slot, signal: Avoid compiler warnings from function pointer conversions

kjellahl · kjellahl · commit ecfa7c2850d1 · 2018-07-16T13:10:21.000+02:00
gcc8 -Wextra prints a warning when reinterpret_cast is used for conversion between different types of function pointers. Avoid that by adding sigc::internal::bitwise_equivalent_cast<>() with a union with members of the two types of function pointers. Fixes #1
diff --git a/sigc++/functors/slot.h b/sigc++/functors/slot.h
@@ -33,6 +33,25 @@ namespace sigc
 namespace internal
 {
 
+// Conversion between different types of function pointers with
+// reinterpret_cast can make gcc8 print a warning.
+// https://github.com/libsigcplusplus/libsigcplusplus/issues/1
+/** Returns the supplied bit pattern, interpreted as another type.
+ *
+ * When reinterpret_cast causes a compiler warning or error, this function
+ * may work. Intended mainly for conversion between different types of pointers.
+ */
+template <typename out_type, typename in_type>
+inline out_type bitwise_equivalent_cast(in_type in)
+{
+  union {
+    in_type in;
+    out_type out;
+  } u;
+  u.in = in;
+  return u.out;
+}
+
 /** A typed slot_rep.
  * A typed slot_rep holds a functor that can be invoked from
  * slot::operator()(). visit_each() is used to visit the functor's
@@ -134,7 +153,7 @@ 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() { return bitwise_equivalent_cast<hook>(&call_it); }
 };
 
 } /* namespace internal */
@@ -192,7 +211,7 @@ 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...);
+      return std::invoke(internal::bitwise_equivalent_cast<call_type>(slot_base::rep_->call_), slot_base::rep_, a...);
     }
 
     return T_return();
diff --git a/sigc++/signal.h b/sigc++/signal.h
@@ -322,12 +322,12 @@ struct signal_emit<T_return, void, T_arg...>
         return T_return();
       }
 
-      r_ = (reinterpret_cast<call_type>(it->rep_->call_))(it->rep_, a...);
+      r_ = (bitwise_equivalent_cast<call_type>(it->rep_->call_))(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...);
+        r_ = (bitwise_equivalent_cast<call_type>(it->rep_->call_))(it->rep_, a...);
       }
     }
 
@@ -365,7 +365,7 @@ struct signal_emit<void, void, T_arg...>
       if (slot.empty() || slot.blocked())
         continue;
 
-      (reinterpret_cast<call_type>(slot.rep_->call_))(slot.rep_, a...);
+      (bitwise_equivalent_cast<call_type>(slot.rep_->call_))(slot.rep_, a...);
     }
   }
 };

Original file line number	Diff line number	Diff line change
`@@ -322,12 +322,12 @@ struct signal_emit<T_return, void, T_arg...>`
`322`	`322`	`return T_return();`
`323`	`323`	`}`
`324`	`324`
`325`		`- r_ = (reinterpret_cast<call_type>(it->rep_->call_))(it->rep_, a...);`
	`325`	`+ r_ = (bitwise_equivalent_cast<call_type>(it->rep_->call_))(it->rep_, a...);`
`326`	`326`	`for (++it; it != slots.end(); ++it)`
`327`	`327`	`{`
`328`	`328`	`if (it->empty() \|\| it->blocked())`
`329`	`329`	`continue;`
`330`		`- r_ = (reinterpret_cast<call_type>(it->rep_->call_))(it->rep_, a...);`
	`330`	`+ r_ = (bitwise_equivalent_cast<call_type>(it->rep_->call_))(it->rep_, a...);`
`331`	`331`	`}`
`332`	`332`	`}`
`333`	`333`
`@@ -365,7 +365,7 @@ struct signal_emit<void, void, T_arg...>`
`365`	`365`	`if (slot.empty() \|\| slot.blocked())`
`366`	`366`	`continue;`
`367`	`367`
`368`		`- (reinterpret_cast<call_type>(slot.rep_->call_))(slot.rep_, a...);`
	`368`	`+ (bitwise_equivalent_cast<call_type>(slot.rep_->call_))(slot.rep_, a...);`
`369`	`369`	`}`
`370`	`370`	`}`
`371`	`371`	`};`