net/IPv[46]Address: add Cast(const sockaddr_in&)

src/net/IPv4Address.cxx

 /*
- * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions

src/net/IPv4Address.hxx

 /*
- * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -158,6 +158,15 @@ public:
 	}
 
 	/**
+	 * Cast a #sockaddr_in6 reference to an IPv6Address reference.
+	 */
+	static constexpr const IPv4Address &Cast(const struct sockaddr_in &src) noexcept {
+		/* this reinterpret_cast works because this class is
+		   just a wrapper for struct sockaddr_in6 */
+		return *(const IPv4Address *)(const void *)&src;
+	}
+
+	/**
 	 * Return a downcasted reference to the address.  This call is
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */

src/net/IPv6Address.cxx

 /*
- * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions

src/net/IPv6Address.hxx

 /*
- * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -127,6 +127,15 @@ public:
 	}
 
 	/**
+	 * Cast a #sockaddr_in6 reference to an IPv6Address reference.
+	 */
+	static constexpr const IPv6Address &Cast(const struct sockaddr_in6 &src) noexcept {
+		/* this reinterpret_cast works because this class is
+		   just a wrapper for struct sockaddr_in6 */
+		return *(const IPv6Address *)(const void *)&src;
+	}
+
+	/**
 	 * Return a downcasted reference to the address.  This call is
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */