1 files changed, 90 insertions, 22 deletions

diff --git a/icu4c/source/common/bytesinkutil.h b/icu4c/source/common/bytesinkutil.h
index 929c71fbe..b3bd487be 100644
--- a/icu4c/source/common/bytesinkutil.h
+++ b/icu4c/source/common/bytesinkutil.h
@@ -7,18 +7,52 @@
 #ifndef BYTESINKUTIL_H
 #define BYTESINKUTIL_H
 
+#include <type_traits>
+
 #include "unicode/utypes.h"
 #include "unicode/bytestream.h"
 #include "unicode/edits.h"
+#include "charstr.h"
 #include "cmemory.h"
 #include "uassert.h"
+#include "ustr_imp.h"
 
 U_NAMESPACE_BEGIN
 
 class ByteSink;
-class CharString;
 class Edits;
 
+class U_COMMON_API CharStringByteSink : public ByteSink {
+public:
+    CharStringByteSink(CharString* dest);
+    ~CharStringByteSink() override;
+
+    CharStringByteSink() = delete;
+    CharStringByteSink(const CharStringByteSink&) = delete;
+    CharStringByteSink& operator=(const CharStringByteSink&) = delete;
+
+    void Append(const char* bytes, int32_t n) override;
+
+    char* GetAppendBuffer(int32_t min_capacity,
+                          int32_t desired_capacity_hint,
+                          char* scratch,
+                          int32_t scratch_capacity,
+                          int32_t* result_capacity) override;
+
+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
@@ -57,30 +91,64 @@ public:
                                  ByteSink &sink, uint32_t options, Edits *edits,
                                  UErrorCode &errorCode);
 
-private:
-    static void appendNonEmptyUnchanged(const uint8_t *s, int32_t length,
-                                        ByteSink &sink, uint32_t options, Edits *edits);
-};
-
-class U_COMMON_API CharStringByteSink : public ByteSink {
-public:
-    CharStringByteSink(CharString* dest);
-    ~CharStringByteSink() override;
-
-    CharStringByteSink() = delete;
-    CharStringByteSink(const CharStringByteSink&) = delete;
-    CharStringByteSink& operator=(const CharStringByteSink&) = delete;
-
-    void Append(const char* bytes, int32_t n) override;
+    /**
+     * Calls a lambda that writes to a ByteSink with a CheckedArrayByteSink
+     * and then returns through u_terminateChars(), in order to implement
+     * the classic ICU4C C API writing to a fix sized buffer on top of a
+     * contemporary C++ API.
+     *
+     * @param buffer receiving buffer
+     * @param capacity capacity of receiving buffer
+     * @param lambda that gets called with the sink as an argument
+     * @param status set to U_BUFFER_OVERFLOW_ERROR on overflow
+     * @return number of bytes written, or needed (in case of overflow)
+     * @internal
+     */
+    template <typename F,
+              typename = std::enable_if_t<
+                  std::is_invocable_r_v<void, F, ByteSink&, UErrorCode&>>>
+    static int32_t viaByteSinkToTerminatedChars(char* buffer, int32_t capacity,
+                                                F&& lambda,
+                                                UErrorCode& status) {
+        if (U_FAILURE(status)) { return 0; }
+        CheckedArrayByteSink sink(buffer, capacity);
+        lambda(sink, status);
+        if (U_FAILURE(status)) { return 0; }
+
+        int32_t reslen = sink.NumberOfBytesAppended();
+
+        if (sink.Overflowed()) {
+            status = U_BUFFER_OVERFLOW_ERROR;
+            return reslen;
+        }
+
+        return u_terminateChars(buffer, capacity, reslen, &status);
+    }
 
-    char* GetAppendBuffer(int32_t min_capacity,
-                          int32_t desired_capacity_hint,
-                          char* scratch,
-                          int32_t scratch_capacity,
-                          int32_t* result_capacity) override;
+    /**
+     * Calls a lambda that writes to a ByteSink with a CharStringByteSink and
+     * then returns a CharString, in order to implement a contemporary C++ API
+     * on top of a C/C++ compatibility ByteSink API.
+     *
+     * @param lambda that gets called with the sink as an argument
+     * @param status to check and report
+     * @return the resulting string, or an empty string (in case of error)
+     * @internal
+     */
+    template <typename F,
+              typename = std::enable_if_t<
+                  std::is_invocable_r_v<void, F, ByteSink&, UErrorCode&>>>
+    static CharString viaByteSinkToCharString(F&& lambda, UErrorCode& status) {
+        if (U_FAILURE(status)) { return {}; }
+        CharString result;
+        CharStringByteSink sink(&result);
+        lambda(sink, status);
+        return result;
+    }
 
 private:
-    CharString& dest_;
+    static void appendNonEmptyUnchanged(const uint8_t *s, int32_t length,
+                                        ByteSink &sink, uint32_t options, Edits *edits);
 };
 
 U_NAMESPACE_END