rewrite unsafe_cast

test=develop

paddle/fluid/framework/ddim.cc

@@ -131,8 +131,6 @@ DDim slice_ddim(const DDim& dim, int begin, int end) {
 
 int arity(const DDim& d) { return d.size(); }
 
-/// \cond HIDDEN
-
 struct DDimPrinter {
   std::ostream& os;
   explicit DDimPrinter(std::ostream& os_) : os(os_) {}
@@ -143,8 +141,6 @@ struct DDimPrinter {
   }
 };
 
-/// \endcond
-
 std::ostream& operator<<(std::ostream& os, const DDim& ddim) {
   ddim.apply_visitor(DDimPrinter(os));
   return os;

paddle/fluid/framework/ddim.h

@@ -126,7 +126,9 @@ class DDim {
  private:
   template <int D>
   inline Dim<D>& UnsafeCast() {
-    return const_cast<Dim<D>&>(const_cast<const DDim*>(this)->UnsafeCast<D>());
+    static_assert(D >= 0 && D <= kMaxRank, "Invalid rank");
+    auto* p = static_cast<void*>(&dim_);
+    return *reinterpret_cast<Dim<D>*>(p);
   }
 
   template <int D>