unorm2_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/ures.h b/icu4c/source/common/unicode/ures.h
index babc01d426a6..c8cdecd6a4ea 100644
--- a/icu4c/source/common/unicode/ures.h
+++ b/icu4c/source/common/unicode/ures.h
@@ -252,9 +252,8 @@ ures_countArrayItems(const UResourceBundle* resourceBundle,
 U_CAPI void U_EXPORT2
 ures_close(UResourceBundle* resourceBundle);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUResourceBundlePointer
@@ -267,8 +266,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUResourceBundlePointer, UResourceBundle, ures_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 #ifndef U_HIDE_DEPRECATED_API
diff --git a/icu4c/source/common/unicode/uset.h b/icu4c/source/common/unicode/uset.h
index 345d62445448..c0ab617eccbf 100644
--- a/icu4c/source/common/unicode/uset.h
+++ b/icu4c/source/common/unicode/uset.h
@@ -347,7 +347,6 @@ U_CAPI void U_EXPORT2
 uset_close(USet* set);
 
 #if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
-
 namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
@@ -361,8 +360,7 @@ namespace U_ICU_NAMESPACE_OR_INTERNAL {
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUSetPointer, USet, uset_close);
 
-}  // U_ICU_NAMESPACE_OR_INTERNAL
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/usprep.h b/icu4c/source/common/unicode/usprep.h
index f8a0f58e0de2..d9c3e3d00dc0 100644
--- a/icu4c/source/common/unicode/usprep.h
+++ b/icu4c/source/common/unicode/usprep.h
@@ -212,9 +212,8 @@ usprep_openByType(UStringPrepProfileType type,
 U_CAPI void U_EXPORT2
 usprep_close(UStringPrepProfile* profile);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUStringPrepProfilePointer
@@ -227,8 +226,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUStringPrepProfilePointer, UStringPrepProfile, usprep_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/common/unicode/utext.h b/icu4c/source/common/unicode/utext.h
index 423b281631db..c0189feb5d39 100644
--- a/icu4c/source/common/unicode/utext.h
+++ b/icu4c/source/common/unicode/utext.h
@@ -1580,9 +1580,8 @@ enum {
 U_CDECL_END
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUTextPointer
@@ -1595,8 +1594,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUTextPointer, UText, utext_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 
diff --git a/icu4c/source/i18n/unicode/ucal.h b/icu4c/source/i18n/unicode/ucal.h
index d9bce21419a1..1677762584b4 100644
--- a/icu4c/source/i18n/unicode/ucal.h
+++ b/icu4c/source/i18n/unicode/ucal.h
@@ -787,9 +787,8 @@ ucal_open(const UChar*   zoneID,
 U_CAPI void U_EXPORT2 
 ucal_close(UCalendar *cal);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUCalendarPointer
@@ -802,8 +801,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUCalendarPointer, UCalendar, ucal_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/ucol.h b/icu4c/source/i18n/unicode/ucol.h
index 8b6dfeaa0a8c..d5a6dafc9a97 100644
--- a/icu4c/source/i18n/unicode/ucol.h
+++ b/icu4c/source/i18n/unicode/ucol.h
@@ -537,9 +537,8 @@ ucol_getContractionsAndExpansions( const UCollator *coll,
 U_CAPI void U_EXPORT2 
 ucol_close(UCollator *coll);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUCollatorPointer
@@ -552,8 +551,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUCollatorPointer, UCollator, ucol_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/ucsdet.h b/icu4c/source/i18n/unicode/ucsdet.h
index 8c62fde9d2ee..41af6a66a476 100644
--- a/icu4c/source/i18n/unicode/ucsdet.h
+++ b/icu4c/source/i18n/unicode/ucsdet.h
@@ -93,9 +93,8 @@ ucsdet_open(UErrorCode   *status);
 U_CAPI void U_EXPORT2
 ucsdet_close(UCharsetDetector *ucsd);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUCharsetDetectorPointer
@@ -108,8 +107,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUCharsetDetectorPointer, UCharsetDetector, ucsdet_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/udat.h b/icu4c/source/i18n/unicode/udat.h
index 3823dc88c4fa..b6320ec23ef5 100644
--- a/icu4c/source/i18n/unicode/udat.h
+++ b/icu4c/source/i18n/unicode/udat.h
@@ -1006,9 +1006,8 @@ typedef enum UDateFormatHourCycle {
     UDAT_HOUR_CYCLE_24
 } UDateFormatHourCycle;
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUDateFormatPointer
@@ -1021,8 +1020,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUDateFormatPointer, UDateFormat, udat_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/udateintervalformat.h b/icu4c/source/i18n/unicode/udateintervalformat.h
index 8439444d0796..061df7fcd762 100644
--- a/icu4c/source/i18n/unicode/udateintervalformat.h
+++ b/icu4c/source/i18n/unicode/udateintervalformat.h
@@ -182,9 +182,8 @@ U_CAPI void U_EXPORT2
 udtitvfmt_closeResult(UFormattedDateInterval* uresult);
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUDateIntervalFormatPointer
@@ -208,8 +207,7 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUDateIntervalFormatPointer, UDateIntervalFormat
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattedDateIntervalPointer, UFormattedDateInterval, udtitvfmt_closeResult);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 
diff --git a/icu4c/source/i18n/unicode/udatpg.h b/icu4c/source/i18n/unicode/udatpg.h
index 1d3060350ec0..c553781813b2 100644
--- a/icu4c/source/i18n/unicode/udatpg.h
+++ b/icu4c/source/i18n/unicode/udatpg.h
@@ -185,9 +185,8 @@ udatpg_openEmpty(UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 udatpg_close(UDateTimePatternGenerator *dtpg);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUDateTimePatternGeneratorPointer
@@ -200,8 +199,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUDateTimePatternGeneratorPointer, UDateTimePatternGenerator, udatpg_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/ufieldpositer.h b/icu4c/source/i18n/unicode/ufieldpositer.h
index 83df184f0a59..d41bc6f99227 100644
--- a/icu4c/source/i18n/unicode/ufieldpositer.h
+++ b/icu4c/source/i18n/unicode/ufieldpositer.h
@@ -67,9 +67,8 @@ U_CAPI void U_EXPORT2
 ufieldpositer_close(UFieldPositionIterator *fpositer);
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUFieldPositionIteratorPointer
@@ -82,8 +81,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFieldPositionIteratorPointer, UFieldPositionIterator, ufieldpositer_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/uformattable.h b/icu4c/source/i18n/unicode/uformattable.h
index 4c4a307a24b4..e282fa76749a 100644
--- a/icu4c/source/i18n/unicode/uformattable.h
+++ b/icu4c/source/i18n/unicode/uformattable.h
@@ -93,9 +93,8 @@ ufmt_open(UErrorCode* status);
 U_CAPI void U_EXPORT2
 ufmt_close(UFormattable* fmt);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUFormattablePointer
@@ -108,8 +107,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattablePointer, UFormattable, ufmt_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/uformattednumber.h b/icu4c/source/i18n/unicode/uformattednumber.h
index 174a040a0833..8518c5124dea 100644
--- a/icu4c/source/i18n/unicode/uformattednumber.h
+++ b/icu4c/source/i18n/unicode/uformattednumber.h
@@ -196,8 +196,8 @@ U_CAPI void U_EXPORT2
 unumf_closeResult(UFormattedNumber* uresult);
 
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUFormattedNumberPointer
@@ -216,8 +216,8 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattedNumberPointer, UFormattedNumber, unumf_closeResult);
 
-U_NAMESPACE_END
-#endif // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unicode/uformattedvalue.h b/icu4c/source/i18n/unicode/uformattedvalue.h
index af6d18f3bc99..2d8b551be5d8 100644
--- a/icu4c/source/i18n/unicode/uformattedvalue.h
+++ b/icu4c/source/i18n/unicode/uformattedvalue.h
@@ -418,8 +418,8 @@ ufmtval_nextPosition(
     UErrorCode* ec);
 
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUConstrainedFieldPositionPointer
@@ -437,8 +437,8 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUConstrainedFieldPositionPointer,
     UConstrainedFieldPosition,
     ucfpos_close);
 
-U_NAMESPACE_END
-#endif // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unicode/ulistformatter.h b/icu4c/source/i18n/unicode/ulistformatter.h
index 8334c7852f0c..b1f4c6b5e682 100644
--- a/icu4c/source/i18n/unicode/ulistformatter.h
+++ b/icu4c/source/i18n/unicode/ulistformatter.h
@@ -220,9 +220,8 @@ U_CAPI void U_EXPORT2
 ulistfmt_closeResult(UFormattedList* uresult);
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUListFormatterPointer
@@ -246,8 +245,7 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUListFormatterPointer, UListFormatter, ulistfmt
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattedListPointer, UFormattedList, ulistfmt_closeResult);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/ulocdata.h b/icu4c/source/i18n/unicode/ulocdata.h
index 3647df9596ae..c6e9000f6ad1 100644
--- a/icu4c/source/i18n/unicode/ulocdata.h
+++ b/icu4c/source/i18n/unicode/ulocdata.h
@@ -102,9 +102,8 @@ ulocdata_open(const char *localeID, UErrorCode *status);
 U_CAPI void U_EXPORT2
 ulocdata_close(ULocaleData *uld);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalULocaleDataPointer
@@ -117,8 +116,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalULocaleDataPointer, ULocaleData, ulocdata_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/umsg.h b/icu4c/source/i18n/unicode/umsg.h
index c955dc18ac4d..fb3cdf77e7fd 100644
--- a/icu4c/source/i18n/unicode/umsg.h
+++ b/icu4c/source/i18n/unicode/umsg.h
@@ -415,9 +415,8 @@ umsg_open(  const UChar     *pattern,
 U_CAPI void U_EXPORT2 
 umsg_close(UMessageFormat* format);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUMessageFormatPointer
@@ -430,8 +429,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUMessageFormatPointer, UMessageFormat, umsg_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/unum.h b/icu4c/source/i18n/unicode/unum.h
index 2dd2badc1bd7..65aec8cbab00 100644
--- a/icu4c/source/i18n/unicode/unum.h
+++ b/icu4c/source/i18n/unicode/unum.h
@@ -444,9 +444,8 @@ unum_open(  UNumberFormatStyle    style,
 U_CAPI void U_EXPORT2 
 unum_close(UNumberFormat* fmt);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUNumberFormatPointer
@@ -459,8 +458,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUNumberFormatPointer, UNumberFormat, unum_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/unumberformatter.h b/icu4c/source/i18n/unicode/unumberformatter.h
index 09fa000b826b..27c95ed59cf5 100644
--- a/icu4c/source/i18n/unicode/unumberformatter.h
+++ b/icu4c/source/i18n/unicode/unumberformatter.h
@@ -549,8 +549,8 @@ unumf_close(UNumberFormatter* uformatter);
 
 
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUNumberFormatterPointer
@@ -569,8 +569,8 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUNumberFormatterPointer, UNumberFormatter, unumf_close);
 
-U_NAMESPACE_END
-#endif // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
 #endif //__UNUMBERFORMATTER_H__
diff --git a/icu4c/source/i18n/unicode/unumberrangeformatter.h b/icu4c/source/i18n/unicode/unumberrangeformatter.h
index 106942f25af0..3a58b4d72129 100644
--- a/icu4c/source/i18n/unicode/unumberrangeformatter.h
+++ b/icu4c/source/i18n/unicode/unumberrangeformatter.h
@@ -426,8 +426,8 @@ U_CAPI void U_EXPORT2
 unumrf_closeResult(UFormattedNumberRange* uresult);
 
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUNumberRangeFormatterPointer
@@ -464,8 +464,8 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUNumberRangeFormatterPointer, UNumberRangeForma
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattedNumberRangePointer, UFormattedNumberRange, unumrf_closeResult);
 
-U_NAMESPACE_END
-#endif // U_SHOW_CPLUSPLUS_API
+}
+#endif
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
 #endif //__UNUMBERRANGEFORMATTER_H__
diff --git a/icu4c/source/i18n/unicode/unumsys.h b/icu4c/source/i18n/unicode/unumsys.h
index fe713ea77a6c..ad4f548ac73d 100644
--- a/icu4c/source/i18n/unicode/unumsys.h
+++ b/icu4c/source/i18n/unicode/unumsys.h
@@ -89,8 +89,8 @@ unumsys_openByName(const char *name, UErrorCode *status);
 U_CAPI void U_EXPORT2
 unumsys_close(UNumberingSystem *unumsys);
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUNumberingSystemPointer
@@ -102,7 +102,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUNumberingSystemPointer, UNumberingSystem, unumsys_close);
 
-U_NAMESPACE_END
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/upluralrules.h b/icu4c/source/i18n/unicode/upluralrules.h
index 983651b1cac0..7d9f97141a36 100644
--- a/icu4c/source/i18n/unicode/upluralrules.h
+++ b/icu4c/source/i18n/unicode/upluralrules.h
@@ -120,9 +120,8 @@ U_CAPI void U_EXPORT2
 uplrules_close(UPluralRules *uplrules);
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUPluralRulesPointer
@@ -135,8 +134,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUPluralRulesPointer, UPluralRules, uplrules_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 
diff --git a/icu4c/source/i18n/unicode/uregex.h b/icu4c/source/i18n/unicode/uregex.h
index e946e632623a..a9d510646143 100644
--- a/icu4c/source/i18n/unicode/uregex.h
+++ b/icu4c/source/i18n/unicode/uregex.h
@@ -213,9 +213,8 @@ uregex_openC( const char           *pattern,
 U_CAPI void U_EXPORT2 
 uregex_close(URegularExpression *regexp);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalURegularExpressionPointer
@@ -228,8 +227,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalURegularExpressionPointer, URegularExpression, uregex_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/ureldatefmt.h b/icu4c/source/i18n/unicode/ureldatefmt.h
index 0882360d147a..9d6899624998 100644
--- a/icu4c/source/i18n/unicode/ureldatefmt.h
+++ b/icu4c/source/i18n/unicode/ureldatefmt.h
@@ -299,9 +299,8 @@ U_CAPI void U_EXPORT2
 ureldatefmt_closeResult(UFormattedRelativeDateTime* ufrdt);
 
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalURelativeDateTimeFormatterPointer
@@ -325,8 +324,7 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalURelativeDateTimeFormatterPointer, URelativeDat
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFormattedRelativeDateTimePointer, UFormattedRelativeDateTime, ureldatefmt_closeResult);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/i18n/unicode/usearch.h b/icu4c/source/i18n/unicode/usearch.h
index fd0b84f67235..f3d917c969cb 100644
--- a/icu4c/source/i18n/unicode/usearch.h
+++ b/icu4c/source/i18n/unicode/usearch.h
@@ -360,9 +360,8 @@ U_CAPI UStringSearch * U_EXPORT2 usearch_openFromCollator(
  */
 U_CAPI void U_EXPORT2 usearch_close(UStringSearch *searchiter);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUStringSearchPointer
@@ -375,8 +374,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUStringSearchPointer, UStringSearch, usearch_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /* get and set methods -------------------------------------------------- */
diff --git a/icu4c/source/i18n/unicode/usimplenumberformatter.h b/icu4c/source/i18n/unicode/usimplenumberformatter.h
index 22e81ba2c939..b28ebe394235 100644
--- a/icu4c/source/i18n/unicode/usimplenumberformatter.h
+++ b/icu4c/source/i18n/unicode/usimplenumberformatter.h
@@ -250,8 +250,8 @@ U_CAPI void U_EXPORT2
 usnumf_close(USimpleNumberFormatter* uformatter);
 
 
-#if U_SHOW_CPLUSPLUS_API
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUSimpleNumberPointer
@@ -290,7 +290,7 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUSimpleNumberPointer, USimpleNumber, usnum_clos
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUSimpleNumberFormatterPointer, USimpleNumberFormatter, usnumf_close);
 
-U_NAMESPACE_END
+}
 #endif // U_SHOW_CPLUSPLUS_API
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
diff --git a/icu4c/source/i18n/unicode/uspoof.h b/icu4c/source/i18n/unicode/uspoof.h
index 4e029fe951c9..41817f7e6e06 100644
--- a/icu4c/source/i18n/unicode/uspoof.h
+++ b/icu4c/source/i18n/unicode/uspoof.h
@@ -1550,9 +1550,8 @@ uspoof_serialize(USpoofChecker *sc,
 
 U_CDECL_END
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUSpoofCheckerPointer
@@ -1589,7 +1588,10 @@ U_DEFINE_LOCAL_OPEN_POINTER(LocalUSpoofCheckerPointer, USpoofChecker, uspoof_clo
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUSpoofCheckResultPointer, USpoofCheckResult, uspoof_closeCheckResult);
 /** \endcond */
 
-U_NAMESPACE_END
+}
+#endif
+
+#if U_SHOW_CPLUSPLUS_API
 
 /**
  * Limit the acceptable characters to those specified by a Unicode Set.
diff --git a/icu4c/source/i18n/unicode/utrans.h b/icu4c/source/i18n/unicode/utrans.h
index 1ad7dbda62f6..b33ec5d6682d 100644
--- a/icu4c/source/i18n/unicode/utrans.h
+++ b/icu4c/source/i18n/unicode/utrans.h
@@ -242,9 +242,8 @@ utrans_clone(const UTransliterator* trans,
 U_CAPI void U_EXPORT2 
 utrans_close(UTransliterator* trans);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUTransliteratorPointer
@@ -257,8 +256,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUTransliteratorPointer, UTransliterator, utrans_close);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**
diff --git a/icu4c/source/io/unicode/ustdio.h b/icu4c/source/io/unicode/ustdio.h
index 5aad6b9bbead..e5ec930c12c0 100644
--- a/icu4c/source/io/unicode/ustdio.h
+++ b/icu4c/source/io/unicode/ustdio.h
@@ -341,9 +341,8 @@ u_fstropen(UChar      *stringBuf,
 U_CAPI void U_EXPORT2
 u_fclose(UFILE *file);
 
-#if U_SHOW_CPLUSPLUS_API
-
-U_NAMESPACE_BEGIN
+#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+namespace U_ICU_NAMESPACE_OR_INTERNAL {
 
 /**
  * \class LocalUFILEPointer
@@ -356,8 +355,7 @@ U_NAMESPACE_BEGIN
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUFILEPointer, UFILE, u_fclose);
 
-U_NAMESPACE_END
-
+}
 #endif
 
 /**