net/SocketAddress: add CastTo()

src/net/IPv4Address.cxx

@@ -31,15 +31,9 @@
 
 #include <cassert>
 
-static const struct sockaddr_in *
-CastToIPv4(const struct sockaddr *p) noexcept
+IPv4Address::IPv4Address(SocketAddress src) noexcept
+	:address(src.CastTo<struct sockaddr_in>())
 {
-	assert(p->sa_family == AF_INET);
-
-	/* cast through void to work around the bogus alignment warning */
-	const void *q = reinterpret_cast<const void *>(p);
-	return reinterpret_cast<const struct sockaddr_in *>(q);
+	assert(!src.IsNull());
+	assert(src.GetFamily() == AF_INET);
 }
-
-IPv4Address::IPv4Address(SocketAddress src) noexcept
-	:address(*CastToIPv4(src.GetAddress())) {}

src/net/IPv4Address.hxx

@@ -171,9 +171,7 @@ public:
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */
 	static constexpr const IPv4Address &Cast(const SocketAddress src) noexcept {
-		/* this reinterpret_cast works because this class is
-		   just a wrapper for struct sockaddr_in */
-		return *(const IPv4Address *)(const void *)src.GetAddress();
+		return Cast(src.CastTo<struct sockaddr_in>());
 	}
 
 	constexpr operator SocketAddress() const noexcept {

src/net/IPv6Address.cxx

@@ -34,19 +34,13 @@
 
 #include <string.h>
 
-static const struct sockaddr_in6 *
-CastToIPv6(const struct sockaddr *p) noexcept
+IPv6Address::IPv6Address(SocketAddress src) noexcept
+	:address(src.CastTo<struct sockaddr_in6>())
 {
-	assert(p->sa_family == AF_INET6);
-
-	/* cast through void to work around the bogus alignment warning */
-	const void *q = reinterpret_cast<const void *>(p);
-	return reinterpret_cast<const struct sockaddr_in6 *>(q);
+	assert(!src.IsNull());
+	assert(src.GetFamily() == AF_INET6);
 }
 
-IPv6Address::IPv6Address(SocketAddress src) noexcept
-	:address(*CastToIPv6(src.GetAddress())) {}
-
 bool
 IPv6Address::IsAny() const noexcept
 {

src/net/IPv6Address.hxx

@@ -140,9 +140,7 @@ public:
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */
 	static constexpr const IPv6Address &Cast(const SocketAddress src) noexcept {
-		/* this reinterpret_cast works because this class is
-		   just a wrapper for struct sockaddr_in6 */
-		return *(const IPv6Address *)(const void *)src.GetAddress();
+		return Cast(src.CastTo<struct sockaddr_in6>());
 	}
 
 	constexpr operator SocketAddress() const noexcept {

src/net/SocketAddress.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
@@ -63,7 +63,7 @@ SocketAddress::GetLocalRaw() const noexcept
 		/* not applicable */
 		return nullptr;
 
-	const auto sun = (const struct sockaddr_un *)GetAddress();
+	const auto *sun = &CastTo<struct sockaddr_un>();
 	const auto start = (const char *)sun;
 	const auto path = sun->sun_path;
 	const size_t header_size = path - start;
@@ -159,10 +159,10 @@ SocketAddress::GetSteadyPart() const noexcept
 
 #ifdef HAVE_TCP
 	case AF_INET:
-		return ::GetSteadyPart(*(const struct sockaddr_in *)(const void *)GetAddress());
+		return ::GetSteadyPart(CastTo<struct sockaddr_in>());
 
 	case AF_INET6:
-		return ::GetSteadyPart(*(const struct sockaddr_in6 *)(const void *)GetAddress());
+		return ::GetSteadyPart(CastTo<struct sockaddr_in6>());
 #endif
 
 	default:

src/net/SocketAddress.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
@@ -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;
 	}

src/net/StaticSocketAddress.hxx

@@ -68,6 +68,19 @@ public:
 		return (const struct sockaddr *)(const void *)&address;
 	}
 
+	/**
+	 * Cast the "sockaddr" pointer to a different address type,
+	 * e.g. "sockaddr_in".  This is only legal after checking
+	 * 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 GetCapacity() const noexcept {
 		return sizeof(address);
 	}

src/net/ToString.cxx

 /*
- * Copyright 2011-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2011-2020 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
@@ -85,7 +85,7 @@ ToString(SocketAddress address) noexcept
 #ifdef HAVE_UN
 	if (address.GetFamily() == AF_LOCAL)
 		/* return path of local socket */
-		return LocalAddressToString(*(const sockaddr_un *)address.GetAddress(),
+		return LocalAddressToString(address.CastTo<struct sockaddr_un>(),
 					    address.GetSize());
 #endif
 

src/net/ToString.hxx

 /*
- * Copyright 2011-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2011-2020 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