slot, signal: Avoid compiler warnings from function pointer conversions

gcc8 -Wextra prints a warning when reinterpret_cast is used for conversion
between different types of function pointers. Avoid that by instead using
a union with members of the two types of function pointers.

Fixes #1

2 files changed, 98 insertions, 28 deletions

diff --git a/sigc++/functors/macros/slot.h.m4 b/sigc++/functors/macros/slot.h.m4
index 60cccbd..aa65677 100644
--- a/sigc++/functors/macros/slot.h.m4
+++ b/sigc++/functors/macros/slot.h.m4
@@ -63,7 +63,17 @@ FOR(1, $1,[
   inline T_return operator()(LOOP(arg%1_type_ _A_a%1, $1)) const
     {
       if (!empty() && !blocked())
-        return (reinterpret_cast<call_type>(slot_base::rep_->call_))(LIST(slot_base::rep_, LOOP(_A_a%1, $1)));
+      {
+        // Conversion between different types of function pointers with
+        // reinterpret_cast can make gcc8 print a warning.
+        // https://github.com/libsigcplusplus/libsigcplusplus/issues/1
+        union {
+          internal::hook ph;
+          call_type pc;
+        } u;
+        u.ph = slot_base::rep_->call_;
+        return (u.pc)(LIST(slot_base::rep_, LOOP(_A_a%1, $1)));
+      }
       return T_return();
     }
 
@@ -355,7 +365,14 @@ ifelse($1,0,[
    * @return A function pointer formed from call_it().
    */
   static hook address()
-    { return reinterpret_cast<hook>(&call_it); }
+  {
+    union {
+      hook ph;
+      decltype(&call_it) pc;
+    } u;
+    u.pc = &call_it;
+    return u.ph;
+  }
 };
 
 ])
@@ -483,7 +500,14 @@ struct slot_call
    * @return A function pointer formed from call_it().
    */
   static hook address()
-    { return reinterpret_cast<hook>(&call_it); }
+  {
+    union {
+      hook ph;
+      decltype(&call_it) pc;
+    } u;
+    u.pc = &call_it;
+    return u.ph;
+  }
 };
 
 /** Abstracts functor execution.
@@ -515,7 +539,14 @@ struct slot_call<T_functor, T_return>
    * @return A function pointer formed from call_it().
    */
   static hook address()
-    { return reinterpret_cast<hook>(&call_it); }
+  {
+    union {
+      hook ph;
+      decltype(&call_it) pc;
+    } u;
+    u.pc = &call_it;
+    return u.ph;
+  }
 };
 
 } /* namespace internal */
@@ -575,7 +606,14 @@ public:
   inline T_return operator()(type_trait_take_t<T_arg>... _A_a) const
     {
       if (!empty() && !blocked())
-        return (reinterpret_cast<call_type>(slot_base::rep_->call_))(slot_base::rep_, _A_a...);
+      {
+        union {
+          internal::hook ph;
+          call_type pc;
+        } u;
+        u.ph = slot_base::rep_->call_;
+        return (u.pc)(slot_base::rep_, _A_a...);
+      }
       return T_return();
     }
 
diff --git a/sigc++/macros/signal.h.m4 b/sigc++/macros/signal.h.m4
index 8f7ed2a..8f7f7d3 100644
--- a/sigc++/macros/signal.h.m4
+++ b/sigc++/macros/signal.h.m4
@@ -51,7 +51,17 @@ ifelse($1,0,[dnl
    * @return The slot's return value.
    */
   T_return operator()(const slot_type& _A_slot) const
-    { return (reinterpret_cast<typename slot_type::call_type>(_A_slot.rep_->call_))(LIST(_A_slot.rep_, LOOP(_A_a%1_, $1))); }
+  {
+    // Conversion between different types of function pointers with
+    // reinterpret_cast can make gcc8 print a warning.
+    // https://github.com/libsigcplusplus/libsigcplusplus/issues/1
+    union {
+      internal::hook ph;
+      typename slot_type::call_type pc;
+    } u;
+    u.ph = _A_slot.rep_->call_;
+    return (u.pc)(LIST(_A_slot.rep_, LOOP(_A_a%1_, $1)));
+  }
 dnl  T_return operator()(const slot_type& _A_slot) const
 dnl    { return _A_slot(LOOP(_A_a%1_, $1)); }
 
@@ -150,13 +160,19 @@ FOR(1, $1,[
         if (it == slots.end())
           return T_return(); // note that 'T_return r_();' doesn't work => define 'r_' after this line and initialize as follows:
   
-        r_ = (reinterpret_cast<call_type>(it->rep_->call_))(LIST(it->rep_, LOOP(_A_a%1, $1)));
+        union {
+          internal::hook ph;
+          call_type pc;
+        } u;
+        u.ph = it->rep_->call_;
+        r_ = (u.pc)(LIST(it->rep_, LOOP(_A_a%1, $1)));
         for (++it; it != slots.end(); ++it)
-          {
-            if (it->empty() || it->blocked())
-              continue;
-            r_ = (reinterpret_cast<call_type>(it->rep_->call_))(LIST(it->rep_, LOOP(_A_a%1, $1)));
-          }
+        {
+          if (it->empty() || it->blocked())
+            continue;
+          u.ph = it->rep_->call_;
+          r_ = (u.pc)(LIST(it->rep_, LOOP(_A_a%1, $1)));
+        }
       }
       
       return r_;
@@ -201,13 +217,19 @@ FOR(1, $1,[
         if (it == reverse_iterator_type(slots.begin()))
           return T_return(); // note that 'T_return r_();' doesn't work => define 'r_' after this line and initialize as follows:
   
-        r_ = (reinterpret_cast<call_type>(it->rep_->call_))(LIST(it->rep_, LOOP(_A_a%1, $1)));
+        union {
+          internal::hook ph;
+          call_type pc;
+        } u;
+        u.ph = it->rep_->call_;
+        r_ = (u.pc)(LIST(it->rep_, LOOP(_A_a%1, $1)));
         for (++it; it != reverse_iterator_type(slots.begin()); ++it)
-          {
-            if (it->empty() || it->blocked())
-              continue;
-            r_ = (reinterpret_cast<call_type>(it->rep_->call_))(LIST(it->rep_, LOOP(_A_a%1, $1)));
-          }
+        {
+          if (it->empty() || it->blocked())
+            continue;
+          u.ph = it->rep_->call_;
+          r_ = (u.pc)(LIST(it->rep_, LOOP(_A_a%1, $1)));
+        }
       }
       
       return r_;
@@ -243,12 +265,17 @@ FOR(1, $1,[
       signal_exec exec(impl);
       temp_slot_list slots(impl->slots_);
 
+      union {
+        internal::hook ph;
+        call_type pc;
+      } u;
       for (const auto& slot : slots)
-        {
-          if (slot.empty() || slot.blocked())
-            continue;
-          (reinterpret_cast<call_type>(slot.rep_->call_))(LIST(slot.rep_, LOOP(_A_a%1, $1)));
-        }
+      {
+        if (slot.empty() || slot.blocked())
+          continue;
+        u.ph = slot.rep_->call_;
+        (u.pc)(LIST(slot.rep_, LOOP(_A_a%1, $1)));
+      }
     }
 
 _DEPRECATE_IFDEF_START
@@ -274,12 +301,17 @@ FOR(1, $1,[
       typedef std::reverse_iterator<signal_impl::iterator_type, std::random_access_iterator_tag,
                                      slot_base, slot_base&, slot_base*, std::ptrdiff_t> reverse_iterator_type;
 #endif
+      union {
+        internal::hook ph;
+        call_type pc;
+      } u;
       for (auto it = reverse_iterator_type(slots.end()); it != reverse_iterator_type(slots.begin()); ++it)
-        {
-          if (it->empty() || it->blocked())
-            continue;
-          (reinterpret_cast<call_type>(it->rep_->call_))(LIST(it->rep_, LOOP(_A_a%1, $1)));
-        }
+      {
+        if (it->empty() || it->blocked())
+          continue;
+        u.ph = it->rep_->call_;
+        (u.pc)(LIST(it->rep_, LOOP(_A_a%1, $1)));
+      }
     }
 _DEPRECATE_IFDEF_END
 };