rewrite(IPAddress): fix implementation

fix implementation and add discussion questions (safe using of
IPAddress).

Author: safocl

api/IPAddress.cpp

@@ -39,8 +39,34 @@ IPAddress::IPAddress(uint8_t o1, uint8_t o2, uint8_t o3, uint8_t o4, uint8_t o5,
 // IPv4 only
 IPAddress::IPAddress(uint32_t address) 
 {
-    uint32_t& addressRef = reinterpret_cast<uint32_t&>(_address[IPADDRESS_V4_BYTES_INDEX]);
-    addressRef = address;
+	memcpy(&_address[IPADDRESS_V4_BYTES_INDEX], &address, 4); // This method guarantees a defined behavior. Any pointer conversions to write to ADDRESS storage (as a multibyte integer) are undefined behavior when the lifetime of the multibyte type has not previously started.
+
+	// C++ standard draft [basic.life#7](https://eel.is/c++draft/basic.life#7)
+// Before the lifetime of an object has started but after the storage which the object 
+// will occupy has been allocated or, after the lifetime of an object has ended and 
+// before the storage which the object occupied is reused or released, any pointer that 
+// represents the address of the storage location where the object will be or was 
+// located may be used but only in limited ways. For an object under construction or 
+// destruction, see [class.cdtor]. Otherwise, such a pointer refers to allocated storage 
+// ([basic.stc.dynamic.allocation]), and using the pointer as if the pointer were of 
+// type void* is well-defined. Indirection through such a pointer is permitted but the 
+// resulting lvalue may only be used in limited ways, as described below. 
+// The program has undefined behavior if
+//  --the pointer is used as the operand of a delete-expression,
+//  --the pointer is used as the operand of a static_cast ([expr.static.cast]), except 
+//  when the conversion is to pointer to cv void, or to pointer to cv void and subsequently 
+//  to pointer to cv char, cv unsigned char, or cv std::byte ([cstddef.syn]), or
+
+	// C++ standard draft [basic.life#8](https://eel.is/c++draft/basic.life#8)
+// Similarly, before the lifetime of an object has started but after the storage which 
+// the object will occupy has been allocated or, after the lifetime of an object has 
+// ended and before the storage which the object occupied is reused or released, any 
+// glvalue that refers to the original object may be used but only in limited ways. 
+// For an object under construction or destruction, see [class.cdtor]. Otherwise, such 
+// a glvalue refers to allocated storage ([basic.stc.dynamic.allocation]), and using 
+// the properties of the glvalue that do not depend on its value is well-defined. 
+// The program has undefined behavior if
+//  -- the glvalue is used to access the object, or
 
     // NOTE on conversion/comparison and uint32_t:
     // These conversions are host platform dependent.
@@ -244,8 +270,7 @@ IPAddress& IPAddress::operator=(uint32_t address)
     // See note on conversion/comparison and uint32_t
     _type = IPv4;
     memset(_address, 0, sizeof(_address));
-    uint32_t& addressRef = reinterpret_cast<uint32_t&>(_address[IPADDRESS_V4_BYTES_INDEX]);
-    addressRef = address;
+	memcpy(&_address[IPADDRESS_V4_BYTES_INDEX], &address, 4); 
     return *this;
 }
 

api/IPAddress.h

@@ -19,6 +19,7 @@
 
 #pragma once
 
+#include <string.h>
 #include <stdint.h>
 #include "Printable.h"
 #include "String.h"
@@ -41,7 +42,10 @@ enum IPType {
 
 class IPAddress : public Printable {
 private:
-    alignas(alignof(uint32_t)) uint8_t _address[16]{};
+    alignas(alignof(uint32_t)) uint8_t _address[16]{}; // If the implementation does not require 
+													   // storage as a multibyte integer, you can 
+													   // remove the storage field alignment.
+													   // Address (as uint32) is accessed by copying.
     IPType _type{IPv4};
 
     // Access the raw byte array containing the address.  Because this returns a pointer
@@ -59,6 +63,7 @@ class IPAddress : public Printable {
     IPAddress(uint8_t first_octet, uint8_t second_octet, uint8_t third_octet, uint8_t fourth_octet);
     IPAddress(uint8_t o1, uint8_t o2, uint8_t o3, uint8_t o4, uint8_t o5, uint8_t o6, uint8_t o7, uint8_t o8, uint8_t o9, uint8_t o10, uint8_t o11, uint8_t o12, uint8_t o13, uint8_t o14, uint8_t o15, uint8_t o16);
      // IPv4; see implementation note
+	 // NOTE: address MUST BE BigEndian.
     IPAddress(uint32_t address);
      // Default IPv4
     IPAddress(const uint8_t *address);
@@ -71,7 +76,15 @@ class IPAddress : public Printable {
 
     // Overloaded cast operator to allow IPAddress objects to be used where a uint32_t is expected
     // NOTE: IPv4 only; see implementation note
-    operator uint32_t() const { return _type == IPv4 ? *reinterpret_cast<const uint32_t*>(&_address[IPADDRESS_V4_BYTES_INDEX]) : 0; };
+    // NOTE: Data of the returned integer in the native endiannes, but relevant ordering is a BigEndian.
+	//       The user is responsible for ensuring that the value is converted to BigEndian.
+    operator uint32_t() const {
+		uint32_t ret;
+		memcpy(&ret, &_address[IPADDRESS_V4_BYTES_INDEX], 4);
+		// NOTE: maybe use the placement-new for starting of the integer type lifetime in the storage when constructing an IPAddress?
+		// FIXME: need endiannes checking? how do this with the arduino-api?
+		return _type == IPv4 ? ret : 0;
+	};
 
     bool operator==(const IPAddress& addr) const;
     bool operator!=(const IPAddress& addr) const { return !(*this == addr); };