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On Sun, Nov 7, 2010 at 7:35 PM, Stewart Smith <[email protected]>wrote: > 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 >
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