ICU-22520 Add a StringByteSink<CharString> template specialization.

This makes it possible to call public functions that take a string class
as a template parameter and return an object of that class (implemented
through the StringByteSink helper class) also with the CharString class,
even though this class doesn't actually provide the public API required
by StringByteSink.

This makes it possible to use such more modern APIs also internally.

icu4c/source/common/brkiter.cpp

@@ -438,17 +438,14 @@ BreakIterator::makeInstance(const Locale& loc, int32_t kind, UErrorCode& status)
             UTRACE_ENTRY(UTRACE_UBRK_CREATE_LINE);
             uprv_strcpy(lb_lw, "line");
             UErrorCode kvStatus = U_ZERO_ERROR;
-            CharString value;
-            CharStringByteSink valueSink(&value);
-            loc.getKeywordValue("lb", valueSink, kvStatus);
+            auto value = loc.getKeywordValue<CharString>("lb", kvStatus);
             if (U_SUCCESS(kvStatus) && (value == "strict" || value == "normal" || value == "loose")) {
                 uprv_strcat(lb_lw, "_");
                 uprv_strcat(lb_lw, value.data());
             }
             // lw=phrase is only supported in Japanese and Korean
             if (uprv_strcmp(loc.getLanguage(), "ja") == 0 || uprv_strcmp(loc.getLanguage(), "ko") == 0) {
-                value.clear();
-                loc.getKeywordValue("lw", valueSink, kvStatus);
+                value = loc.getKeywordValue<CharString>("lw", kvStatus);
                 if (U_SUCCESS(kvStatus) && value == "phrase") {
                     uprv_strcat(lb_lw, "_");
                     uprv_strcat(lb_lw, value.data());

icu4c/source/common/bytesinkutil.h

@@ -43,6 +43,16 @@ private:
     CharString& dest_;
 };
 
+// CharString doesn't provide the public API that StringByteSink requires a
+// string class to have so this template specialization replaces the default
+// implementation of StringByteSink<CharString> with CharStringByteSink.
+template<>
+class StringByteSink<CharString> : public CharStringByteSink {
+ public:
+  StringByteSink(CharString* dest) : CharStringByteSink(dest) { }
+  StringByteSink(CharString* dest, int32_t /*initialAppendCapacity*/) : CharStringByteSink(dest) { }
+};
+
 class U_COMMON_API ByteSinkUtil {
 public:
     ByteSinkUtil() = delete;  // all static

icu4c/source/common/localebuilder.cpp

@@ -3,7 +3,7 @@
 
 #include <utility>
 
-#include "bytesinkutil.h"  // CharStringByteSink
+#include "bytesinkutil.h"  // StringByteSink<CharString>
 #include "charstr.h"
 #include "cstring.h"
 #include "ulocimp.h"
@@ -183,9 +183,7 @@ _copyExtensions(const Locale& from, icu::StringEnumeration *keywords,
     }
     const char* key;
     while ((key = keywords->next(nullptr, errorCode)) != nullptr) {
-        CharString value;
-        CharStringByteSink sink(&value);
-        from.getKeywordValue(key, sink, errorCode);
+        auto value = from.getKeywordValue<CharString>(key, errorCode);
         if (U_FAILURE(errorCode)) { return; }
         if (uprv_strcmp(key, kAttributeKey) == 0) {
             transform(value.data(), value.length());
@@ -307,10 +305,8 @@ LocaleBuilder& LocaleBuilder::addUnicodeLocaleAttribute(
         return *this;
     }
 
-    CharString attributes;
-    CharStringByteSink sink(&attributes);
     UErrorCode localErrorCode = U_ZERO_ERROR;
-    extensions_->getKeywordValue(kAttributeKey, sink, localErrorCode);
+    auto attributes = extensions_->getKeywordValue<CharString>(kAttributeKey, localErrorCode);
     if (U_FAILURE(localErrorCode)) {
         CharString new_attributes(value_str.data(), status_);
         // No attributes, set the attribute.
@@ -362,9 +358,7 @@ LocaleBuilder& LocaleBuilder::removeUnicodeLocaleAttribute(
     }
     if (extensions_ == nullptr) { return *this; }
     UErrorCode localErrorCode = U_ZERO_ERROR;
-    CharString attributes;
-    CharStringByteSink sink(&attributes);
-    extensions_->getKeywordValue(kAttributeKey, sink, localErrorCode);
+    auto attributes = extensions_->getKeywordValue<CharString>(kAttributeKey, localErrorCode);
     // get failure, just return
     if (U_FAILURE(localErrorCode)) { return *this; }
     // Do not have any attributes, just return.

icu4c/source/common/locdspnm.cpp

@@ -634,13 +634,9 @@ LocaleDisplayNamesImpl::localeDisplayName(const Locale& loc,
     UnicodeString temp2;
     const char* key;
     while ((key = e->next((int32_t *)0, status)) != nullptr) {
-      CharString value;
-      {
-        CharStringByteSink sink(&value);
-        loc.getKeywordValue(key, sink, status);
-      }
-      if (U_FAILURE(status)) {
-        return result;
+        auto value = loc.getKeywordValue<CharString>(key, status);
+        if (U_FAILURE(status)) {
+            return result;
       }
       keyDisplayName(key, temp, true);
       temp.findAndReplace(formatOpenParen, formatReplaceOpenParen);

icu4c/source/common/locid.cpp

@@ -1521,13 +1521,12 @@ AliasReplacer::replaceTransformedExtensions(
     const char* tkey = ultag_getTKeyStart(str);
     int32_t tlangLen = (tkey == str) ? 0 :
         ((tkey == nullptr) ? len : static_cast<int32_t>((tkey - str - 1)));
-    CharStringByteSink sink(&output);
     if (tlangLen > 0) {
         Locale tlang = LocaleBuilder()
             .setLanguageTag(StringPiece(str, tlangLen))
             .build(status);
         tlang.canonicalize(status);
-        tlang.toLanguageTag(sink, status);
+        output = tlang.toLanguageTag<CharString>(status);
         if (U_FAILURE(status)) {
             return false;
         }
@@ -1728,9 +1727,7 @@ AliasReplacer::replace(const Locale& locale, CharString& out, UErrorCode& status
             while ((key = iter->next(nullptr, status)) != nullptr) {
                 if (uprv_strcmp("sd", key) == 0 || uprv_strcmp("rg", key) == 0 ||
                         uprv_strcmp("t", key) == 0) {
-                    CharString value;
-                    CharStringByteSink valueSink(&value);
-                    locale.getKeywordValue(key, valueSink, status);
+                    auto value = locale.getKeywordValue<CharString>(key, status);
                     if (U_FAILURE(status)) {
                         status = U_ZERO_ERROR;
                         continue;
@@ -2628,11 +2625,7 @@ Locale::getUnicodeKeywordValue(StringPiece keywordName,
         return;
     }
 
-    CharString legacy_value;
-    {
-        CharStringByteSink sink(&legacy_value);
-        getKeywordValue(legacy_key, sink, status);
-    }
+    auto legacy_value = getKeywordValue<CharString>(legacy_key, status);
 
     if (U_FAILURE(status)) {
         return;

icu4c/source/i18n/units_data.cpp

@@ -403,12 +403,8 @@ U_I18N_API UnitPreferences::UnitPreferences(UErrorCode &status) {
 }
 
 CharString getKeyWordValue(const Locale &locale, StringPiece kw, UErrorCode &status) {
-    CharString result;
-    if (U_FAILURE(status)) { return result; }
-    {
-        CharStringByteSink sink(&result);
-        locale.getKeywordValue(kw, sink, status);
-    }
+    if (U_FAILURE(status)) { return {}; }
+    auto result = locale.getKeywordValue<CharString>(kw, status);
     if (U_SUCCESS(status) && result.isEmpty()) {
         status = U_MISSING_RESOURCE_ERROR;
     }