[common] replace enum constants with constexpr (#6845)

This commit replaces the `enum` constants with `constexpr` definitions
in all the modules under `core/common`. It also adds type to the
names `enum` definitions.

src/core/common/crc16.hpp

@@ -47,7 +47,7 @@ namespace ot {
 class Crc16
 {
 public:
-    enum Polynomial
+    enum Polynomial : uint16_t
     {
         kCcitt = 0x1021, ///< CRC16_CCITT
         kAnsi  = 0x8005, ///< CRC16-ANSI

src/core/common/logging.cpp

@@ -226,11 +226,8 @@ exit:
 }
 #endif
 
-enum : uint8_t
-{
-    kStringLineLength = 80,
-    kDumpBytesPerLine = 16,
-};
+static constexpr uint8_t kStringLineLength = 80;
+static constexpr uint8_t kDumpBytesPerLine = 16;
 
 static void DumpLine(otLogLevel aLogLevel, otLogRegion aLogRegion, const uint8_t *aBytes, const size_t aLength)
 {
@@ -279,10 +276,7 @@ static void DumpLine(otLogLevel aLogLevel, otLogRegion aLogRegion, const uint8_t
 
 void otDump(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aId, const void *aBuf, const size_t aLength)
 {
-    enum : uint8_t
-    {
-        kWidth = 72,
-    };
+    constexpr uint8_t kWidth = 72;
 
     size_t                        idLen = strlen(aId);
     ot::String<kStringLineLength> string;

src/core/common/message.hpp

@@ -134,11 +134,8 @@ class HmacSha256;
         }                                                        \
     } while (false)
 
-enum
-{
-    kNumBuffers = OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS,
-    kBufferSize = OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE,
-};
+constexpr uint16_t kNumBuffers = OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS;
+constexpr uint16_t kBufferSize = OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE;
 
 class Message;
 class MessagePool;
@@ -285,11 +282,8 @@ private:
      */
     const uint8_t *GetData(void) const { return mBuffer.mData; }
 
-    enum
-    {
-        kBufferDataSize     = kBufferSize - sizeof(otMessageBuffer),
-        kHeadBufferDataSize = kBufferDataSize - sizeof(MessageMetadata),
-    };
+    static constexpr uint16_t kBufferDataSize     = kBufferSize - sizeof(otMessageBuffer);
+    static constexpr uint16_t kHeadBufferDataSize = kBufferDataSize - sizeof(MessageMetadata);
 
 protected:
     union
@@ -321,7 +315,7 @@ public:
      * This enumeration represents the message type.
      *
      */
-    enum Type
+    enum Type : uint8_t
     {
         kTypeIp6          = 0, ///< A full uncompressed IPv6 packet
         kType6lowpan      = 1, ///< A 6lowpan frame
@@ -334,7 +328,7 @@ public:
      * This enumeration represents the message sub-type.
      *
      */
-    enum SubType
+    enum SubType : uint8_t
     {
         kSubTypeNone                   = 0,  ///< None
         kSubTypeMleAnnounce            = 1,  ///< MLE Announce
@@ -349,7 +343,7 @@ public:
         kSubTypeMleChildIdRequest      = 10, ///< MLE Child ID Request
     };
 
-    enum Priority
+    enum Priority : uint8_t
     {
         kPriorityLow    = OT_MESSAGE_PRIORITY_LOW,      ///< Low priority level.
         kPriorityNormal = OT_MESSAGE_PRIORITY_NORMAL,   ///< Normal priority level.
@@ -357,16 +351,13 @@ public:
         kPriorityNet    = OT_MESSAGE_PRIORITY_HIGH + 1, ///< Network Control priority level.
     };
 
-    enum
-    {
-        kNumPriorities = 4, ///< Number of priority levels.
-    };
+    static constexpr uint8_t kNumPriorities = 4; ///< Number of priority levels.
 
     /**
      * This enumeration represents the link security mode (used by `Settings` constructor).
      *
      */
-    enum LinkSecurityMode
+    enum LinkSecurityMode : uint8_t
     {
         kNoLinkSecurity,   ///< Link security disabled (no link security).
         kWithLinkSecurity, ///< Link security enabled.
@@ -1400,7 +1391,7 @@ public:
      * should be added in the queue.
      *
      */
-    enum QueuePosition
+    enum QueuePosition : uint8_t
     {
         kQueuePositionHead, ///< Indicates the head (front) of the list.
         kQueuePositionTail, ///< Indicates the tail (end) of the list.

src/core/common/notifier.hpp

@@ -297,13 +297,15 @@ public:
     }
 
 private:
-    enum
-    {
-        kMaxExternalHandlers   = OPENTHREAD_CONFIG_MAX_STATECHANGE_HANDLERS,
-        kFlagsStringLineLimit  = 70, // Character limit to divide the log into multiple lines in `LogChangedFlags()`.
-        kMaxFlagNameLength     = 25, // Max length for string representation of a flag by `FlagToString()`.
-        kFlagsStringBufferSize = kFlagsStringLineLimit + kMaxFlagNameLength,
-    };
+    static constexpr uint16_t kMaxExternalHandlers = OPENTHREAD_CONFIG_MAX_STATECHANGE_HANDLERS;
+
+    // Character limit to divide the log into multiple lines in `LogChangedFlags()`.
+    static constexpr uint16_t kFlagsStringLineLimit = 70;
+
+    // Max length for string representation of a flag by `FlagToString()`.
+    static constexpr uint8_t kMaxFlagNameLength = 25;
+
+    static constexpr uint16_t kFlagsStringBufferSize = kFlagsStringLineLimit + kMaxFlagNameLength;
 
     struct ExternalCallback
     {

src/core/common/settings.hpp

@@ -1062,7 +1062,7 @@ public:
         }
 
     private:
-        enum IteratorType
+        enum IteratorType : uint8_t
         {
             kEndIterator,
         };

src/core/common/time.hpp

@@ -249,10 +249,7 @@ public:
     static uint32_t MsecToSec(uint32_t aMilliseconds) { return aMilliseconds / 1000u; }
 
 private:
-    enum
-    {
-        kDistantFuture = (1UL << 31),
-    };
+    static constexpr uint32_t kDistantFuture = (1UL << 31);
 
     uint32_t mValue;
 };

src/core/common/time_ticker.hpp

@@ -109,11 +109,8 @@ public:
     bool IsReceiverRegistered(Receiver aReceiver) const { return (mReceivers & Mask(aReceiver)) != 0; }
 
 private:
-    enum : uint32_t
-    {
-        kTickInterval  = 1000, // in msec.
-        kRestartJitter = 4,    // in msec, jitter added when restarting the timer [-4,+4] ms.
-    };
+    static constexpr uint32_t kTickInterval  = 1000; // in msec.
+    static constexpr uint32_t kRestartJitter = 4;    // in msec, jitter added when restarting the timer [-4,+4] ms.
 
     constexpr static uint32_t Mask(Receiver aReceiver) { return static_cast<uint32_t>(1U) << aReceiver; }
 

src/core/common/tlvs.hpp

@@ -58,13 +58,10 @@ class Tlv
 {
 public:
     /**
-     * Length values.
+     * The maximum length of the Base TLV format.
      *
      */
-    enum
-    {
-        kBaseTlvMaxLength = OT_NETWORK_BASE_TLV_MAX_LENGTH, ///< The maximum length of the Base TLV format.
-    };
+    static constexpr uint8_t kBaseTlvMaxLength = OT_NETWORK_BASE_TLV_MAX_LENGTH;
 
     /**
      * This method returns the Type value.
@@ -431,10 +428,7 @@ public:
     }
 
 protected:
-    enum
-    {
-        kExtendedLength = 255, ///< Extended Length value
-    };
+    static const uint8_t kExtendedLength = 255; // Extended Length value.
 
 private:
     static Error Find(const Message &aMessage, uint8_t aType, uint16_t *aOffset, uint16_t *aSize, bool *aIsExtendedTlv);
@@ -483,10 +477,7 @@ private:
 template <uint8_t kTlvTypeValue> class TlvInfo
 {
 public:
-    enum : uint8_t
-    {
-        kType = kTlvTypeValue, ///< The TLV Type value.
-    };
+    static constexpr uint8_t kType = kTlvTypeValue; ///< The TLV Type value.
 };
 
 /**

src/core/common/trickle_timer.hpp

@@ -68,15 +68,12 @@ public:
         kModePlainTimer, ///< Operate as a plain periodic timer with random interval selected within min/max intervals.
     };
 
-    enum : uint16_t
-    {
-        /**
-         * Special value for redundancy constant (aka `k`) to indicate infinity (when used, it disables trickle timer's
-         * suppression behavior, invoking the handler callback independent of number of "consistent" events).
-         *
-         */
-        kInfiniteRedundancyConstant = NumericLimits<uint16_t>::kMax,
-    };
+    /**
+     * Special value for redundancy constant (aka `k`) to indicate infinity (when used, it disables trickle timer's
+     * suppression behavior, invoking the handler callback independent of number of "consistent" events).
+     *
+     */
+    static constexpr uint16_t kInfiniteRedundancyConstant = NumericLimits<uint16_t>::kMax;
 
     /**
      * This function pointer is called when the timer expires (i.e., transmission should happen).