[string] introduce StringWriter class (#6564)

This commit moves methods of String out of the header file to save code
size.

Since most use cases of String writers ignores errors, this commit
changes the return of writers to the Writer object itself for chained
writes.

src/core/common/logging.cpp

@@ -59,6 +59,7 @@ static void Log(otLogLevel  aLogLevel,
                 va_list     aArgs)
 {
     ot::String<OPENTHREAD_CONFIG_LOG_MAX_SIZE> logString;
+    ot::StringWriter                           writer(logString);
 
 #if OPENTHREAD_CONFIG_LOG_LEVEL_DYNAMIC_ENABLE
     VerifyOrExit(otLoggingGetLevel() >= aLogLevel);
@@ -96,16 +97,18 @@ static void Log(otLogLevel  aLogLevel,
             break;
         }
 
-        IgnoreError(logString.Append("%s", levelStr));
+        writer.Append("%s", levelStr);
     }
 #endif // OPENTHREAD_CONFIG_LOG_PREPEND_LEVEL
 
-    IgnoreError(logString.Append("%s", aRegionPrefix));
-    VerifyOrExit(logString.AppendVarArgs(aFormat, aArgs) != ot::kErrorInvalidArgs);
+    writer.Append("%s", aRegionPrefix);
+    writer.AppendVarArgs(aFormat, aArgs);
     otPlatLog(aLogLevel, aLogRegion, "%s" OPENTHREAD_CONFIG_LOG_SUFFIX, logString.AsCString());
 
+#if OPENTHREAD_CONFIG_LOG_LEVEL_DYNAMIC_ENABLE
 exit:
     return;
+#endif
 }
 
 #if OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_CRIT
@@ -232,26 +235,29 @@ enum : uint8_t
 
 static void DumpLine(otLogLevel aLogLevel, otLogRegion aLogRegion, const uint8_t *aBytes, const size_t aLength)
 {
-    ot::String<kStringLineLength> string("|");
+    ot::String<kStringLineLength> string;
+    ot::StringWriter              writer(string);
+
+    writer.Append("|");
 
     for (uint8_t i = 0; i < kDumpBytesPerLine; i++)
     {
         if (i < aLength)
         {
-            IgnoreError(string.Append(" %02X", aBytes[i]));
+            writer.Append(" %02X", aBytes[i]);
         }
         else
         {
-            IgnoreError(string.Append(" .."));
+            writer.Append(" ..");
         }
 
         if (!((i + 1) % 8))
         {
-            IgnoreError(string.Append(" |"));
+            writer.Append(" |");
         }
     }
 
-    IgnoreError(string.Append(" "));
+    writer.Append(" ");
 
     for (uint8_t i = 0; i < kDumpBytesPerLine; i++)
     {
@@ -267,7 +273,7 @@ static void DumpLine(otLogLevel aLogLevel, otLogRegion aLogRegion, const uint8_t
             }
         }
 
-        IgnoreError(string.Append("%c", c));
+        writer.Append("%c", c);
     }
 
     otLogDump(aLogLevel, aLogRegion, "%s", string.AsCString());
@@ -282,17 +288,18 @@ void otDump(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aId, const
 
     size_t                        idLen = strlen(aId);
     ot::String<kStringLineLength> string;
+    ot::StringWriter              writer(string);
 
     for (size_t i = 0; i < (kWidth - idLen) / 2 - 5; i++)
     {
-        IgnoreError(string.Append("="));
+        writer.Append("=");
     }
 
-    IgnoreError(string.Append("[%s len=%03u]", aId, static_cast<unsigned>(aLength)));
+    writer.Append("[%s len=%03u]", aId, static_cast<unsigned>(aLength));
 
     for (size_t i = 0; i < (kWidth - idLen) / 2 - 4; i++)
     {
-        IgnoreError(string.Append("="));
+        writer.Append("=");
     }
 
     otLogDump(aLogLevel, aLogRegion, "%s", string.AsCString());
@@ -303,11 +310,11 @@ void otDump(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aId, const
                  OT_MIN((aLength - i), static_cast<size_t>(kDumpBytesPerLine)));
     }
 
-    string.Clear();
+    writer.Clear();
 
     for (size_t i = 0; i < kWidth; i++)
     {
-        IgnoreError(string.Append("-"));
+        writer.Append("-");
     }
 
     otLogDump(aLogLevel, aLogRegion, "%s", string.AsCString());

src/core/common/notifier.cpp

@@ -212,6 +212,7 @@ void Notifier::LogEvents(Events aEvents) const
     bool                           addSpace = false;
     bool                           didLog   = false;
     String<kFlagsStringBufferSize> string;
+    StringWriter                   writer(string);
 
     for (uint8_t bit = 0; bit < sizeof(Events::Flags) * CHAR_BIT; bit++)
     {
@@ -219,16 +220,16 @@ void Notifier::LogEvents(Events aEvents) const
 
         if (flags & (1 << bit))
         {
-            if (string.GetLength() >= kFlagsStringLineLimit)
+            if (writer.GetLength() >= kFlagsStringLineLimit)
             {
                 otLogInfoCore("Notifier: StateChanged (0x%08x) %s%s ...", aEvents.GetAsFlags(), didLog ? "... " : "[",
                               string.AsCString());
-                string.Clear();
+                writer.Clear();
                 didLog   = true;
                 addSpace = false;
             }
 
-            IgnoreError(string.Append("%s%s", addSpace ? " " : "", EventToString(static_cast<Event>(1 << bit))));
+            writer.Append("%s%s", addSpace ? " " : "", EventToString(static_cast<Event>(1 << bit)));
             addSpace = true;
 
             flags ^= (1 << bit);

src/core/common/string.cpp

@@ -32,6 +32,7 @@
  */
 
 #include "string.hpp"
+#include "debug.hpp"
 
 #include <string.h>
 
@@ -72,30 +73,56 @@ bool StringEndsWith(const char *aString, char aChar)
     return len > 0 && aString[len - 1] == aChar;
 }
 
-Error StringBase::Write(char *aBuffer, uint16_t aSize, uint16_t &aLength, const char *aFormat, va_list aArgs)
+StringWriter::StringWriter(char *aBuffer, uint16_t aSize)
+    : mBuffer(aBuffer)
+    , mLength(0)
+    , mSize(aSize)
 {
-    Error error = kErrorNone;
-    int   len;
+    mBuffer[0] = '\0';
+}
 
-    len = vsnprintf(aBuffer + aLength, aSize - aLength, aFormat, aArgs);
+StringWriter &StringWriter::Clear(void)
+{
+    mBuffer[0] = '\0';
+    mLength    = 0;
+    return *this;
+}
 
-    if (len < 0)
+StringWriter &StringWriter::Append(const char *aFormat, ...)
+{
+    va_list args;
+    va_start(args, aFormat);
+    AppendVarArgs(aFormat, args);
+    va_end(args);
+
+    return *this;
+}
+
+StringWriter &StringWriter::AppendVarArgs(const char *aFormat, va_list aArgs)
+{
+    int len;
+
+    len = vsnprintf(mBuffer + mLength, (mSize > mLength ? (mSize - mLength) : 0), aFormat, aArgs);
+    OT_ASSERT(len >= 0);
+
+    mLength += static_cast<uint16_t>(len);
+
+    if (IsTruncated())
     {
-        aLength    = 0;
-        aBuffer[0] = 0;
-        error      = kErrorInvalidArgs;
-    }
-    else if (len >= aSize - aLength)
-    {
-        aLength = aSize - 1;
-        error   = kErrorNoBufs;
-    }
-    else
-    {
-        aLength += static_cast<uint16_t>(len);
+        mBuffer[mSize - 1] = '\0';
     }
 
-    return error;
+    return *this;
+}
+
+StringWriter &StringWriter::AppendHexBytes(const uint8_t *aBytes, uint16_t aLength)
+{
+    while (aLength--)
+    {
+        Append("%02x", *aBytes++);
+    }
+
+    return *this;
 }
 
 bool IsValidUtf8String(const char *aString)

src/core/common/string.hpp

@@ -90,79 +90,75 @@ const char *StringFind(const char *aString, char aChar);
  */
 bool StringEndsWith(const char *aString, char aChar);
 
-/**
- * This class defines the base class for `String`.
- *
- */
-class StringBase
-{
-protected:
-    /**
-     * This method appends `printf()` style formatted data to a given character string buffer.
-     *
-     * @param[in]    aBuffer  A pointer to buffer containing the string.
-     * @param[in]    aSize    The size of the buffer (in bytes).
-     * @param[inout] aLength  A reference to variable containing current length of string. On exit length is updated.
-     * @param[in]    aFormat  A pointer to the format string.
-     * @param[in]    aArgs    Arguments for the format specification.
-     *
-     * @retval kErrorNone          Updated the string successfully.
-     * @retval kErrorNoBufs        String could not fit in the storage.
-     * @retval kErrorInvalidArgs   Arguments do not match the format string.
-     */
-    static Error Write(char *aBuffer, uint16_t aSize, uint16_t &aLength, const char *aFormat, va_list aArgs);
-};
+class StringWriter;
 
 /**
  * This class defines a fixed-size string.
  *
  */
-template <uint16_t SIZE> class String : private StringBase
+template <uint16_t kSize> class String
+{
+    friend class StringWriter;
+
+    static_assert(kSize > 0, "String buffer cannot be empty.");
+
+public:
+    /**
+     * This method returns the string as a null-terminated C string.
+     *
+     * @returns The null-terminated C string.
+     *
+     */
+    const char *AsCString(void) const { return mBuffer; }
+
+private:
+    char mBuffer[kSize];
+};
+
+/**
+ * This class implements writing to a string buffer.
+ *
+ */
+class StringWriter
 {
 public:
-    enum
-    {
-        kSize = SIZE, ///< Size (number of characters) in the buffer storage for the string.
-    };
-
     /**
-     * This constructor initializes the `String` object as empty.
+     * This constructor initializes the object as cleared on the provided buffer.
      *
      */
-    String(void)
-        : mLength(0)
+    StringWriter(char *aBuffer, uint16_t aSize);
+
+    /**
+     * This constructor initializes the object as cleared on a fixed-size string.
+     *
+     */
+    template <uint16_t kSize>
+    explicit StringWriter(String<kSize> &aStringBuffer)
+        : StringWriter(aStringBuffer.mBuffer, kSize)
     {
-        mBuffer[0] = 0;
     }
 
     /**
-     * This constructor initializes the `String` object using `printf()` style formatted data.
+     * This method clears the string writer.
      *
-     * @param[in] aFormat    A pointer to the format string.
-     * @param[in] ...        Arguments for the format specification.
+     * @returns The string writer.
      *
      */
-    explicit String(const char *aFormat, ...)
-        : mLength(0)
-    {
-        va_list args;
-        va_start(args, aFormat);
-        IgnoreError(Write(mBuffer, kSize, mLength, aFormat, args));
-        va_end(args);
-    }
+    StringWriter &Clear(void);
 
     /**
-     * This method clears the string.
+     * This method returns whether the output is truncated.
+     *
+     * @note If the output is truncated, the buffer is still null-terminated.
+     *
+     * @retval  true    The output is truncated.
+     * @retval  false   The output is not truncated.
      *
      */
-    void Clear(void)
-    {
-        mBuffer[0] = 0;
-        mLength    = 0;
-    }
+    bool IsTruncated(void) const { return mLength >= mSize; }
 
     /**
-     * This method gets the length of the string.
+     * This method gets the length of the wanted string.
      *
      * Similar to `strlen()` the length does not include the null character at the end of the string.
      *
@@ -172,107 +168,50 @@ public:
     uint16_t GetLength(void) const { return mLength; }
 
     /**
-     * This method returns the size (number of chars) in the buffer storage for the string.
+     * This method returns the size (number of chars) in the buffer.
      *
-     * @returns The size of the buffer storage for the string.
+     * @returns The size of the buffer.
      *
      */
-    uint16_t GetSize(void) const { return kSize; }
+    uint16_t GetSize(void) const { return mSize; }
 
     /**
-     * This method returns the string as a null-terminated C string.
-     *
-     * @returns The null-terminated C string.
-     *
-     */
-    const char *AsCString(void) const { return mBuffer; }
-
-    /**
-     * This method sets the string using `printf()` style formatted data.
+     * This method appends `printf()` style formatted data to the buffer.
      *
      * @param[in] aFormat    A pointer to the format string.
      * @param[in] ...        Arguments for the format specification.
      *
-     * @retval kErrorNone          Updated the string successfully.
-     * @retval kErrorNoBufs        String could not fit in the storage.
-     * @retval kErrorInvalidArgs   Arguments do not match the format string.
+     * @returns The string writer.
      *
      */
-    Error Set(const char *aFormat, ...)
-    {
-        va_list args;
-        Error   error;
-
-        va_start(args, aFormat);
-        mLength = 0;
-        error   = Write(mBuffer, kSize, mLength, aFormat, args);
-        va_end(args);
-
-        return error;
-    }
+    StringWriter &Append(const char *aFormat, ...);
 
     /**
-     * This method appends `printf()` style formatted data to the `String` object.
-     *
-     * @param[in] aFormat    A pointer to the format string.
-     * @param[in] ...        Arguments for the format specification.
-     *
-     * @retval kErrorNone          Updated the string successfully.
-     * @retval kErrorNoBufs        String could not fit in the storage.
-     * @retval kErrorInvalidArgs   Arguments do not match the format string.
-     *
-     */
-    Error Append(const char *aFormat, ...)
-    {
-        va_list args;
-        Error   error;
-
-        va_start(args, aFormat);
-        error = Write(mBuffer, kSize, mLength, aFormat, args);
-        va_end(args);
-
-        return error;
-    }
-
-    /**
-     * This method appends `printf()` style formatted data to the `String` object.
+     * This method appends `printf()` style formatted data to the buffer.
      *
      * @param[in] aFormat    A pointer to the format string.
      * @param[in] aArgs      Arguments for the format specification (as `va_list`).
      *
-     * @retval kErrorNone          Updated the string successfully.
-     * @retval kErrorNoBufs        String could not fit in the storage.
-     * @retval kErrorInvalidArgs   Arguments do not match the format string.
+     * @returns The string writer.
      *
      */
-    Error AppendVarArgs(const char *aFormat, va_list aArgs) { return Write(mBuffer, kSize, mLength, aFormat, aArgs); }
+    StringWriter &AppendVarArgs(const char *aFormat, va_list aArgs);
 
     /**
-     * This method appends an array of bytes in hex representation (using "%02x" style) to the `String` object.
+     * This method appends an array of bytes in hex representation (using "%02x" style) to the buffer.
      *
      * @param[in] aBytes    A pointer to buffer containing the bytes to append.
      * @param[in] aLength   The length of @p aBytes buffer (in bytes).
      *
-     * @retval kErrorNone          Updated the string successfully.
-     * @retval kErrorNoBufs        String could not fit in the storage.
+     * @returns The string writer.
      *
      */
-    Error AppendHexBytes(const uint8_t *aBytes, uint16_t aLength)
-    {
-        Error error = kErrorNone;
-
-        while (aLength--)
-        {
-            SuccessOrExit(error = Append("%02x", *aBytes++));
-        }
-
-    exit:
-        return error;
-    }
+    StringWriter &AppendHexBytes(const uint8_t *aBytes, uint16_t aLength);
 
 private:
-    uint16_t mLength;
-    char     mBuffer[kSize];
+    char *         mBuffer;
+    uint16_t       mLength;
+    const uint16_t mSize;
 };
 
 /**

src/core/mac/channel_mask.cpp

@@ -95,12 +95,13 @@ exit:
 
 ChannelMask::InfoString ChannelMask::ToString(void) const
 {
-    InfoString string;
-    uint8_t    channel  = kChannelIteratorFirst;
-    bool       addComma = false;
-    Error      error;
+    InfoString   string;
+    StringWriter writer(string);
+    uint8_t      channel  = kChannelIteratorFirst;
+    bool         addComma = false;
+    Error        error;
 
-    IgnoreError(string.Append("{"));
+    writer.Append("{");
 
     error = GetNextChannel(channel);
 
@@ -119,16 +120,16 @@ ChannelMask::InfoString ChannelMask::ToString(void) const
             rangeEnd = channel;
         }
 
-        IgnoreError(string.Append("%s%d", addComma ? ", " : " ", rangeStart));
+        writer.Append("%s%d", addComma ? ", " : " ", rangeStart);
         addComma = true;
 
         if (rangeStart < rangeEnd)
         {
-            IgnoreError(string.Append("%s%d", rangeEnd == rangeStart + 1 ? ", " : "-", rangeEnd));
+            writer.Append("%s%d", rangeEnd == rangeStart + 1 ? ", " : "-", rangeEnd);
         }
     }
 
-    IgnoreError(string.Append(" }"));
+    writer.Append(" }");
 
     return string;
 }

src/core/mac/mac_frame.cpp

@@ -1381,26 +1381,27 @@ exit:
 
 Frame::InfoString Frame::ToInfoString(void) const
 {
-    InfoString string;
-    uint8_t    commandId, type;
-    Address    src, dst;
+    InfoString   string;
+    StringWriter writer(string);
+    uint8_t      commandId, type;
+    Address      src, dst;
 
-    IgnoreError(string.Append("len:%d, seqnum:%d, type:", mLength, GetSequence()));
+    writer.Append("len:%d, seqnum:%d, type:", mLength, GetSequence());
 
     type = GetType();
 
     switch (type)
     {
     case kFcfFrameBeacon:
-        IgnoreError(string.Append("Beacon"));
+        writer.Append("Beacon");
         break;
 
     case kFcfFrameData:
-        IgnoreError(string.Append("Data"));
+        writer.Append("Data");
         break;
 
     case kFcfFrameAck:
-        IgnoreError(string.Append("Ack"));
+        writer.Append("Ack");
         break;
 
     case kFcfFrameMacCmd:
@@ -1412,34 +1413,33 @@ Frame::InfoString Frame::ToInfoString(void) const
         switch (commandId)
         {
         case kMacCmdDataRequest:
-            IgnoreError(string.Append("Cmd(DataReq)"));
+            writer.Append("Cmd(DataReq)");
             break;
 
         case kMacCmdBeaconRequest:
-            IgnoreError(string.Append("Cmd(BeaconReq)"));
+            writer.Append("Cmd(BeaconReq)");
             break;
 
         default:
-            IgnoreError(string.Append("Cmd(%d)", commandId));
+            writer.Append("Cmd(%d)", commandId);
             break;
         }
 
         break;
 
     default:
-        IgnoreError(string.Append("%d", type));
+        writer.Append("%d", type);
         break;
     }
 
     IgnoreError(GetSrcAddr(src));
     IgnoreError(GetDstAddr(dst));
 
-    IgnoreError(string.Append(", src:%s, dst:%s, sec:%s, ackreq:%s", src.ToString().AsCString(),
-                              dst.ToString().AsCString(), GetSecurityEnabled() ? "yes" : "no",
-                              GetAckRequest() ? "yes" : "no"));
+    writer.Append(", src:%s, dst:%s, sec:%s, ackreq:%s", src.ToString().AsCString(), dst.ToString().AsCString(),
+                  GetSecurityEnabled() ? "yes" : "no", GetAckRequest() ? "yes" : "no");
 
 #if OPENTHREAD_CONFIG_MULTI_RADIO
-    IgnoreError(string.Append(", radio:%s", RadioTypeToString(GetRadioType())));
+    writer.Append(", radio:%s", RadioTypeToString(GetRadioType()));
 #endif
 
     return string;
@@ -1448,12 +1448,14 @@ Frame::InfoString Frame::ToInfoString(void) const
 BeaconPayload::InfoString BeaconPayload::ToInfoString(void) const
 {
     NetworkName name;
+    InfoString  string;
 
     IgnoreError(name.Set(GetNetworkName()));
 
-    return InfoString("name:%s, xpanid:%s, id:%d, ver:%d, joinable:%s, native:%s", name.GetAsCString(),
-                      mExtendedPanId.ToString().AsCString(), GetProtocolId(), GetProtocolVersion(),
-                      IsJoiningPermitted() ? "yes" : "no", IsNative() ? "yes" : "no");
+    StringWriter(string).Append("name:%s, xpanid:%s, id:%d, ver:%d, joinable:%s, native:%s", name.GetAsCString(),
+                                mExtendedPanId.ToString().AsCString(), GetProtocolId(), GetProtocolVersion(),
+                                IsJoiningPermitted() ? "yes" : "no", IsNative() ? "yes" : "no");
+    return string;
 }
 
 #endif // #if (OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_NOTE) && (OPENTHREAD_CONFIG_LOG_MAC == 1)

src/core/mac/mac_types.cpp

@@ -67,7 +67,7 @@ ExtAddress::InfoString ExtAddress::ToString(void) const
 {
     InfoString string;
 
-    IgnoreError(string.AppendHexBytes(m8, sizeof(ExtAddress)));
+    StringWriter(string).AppendHexBytes(m8, sizeof(ExtAddress));
 
     return string;
 }
@@ -92,15 +92,29 @@ void ExtAddress::CopyAddress(uint8_t *aDst, const uint8_t *aSrc, CopyByteOrder a
 
 Address::InfoString Address::ToString(void) const
 {
-    return (mType == kTypeExtended) ? GetExtended().ToString()
-                                    : (mType == kTypeNone ? InfoString("None") : InfoString("0x%04x", GetShort()));
+    InfoString string;
+
+    if (mType == kTypeExtended)
+    {
+        string = GetExtended().ToString();
+    }
+    else if (mType == kTypeNone)
+    {
+        StringWriter(string).Append("None");
+    }
+    else
+    {
+        StringWriter(string).Append("0x%04x", GetShort());
+    }
+
+    return string;
 }
 
 ExtendedPanId::InfoString ExtendedPanId::ToString(void) const
 {
     InfoString string;
 
-    IgnoreError(string.AppendHexBytes(m8, sizeof(ExtendedPanId)));
+    StringWriter(string).AppendHexBytes(m8, sizeof(ExtendedPanId));
 
     return string;
 }
@@ -203,13 +217,15 @@ void RadioTypes::AddAll(void)
 
 RadioTypes::InfoString RadioTypes::ToString(void) const
 {
-    InfoString string("{");
-    bool       addComma = false;
+    InfoString   string;
+    StringWriter writer(string);
+    bool         addComma = false;
 
+    writer.Append("{");
 #if OPENTHREAD_CONFIG_RADIO_LINK_IEEE_802_15_4_ENABLE
     if (Contains(kRadioTypeIeee802154))
     {
-        IgnoreError(string.Append("%s%s", addComma ? ", " : " ", RadioTypeToString(kRadioTypeIeee802154)));
+        writer.Append("%s%s", addComma ? ", " : " ", RadioTypeToString(kRadioTypeIeee802154));
         addComma = true;
     }
 #endif
@@ -217,14 +233,14 @@ RadioTypes::InfoString RadioTypes::ToString(void) const
 #if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE
     if (Contains(kRadioTypeTrel))
     {
-        IgnoreError(string.Append("%s%s", addComma ? ", " : " ", RadioTypeToString(kRadioTypeTrel)));
+        writer.Append("%s%s", addComma ? ", " : " ", RadioTypeToString(kRadioTypeTrel));
         addComma = true;
     }
 #endif
 
     OT_UNUSED_VARIABLE(addComma);
 
-    IgnoreError(string.Append(" }"));
+    writer.Append(" }");
 
     return string;
 }

src/core/meshcop/meshcop.cpp

@@ -164,22 +164,23 @@ bool JoinerDiscerner::operator==(const JoinerDiscerner &aOther) const
 
 JoinerDiscerner::InfoString JoinerDiscerner::ToString(void) const
 {
-    InfoString str;
+    InfoString   str;
+    StringWriter writer(str);
 
     if (mLength <= sizeof(uint16_t) * CHAR_BIT)
     {
-        IgnoreError(str.Set("0x%04x", static_cast<uint16_t>(mValue)));
+        writer.Append("0x%04x", static_cast<uint16_t>(mValue));
     }
     else if (mLength <= sizeof(uint32_t) * CHAR_BIT)
     {
-        IgnoreError(str.Set("0x%08x", static_cast<uint32_t>(mValue)));
+        writer.Append("0x%08x", static_cast<uint32_t>(mValue));
     }
     else
     {
-        IgnoreError(str.Set("0x%x-%08x", static_cast<uint32_t>(mValue >> 32), static_cast<uint32_t>(mValue)));
+        writer.Append("0x%x-%08x", static_cast<uint32_t>(mValue >> 32), static_cast<uint32_t>(mValue));
     }
 
-    IgnoreError(str.Append("/len:%d", mLength));
+    writer.Append("/len:%d", mLength);
 
     return str;
 }

src/core/net/ip4_address.cpp

@@ -88,7 +88,10 @@ exit:
 
 Address::InfoString Address::ToString(void) const
 {
-    return InfoString("%d.%d.%d.%d", mBytes[0], mBytes[1], mBytes[2], mBytes[3]);
+    InfoString string;
+
+    StringWriter(string).Append("%d.%d.%d.%d", mBytes[0], mBytes[1], mBytes[2], mBytes[3]);
+    return string;
 }
 
 } // namespace Ip4

src/core/net/ip6_address.cpp

@@ -126,20 +126,21 @@ bool Prefix::IsValidNat64(void) const
 
 Prefix::InfoString Prefix::ToString(void) const
 {
-    InfoString string;
-    uint8_t    sizeInUint16 = (GetBytesSize() + sizeof(uint16_t) - 1) / sizeof(uint16_t);
+    InfoString   string;
+    StringWriter writer(string);
+    uint8_t      sizeInUint16 = (GetBytesSize() + sizeof(uint16_t) - 1) / sizeof(uint16_t);
 
     for (uint16_t i = 0; i < sizeInUint16; i++)
     {
-        IgnoreError(string.Append("%s%x", (i > 0) ? ":" : "", HostSwap16(mPrefix.mFields.m16[i])));
+        writer.Append("%s%x", (i > 0) ? ":" : "", HostSwap16(mPrefix.mFields.m16[i]));
     }
 
     if (GetBytesSize() < Address::kSize - 1)
     {
-        IgnoreError(string.Append("::"));
+        writer.Append("::");
     }
 
-    IgnoreError(string.Append("/%d", mLength));
+    writer.Append("/%d", mLength);
 
     return string;
 }
@@ -248,7 +249,7 @@ InterfaceIdentifier::InfoString InterfaceIdentifier::ToString(void) const
 {
     InfoString string;
 
-    IgnoreError(string.AppendHexBytes(mFields.m8, kSize));
+    StringWriter(string).AppendHexBytes(mFields.m8, kSize);
 
     return string;
 }
@@ -629,9 +630,12 @@ exit:
 
 Address::InfoString Address::ToString(void) const
 {
-    return InfoString("%x:%x:%x:%x:%x:%x:%x:%x", HostSwap16(mFields.m16[0]), HostSwap16(mFields.m16[1]),
-                      HostSwap16(mFields.m16[2]), HostSwap16(mFields.m16[3]), HostSwap16(mFields.m16[4]),
-                      HostSwap16(mFields.m16[5]), HostSwap16(mFields.m16[6]), HostSwap16(mFields.m16[7]));
+    InfoString string;
+
+    StringWriter(string).Append("%x:%x:%x:%x:%x:%x:%x:%x", HostSwap16(mFields.m16[0]), HostSwap16(mFields.m16[1]),
+                                HostSwap16(mFields.m16[2]), HostSwap16(mFields.m16[3]), HostSwap16(mFields.m16[4]),
+                                HostSwap16(mFields.m16[5]), HostSwap16(mFields.m16[6]), HostSwap16(mFields.m16[7]));
+    return string;
 }
 
 const Address &Address::GetLinkLocalAllNodesMulticast(void)

src/core/net/socket.cpp

@@ -38,7 +38,10 @@ namespace Ip6 {
 
 SockAddr::InfoString SockAddr::ToString(void) const
 {
-    return InfoString("[%s]:%u", GetAddress().ToString().AsCString(), GetPort());
+    InfoString string;
+
+    StringWriter(string).Append("[%s]:%u", GetAddress().ToString().AsCString(), GetPort());
+    return string;
 }
 
 } // namespace Ip6

src/core/radio/trel_packet.cpp

@@ -77,35 +77,35 @@ uint16_t Header::GetSize(Type aType)
 
 Header::InfoString Header::ToString(void) const
 {
-    Type       type = GetType();
-    InfoString string;
+    Type         type = GetType();
+    InfoString   string;
+    StringWriter writer(string);
 
     switch (type)
     {
     case kTypeBroadcast:
-        IgnoreError(string.Set("broadcast ch:%d", GetChannel()));
+        writer.Append("broadcast ch:%d", GetChannel());
         break;
 
     case kTypeUnicast:
-        IgnoreError(string.Set("unicast ch:%d", GetChannel()));
+        writer.Append("unicast ch:%d", GetChannel());
         break;
 
     case kTypeAck:
-        IgnoreError(string.Set("ack"));
+        writer.Append("ack");
         break;
     }
 
-    IgnoreError(
-        string.Append(" panid:%04x num:%lu src:%s", GetPanId(), GetPacketNumber(), GetSource().ToString().AsCString()));
+    writer.Append(" panid:%04x num:%lu src:%s", GetPanId(), GetPacketNumber(), GetSource().ToString().AsCString());
 
     if ((type == kTypeUnicast) || (type == kTypeAck))
     {
-        IgnoreError(string.Append(" dst:%s", GetDestination().ToString().AsCString()));
+        writer.Append(" dst:%s", GetDestination().ToString().AsCString());
     }
 
     if ((type == kTypeUnicast) || (type == kTypeBroadcast))
     {
-        IgnoreError(string.Append(GetAckMode() == kNoAck ? " no-ack" : " ack-req"));
+        writer.Append(GetAckMode() == kNoAck ? " no-ack" : " ack-req");
     }
 
     return string;

src/core/thread/link_quality.cpp

@@ -109,7 +109,8 @@ RssAverager::InfoString RssAverager::ToString(void) const
     InfoString string;
 
     VerifyOrExit(mCount != 0);
-    IgnoreError(string.Set("%d.%s", -(mAverage >> kPrecisionBitShift), kDigitsString[mAverage & kPrecisionBitMask]));
+    StringWriter(string).Append("%d.%s", -(mAverage >> kPrecisionBitShift),
+                                kDigitsString[mAverage & kPrecisionBitMask]);
 
 exit:
     return string;
@@ -166,8 +167,11 @@ uint8_t LinkQualityInfo::GetLinkMargin(void) const
 
 LinkQualityInfo::InfoString LinkQualityInfo::ToInfoString(void) const
 {
-    return InfoString("aveRss:%s, lastRss:%d, linkQuality:%d", mRssAverager.ToString().AsCString(), GetLastRss(),
-                      GetLinkQuality());
+    InfoString string;
+
+    StringWriter(string).Append("aveRss:%s, lastRss:%d, linkQuality:%d", mRssAverager.ToString().AsCString(),
+                                GetLastRss(), GetLinkQuality());
+    return string;
 }
 
 uint8_t LinkQualityInfo::ConvertRssToLinkMargin(int8_t aNoiseFloor, int8_t aRss)

src/core/thread/mle.cpp

@@ -4117,7 +4117,7 @@ void Mle::Log(MessageAction aAction, MessageType aType, const Ip6::Address &aAdd
 
     if (aRloc != Mac::kShortAddrInvalid)
     {
-        IgnoreError(rlocString.Set(",0x%04x", aRloc));
+        StringWriter(rlocString).Append(",0x%04x", aRloc);
     }
 
     otLogInfoMle("%s %s%s (%s%s)", MessageActionToString(aAction), MessageTypeToString(aType),

src/core/thread/mle_types.cpp

@@ -55,8 +55,11 @@ void DeviceMode::Set(const ModeConfig &aModeConfig)
 
 DeviceMode::InfoString DeviceMode::ToString(void) const
 {
-    return InfoString("rx-on:%s ftd:%s full-net:%s", IsRxOnWhenIdle() ? "yes" : "no",
-                      IsFullThreadDevice() ? "yes" : "no", IsFullNetworkData() ? "yes" : "no");
+    InfoString string;
+
+    StringWriter(string).Append("rx-on:%s ftd:%s full-net:%s", IsRxOnWhenIdle() ? "yes" : "no",
+                                IsFullThreadDevice() ? "yes" : "no", IsFullNetworkData() ? "yes" : "no");
+    return string;
 }
 
 void MeshLocalPrefix::SetFromExtendedPanId(const Mac::ExtendedPanId &aExtendedPanId)

src/core/thread/radio_selector.cpp

@@ -369,8 +369,9 @@ void RadioSelector::Log(otLogLevel      aLogLevel,
     {
         if (aNeighbor.GetSupportedRadioTypes().Contains(radio))
         {
-            IgnoreError(preferenceString.Append("%s%s:%d", isFirstEntry ? "" : " ", RadioTypeToString(radio),
-                                                aNeighbor.GetRadioPreference(radio)));
+            StringWriter(preferenceString)
+                .Append("%s%s:%d", isFirstEntry ? "" : " ", RadioTypeToString(radio),
+                        aNeighbor.GetRadioPreference(radio));
             isFirstEntry = false;
         }
     }

src/core/utils/channel_monitor.cpp

@@ -195,7 +195,7 @@ void ChannelMonitor::LogResults(void)
 
     for (uint16_t channel : mChannelOccupancy)
     {
-        IgnoreError(logString.Append("%02x ", channel >> 8));
+        StringWriter(logString).Append("%02x ", channel >> 8);
     }
 
     otLogInfoUtil("ChannelMonitor: %u [%s]", mSampleCount, logString.AsCString());

tests/unit/test_string.cpp

@@ -43,76 +43,70 @@ enum
 
 template <uint16_t kSize> void PrintString(const char *aName, const String<kSize> aString)
 {
-    printf("\t%s = [%d] \"%s\"\n", aName, aString.GetLength(), aString.AsCString());
+    printf("\t%s = [%zu] \"%s\"\n", aName, strlen(aString.AsCString()), aString.AsCString());
 }
 
-void TestString(void)
+void TestStringWriter(void)
 {
-    otError             error;
-    String<kStringSize> str1;
-    String<kStringSize> str2("abc");
-    String<kStringSize> str3("%d", 12);
+    String<kStringSize> str;
+    constexpr char      kLongString[] = "abcdefghijklmnopqratuvwxyzabcdefghijklmnopqratuvwxyz";
 
-    printf("\nTest 1: String constructor\n");
+    printf("\nTest 1: StringWriter constructor\n");
 
-    VerifyOrQuit(str1.GetSize() == kStringSize, "GetSize() failed");
+    VerifyOrQuit(StringWriter(str).GetSize() == kStringSize, "GetSize() failed");
+    VerifyOrQuit(StringWriter(str).GetLength() == 0, "GetLength() failed for empty string");
 
-    VerifyOrQuit(str1.GetLength() == 0, "GetLength() failed for empty string");
-    VerifyOrQuit(str2.GetLength() == 3, "GetLength() failed");
-    VerifyOrQuit(str3.GetLength() == 2, "GetLength() failed");
+    VerifyOrQuit(strcmp(str.AsCString(), "") == 0, "String content is incorrect");
 
-    VerifyOrQuit(strcmp(str1.AsCString(), "") == 0, "String content is incorrect");
-    VerifyOrQuit(strcmp(str2.AsCString(), "abc") == 0, "String content is incorrect");
-    VerifyOrQuit(strcmp(str3.AsCString(), "12") == 0, "String content is incorrect");
-
-    PrintString("str1", str1);
-    PrintString("str2", str2);
-    PrintString("str3", str3);
+    PrintString("str", str);
 
     printf(" -- PASS\n");
 
-    printf("\nTest 2: String::Set() and String::Clear() method\n");
+    printf("\nTest 2: String::Append() method\n");
 
-    error = str1.Set("Hello");
-    SuccessOrQuit(error, "String::Set() failed unexpectedly");
-    VerifyOrQuit(str1.GetLength() == 5, "GetLength() failed for empty string");
-    VerifyOrQuit(strcmp(str1.AsCString(), "Hello") == 0, "String content is incorrect");
-    PrintString("str1", str1);
+    {
+        StringWriter writer(str);
 
-    str1.Clear();
-    VerifyOrQuit(str1.GetLength() == 0, "GetLength() failed for empty string");
-    VerifyOrQuit(strcmp(str1.AsCString(), "") == 0, "String content is incorrect");
+        writer.Append("Hi");
+        VerifyOrQuit(writer.GetLength() == 2, "GetLength() failed");
+        VerifyOrQuit(strcmp(str.AsCString(), "Hi") == 0, "String content is incorrect");
+        PrintString("str", str);
 
-    IgnoreError(str1.Set("%d", 12));
-    VerifyOrQuit(str1.GetLength() == 2, "GetLength() failed");
-    VerifyOrQuit(strcmp(str1.AsCString(), "12") == 0, "String content is incorrect");
-    PrintString("str1", str1);
+        writer.Append("%s%d", "!", 12);
+        VerifyOrQuit(writer.GetLength() == 5, "GetLength() failed");
+        VerifyOrQuit(strcmp(str.AsCString(), "Hi!12") == 0, "String content is incorrect");
+        PrintString("str", str);
 
-    error = str1.Set("abcdefghijklmnopqratuvwxyzabcdefghijklmnopqratuvwxyz");
-    VerifyOrQuit(error == OT_ERROR_NO_BUFS, "String::Set() did not handle overflow buffer correctly");
-    PrintString("str1", str1);
+        writer.Append(kLongString);
+        VerifyOrQuit(writer.IsTruncated() && writer.GetLength() == 5 + sizeof(kLongString) - 1,
+                     "String::Append() did not handle overflow buffer correctly");
+        PrintString("str", str);
+    }
 
-    printf("\nTest 3: String::Append() method\n");
+    printf("\nTest 3: String::Clear() method\n");
 
-    str2.Clear();
-    VerifyOrQuit(str2.GetLength() == 0, "GetLength() failed for empty string");
-    VerifyOrQuit(strcmp(str2.AsCString(), "") == 0, "String content is incorrect");
+    {
+        StringWriter writer(str);
 
-    error = str2.Append("Hi");
-    SuccessOrQuit(error, "String::Append() failed unexpectedly");
-    VerifyOrQuit(str2.GetLength() == 2, "GetLength() failed");
-    VerifyOrQuit(strcmp(str2.AsCString(), "Hi") == 0, "String content is incorrect");
-    PrintString("str2", str2);
+        writer.Append("Hello");
+        VerifyOrQuit(writer.GetLength() == 5, "GetLength() failed for empty string");
+        VerifyOrQuit(strcmp(str.AsCString(), "Hello") == 0, "String content is incorrect");
+        PrintString("str", str);
 
-    error = str2.Append("%s%d", "!", 12);
-    SuccessOrQuit(error, "String::Append() failed unexpectedly");
-    VerifyOrQuit(str2.GetLength() == 5, "GetLength() failed");
-    VerifyOrQuit(strcmp(str2.AsCString(), "Hi!12") == 0, "String content is incorrect");
-    PrintString("str2", str2);
+        writer.Clear();
+        VerifyOrQuit(writer.GetLength() == 0, "GetLength() failed for empty string");
+        VerifyOrQuit(strcmp(str.AsCString(), "") == 0, "String content is incorrect");
 
-    error = str2.Append("abcdefghijklmnopqratuvwxyzabcdefghijklmnopqratuvwxyz");
-    VerifyOrQuit(error == OT_ERROR_NO_BUFS, "String::Append() did not handle overflow buffer correctly");
-    PrintString("str2", str2);
+        writer.Append("%d", 12);
+        VerifyOrQuit(writer.GetLength() == 2, "GetLength() failed");
+        VerifyOrQuit(strcmp(str.AsCString(), "12") == 0, "String content is incorrect");
+        PrintString("str", str);
+
+        writer.Clear().Append(kLongString);
+        VerifyOrQuit(writer.IsTruncated() && writer.GetLength() == sizeof(kLongString) - 1,
+                     "String::Clear() + String::Append() did not handle overflow buffer correctly");
+        PrintString("str", str);
+    }
 
     printf(" -- PASS\n");
 }
@@ -179,7 +173,7 @@ void TestStringFind(void)
 
 int main(void)
 {
-    ot::TestString();
+    ot::TestStringWriter();
     ot::TestStringLength();
     ot::TestUtf8();
     ot::TestStringFind();