slot, signal: Avoid compiler warnings from function pointer conversions

kjellahl · kjellahl · commit c7dbcd5df0e6 · 2018-07-13T16:36:19.000+02:00
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 libsigcplusplus#1
diff --git a/sigc++/functors/slot.h b/sigc++/functors/slot.h
@@ -134,7 +134,18 @@ struct slot_call
   /** Forms a function pointer from call_it().
    * @return A function pointer formed from call_it().
    */
-  static hook address() { return reinterpret_cast<hook>(&call_it); }
+  static hook address()
+  {
+    // Conversion between different types of function pointers with
+    // reinterpret_cast can make gcc8 print a warning.
+    // https://github.com/libsigcplusplus/libsigcplusplus/issues/1
+    union {
+      hook ph;
+      decltype(&call_it) pc;
+    } u;
+    u.pc = &call_it;
+    return u.ph;
+  }
 };
 
 } /* namespace internal */
@@ -191,8 +202,17 @@ class slot<T_return(T_arg...)> : public slot_base
    */
   inline T_return operator()(type_trait_take_t<T_arg>... a) const
   {
-    if (!empty() && !blocked()) {
-      return std::invoke(reinterpret_cast<call_type>(slot_base::rep_->call_), slot_base::rep_, a...);
+    if (!empty() && !blocked())
+    {
+      // 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 std::invoke(u.pc, slot_base::rep_, a...);
     }
 
     return T_return();
diff --git a/sigc++/signal.h b/sigc++/signal.h
@@ -322,12 +322,21 @@ struct signal_emit<T_return, void, T_arg...>
         return T_return();
       }
 
-      r_ = (reinterpret_cast<call_type>(it->rep_->call_))(it->rep_, a...);
+      // 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 = it->rep_->call_;
+      r_ = (u.pc)(it->rep_, a...);
       for (++it; it != slots.end(); ++it)
       {
         if (it->empty() || it->blocked())
           continue;
-        r_ = (reinterpret_cast<call_type>(it->rep_->call_))(it->rep_, a...);
+        u.ph = it->rep_->call_;
+        r_ = (u.pc)(it->rep_, a...);
       }
     }
 
@@ -360,12 +369,20 @@ struct signal_emit<void, void, T_arg...>
     signal_impl_holder exec(impl);
     const temp_slot_list slots(impl->slots_);
 
+    // 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;
     for (const auto& slot : slots)
     {
       if (slot.empty() || slot.blocked())
         continue;
 
-      (reinterpret_cast<call_type>(slot.rep_->call_))(slot.rep_, a...);
+      u.ph = slot.rep_->call_;
+      (u.pc)(slot.rep_, a...);
     }
   }
 };
