net/SocketAddress: add CastTo()

src/net/SocketAddress.hxx

@@ -1,5 +1,5 @@
 /*
- * 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
@@ -82,6 +82,19 @@ public:
 		return address;
 	}
 
+	/**
+	 * Cast the "sockaddr" pointer to a different address type,
+	 * e.g. "sockaddr_in".  This is only legal after checking
+	 * !IsNull() and GetFamily().
+	 */
+	template<typename T>
+	constexpr const T &CastTo() const noexcept {
+		/* cast through void to work around the bogus
+		   alignment warning */
+		const void *q = reinterpret_cast<const void *>(address);
+		return *reinterpret_cast<const T *>(q);
+	}
+
 	constexpr size_type GetSize() const noexcept {
 		return size;
 	}