move SocketHandle to TcpTag

include/sp/extensions/tcp/TcpListener.h

@@ -10,6 +10,8 @@ namespace io {
  */
 class TcpListener : private NonCopyable {
   public:
+	using SocketHandle = TcpTag::SocketHandle;
+
 	/**
 	 * \brief Starts listening for guests to connect
 	 * \param port The port to listen to

include/sp/extensions/tcp/TcpSocket.h

@@ -6,9 +6,9 @@
 namespace sp {
 namespace io {
 
-using SocketHandle = int;
-
-struct TcpTag {};
+struct TcpTag {
+	using SocketHandle = int;
+};
 
 class SocketError : public std::exception {
   private:
@@ -25,6 +25,8 @@ class SocketError : public std::exception {
 template <>
 class IOInterface<TcpTag> : private NonCopyable {
   public:
+	using SocketHandle = TcpTag::SocketHandle;
+
 	/**
 	 * \enum Status
 	 * \brief Describes the state of a socket