Add default construction and isNull for Call handlers

Subtrees/beast/modules/beast_asio/basics/beast_CompletionCall.h

@@ -30,6 +30,10 @@ class CompletionCall
 public:
     typedef void result_type;
 
+    CompletionCall () noexcept
+    {
+    }
+
     // Construction from Handler with zero arguments
     //
     template <typename Handler>
@@ -63,6 +67,11 @@ public:
     { 
     }
 
+    bool isNull () const noexcept
+    {
+        return m_call == nullptr;
+    }
+
     void operator() ()
     {
         (*m_call) ();

Subtrees/beast/modules/beast_asio/basics/beast_ErrorCall.h

@@ -39,6 +39,10 @@ class ErrorCall
 public:
     typedef void result_type;
 
+    ErrorCall () noexcept
+    {
+    }
+
     template <class Handler>
     ErrorCall (BOOST_ASIO_MOVE_ARG(Handler) handler)
         : m_call (new CallType <Handler> (BOOST_ASIO_MOVE_CAST(Handler)(handler)))
@@ -50,6 +54,11 @@ public:
     { 
     }
 
+    bool isNull () const noexcept
+    {
+        return m_call == nullptr;
+    }
+
     void operator() (boost::system::error_code const& ec)
     {
         (*m_call) (ec);

Subtrees/beast/modules/beast_asio/basics/beast_TransferCall.h

@@ -36,6 +36,10 @@ class TransferCall
 public:
     typedef void result_type;
 
+    TransferCall () noexcept
+    {
+    }
+
     template <class Handler>
     TransferCall (BOOST_ASIO_MOVE_ARG(Handler) handler)
         : m_call (new CallType <Handler> (BOOST_ASIO_MOVE_CAST(Handler)(handler)))
@@ -47,6 +51,11 @@ public:
     { 
     }
 
+    bool isNull () const noexcept
+    {
+        return m_call == nullptr;
+    }
+
     void operator() (boost::system::error_code const& ec, std::size_t bytes_transferred)
     {
         (*m_call) (ec, bytes_transferred);