Add "null" method to signals so user can query if a signal

has been moved from.  Allow some methods to be called on
null signals.

Author: fmhess

doc/reference/signal_header.xml

@@ -145,9 +145,7 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
             <paramtype>signal &amp;&amp;</paramtype>
           </parameter>
           <description><para>Move constructor.</para></description>
-          <postconditions><para>The signal <computeroutput>other</computeroutput>
-            is in a "moved-from" state where it may only be destroyed, swapped, or move assigned.
-            Any other operation on a "moved-from" signal is invalid.</para></postconditions>
+          <postconditions><para><computeroutput>other.<methodname>null</methodname>()</computeroutput></para></postconditions>
 
           <throws><para>Will not throw.</para></throws>
         </constructor>
@@ -157,9 +155,7 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
             <paramtype>signal &amp;&amp;</paramtype>
           </parameter>
           <description><para>Move assignment.</para></description>
-          <postconditions><para>The signal <computeroutput>rhs</computeroutput>
-            is in a "moved-from" state where it may only be destroyed, swapped, or move assigned.
-            Any other operation on a "moved-from" signal is invalid.</para></postconditions>
+          <postconditions><para><computeroutput>rhs.<methodname>null</methodname>()</computeroutput></para></postconditions>
 
           <throws><para>Will not throw.</para></throws>
         </copy-assignment>
@@ -431,6 +427,41 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
             <throws><para>Will not throw.</para></throws>
           </method>
         </method-group>
+    
+        <method-group name="misc">
+          <method name="null" cv="const">
+            <type>bool</type>
+            <returns><para><computeroutput>false</computeroutput> for a normal signal,
+                <computeroutput>true</computeroutput> after a signal
+                has been moved from.  Null signals are nonfunctional husks left
+                behind after a signal is moved.  They may not be invoked or have
+                slots connected to them.</para>
+            </returns>
+
+            <throws><para>Will not throw.</para></throws>
+
+            <notes>
+                <para>A null signal may be destroyed, swapped, or move assigned.  Also,
+                    the following methods may be called on a null signal:
+                </para>
+                <variablelist>
+                    <varlistentry><term><methodname>disconnect</methodname> and <methodname>disconnect_all_slots</methodname></term>
+                        <listitem>no effect</listitem>
+                    </varlistentry>
+                    <varlistentry><term><methodname>empty</methodname></term>
+                        <listitem>returns <computeroutput>true</computeroutput></listitem>
+                    </varlistentry>
+                    <varlistentry><term><methodname>null</methodname></term>
+                        <listitem>returns <computeroutput>true</computeroutput></listitem>
+                    </varlistentry>
+                    <varlistentry><term><methodname>num_slots</methodname></term>
+                        <listitem>returns <computeroutput>0</computeroutput></listitem>
+                    </varlistentry>
+                </variablelist>
+                <para>Any other operation on a null signal is invalid.</para>
+            </notes>
+          </method>
+        </method-group>
 
         <free-function-group name="specialized algorithms">
           <function name="swap">

include/boost/signals2/detail/signal_template.hpp

@@ -714,15 +714,18 @@ namespace boost
       }
       void disconnect_all_slots()
       {
+        if (_pimpl.get() == 0) return;
         (*_pimpl).disconnect_all_slots();
       }
       void disconnect(const group_type &group)
       {
+        if (_pimpl.get() == 0) return;
         (*_pimpl).disconnect(group);
       }
       template <typename T>
       void disconnect(const T &slot)
       {
+        if (_pimpl.get() == 0) return;
         (*_pimpl).disconnect(slot);
       }
       result_type operator ()(BOOST_SIGNALS2_SIGNATURE_FULL_ARGS(BOOST_SIGNALS2_NUM_ARGS))
@@ -735,10 +738,12 @@ namespace boost
       }
       std::size_t num_slots() const
       {
+        if (_pimpl.get() == 0) return 0;
         return (*_pimpl).num_slots();
       }
       bool empty() const
       {
+        if (_pimpl.get() == 0) return true;
         return (*_pimpl).empty();
       }
       combiner_type combiner() const
@@ -758,6 +763,10 @@ namespace boost
       {
         return _pimpl.get() == other._pimpl.get();
       }
+      bool null() const
+      {
+        return _pimpl.get() == 0;
+      }
     protected:
       virtual shared_ptr<void> lock_pimpl() const
       {

test/signal_test.cpp

@@ -339,8 +339,13 @@ void test_move()
   signal_type sig2(dummy_combiner(2));
   BOOST_CHECK(sig2() == 2);
 
+  BOOST_CHECK(sig2.null() == false);
   sig1 = std::move(sig2);
   BOOST_CHECK(sig1() == 2);
+  BOOST_CHECK(sig2.null() == true);
+  BOOST_CHECK(sig2.empty() == true);
+  BOOST_CHECK(sig2.num_slots() == 0);
+  sig2.disconnect_all_slots();
 
   signal_type sig3(std::move(sig1));
   BOOST_CHECK(sig3() == 2);