Typically, when casting Foo* to SubclassOfFoo*, we use static_cast instead of dynamic_cast.
Why? dynamic_cast<> has runtime overhead. It does however do really nice things, like blow up in your face if Foo* is not in fact a object that is SubclassOfFoo* (it could be OtherSubclassOfFoo*). We have had one of these in the tree for approximately forever: Field_blob* is not on the same branch as Field_varstring*, but has been being cast to it in field_conv.cc for ever. It just "worked" because the first member of Field_blob kinda made sense in that codepath. Monty pointed out that protobuf has their own down_cast that does the check on debug builds (included below). What are people's thoughts? also see http://smolsky.net/index.php/2009/09/14/down_cast-v2 >From protobuf source: // When you upcast (that is, cast a pointer from type Foo to type // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts // always succeed. When you downcast (that is, cast a pointer from // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because // how do you know the pointer is really of type SubclassOfFoo? It // could be a bare Foo, or of type DifferentSubclassOfFoo. Thus, // when you downcast, you should use this macro. In debug mode, we // use dynamic_cast<> to double-check the downcast is legal (we die // if it's not). In normal mode, we do the efficient static_cast<> // instead. Thus, it's important to test in debug mode to make sure // the cast is legal! // This is the only place in the code we should use dynamic_cast<>. // In particular, you SHOULDN'T be using dynamic_cast<> in order to // do RTTI (eg code like this: // if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo); // if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo); // You should design the code some other way not to need this. template<typename To, typename From> // use like this: down_cast<T*>(foo); inline To down_cast(From* f) { // so we only accept pointers // Ensures that To is a sub-type of From *. This test is here only // for compile-time type checking, and has no overhead in an // optimized build at run-time, as it will be optimized away // completely. if (false) { implicit_cast<From*, To>(0); } -- Stewart Smith _______________________________________________ Mailing list: https://launchpad.net/~drizzle-discuss Post to : [email protected] Unsubscribe : https://launchpad.net/~drizzle-discuss More help : https://help.launchpad.net/ListHelp
