C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  static_cast confusion          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Chandra Shekhar Kumar Guest Posted: Tue Jun 24, 2003 10:49 pm    Post subject: Re: static_cast confusion Quote: Derived& Derived::operator=(const Derived& inDerived) { //to assign to the base class object the following statement static_cast<Base&>(*this)=inDerived;//works fine static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. coz, static_cast<Base> results into creation of a temporary obj of type Base Quote: *((Base*)this) = inDerived;//works fine this is same as *(static_cast<Base*>(this)) = inDerived; Quote: return (*this); } I thought the static_cast was equivalent to the C-style cast. yes, u r right, see the above. but ideally u shud use dynamic_cast for polymorphic classes.. Back to top buds Guest Posted: Wed Jun 25, 2003 10:36 am    Post subject: static_cast confusion Hi all, Following is the assigment operator of a derived class Derived& Derived::operator=(const Derived& inDerived) { //to assign to the base class object the following statement static_cast<Base&>(*this)=inDerived;//works fine static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. *((Base*)this) = inDerived;//works fine return (*this); } I thought the static_cast was equivalent to the C-style cast. Then why this behavior. Can anyone please help me with this TIA Buds Back to top Rolf Magnus Guest Posted: Wed Jun 25, 2003 10:39 am    Post subject: Re: static_cast confusion buds wrote: Quote: Hi all, Following is the assigment operator of a derived class Derived& Derived::operator=(const Derived& inDerived) { //to assign to the base class object the following statement static_cast<Base&>(*this)=inDerived;//works fine This invokes the base class's assignment operator. Quote: static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. Right. Quote: *((Base*)this) = inDerived;//works fine With this, you tell the compiler "hey, I know that 'this' is a pointer to Derived, but that's not really true. Actually it's a Base, so treat the pointer as if it were a Base.". It's equivalent to: *reinterpret_cast<Base*>(this) = inDerived;//works fine Don't expect that to work generally. Quote: return (*this); } I thought the static_cast was equivalent to the C-style cast. No. The C style cast is equivalent to any combination of static_cast, const_cast and reinterpret_cast that would be needed for the specific conversion. Back to top Rolf Magnus Guest Posted: Wed Jun 25, 2003 12:01 pm    Post subject: Re: static_cast confusion Chandra Shekhar Kumar wrote: Quote: Derived& Derived::operator=(const Derived& inDerived) { //to assign to the base class object the following statement static_cast<Base&>(*this)=inDerived;//works fine static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. coz, static_cast<Base> results into creation of a temporary obj of type Base *((Base*)this) = inDerived;//works fine this is same as *(static_cast<Base*>(this)) = inDerived; Hmm. I guess I was wrong then. Quote: return (*this); } I thought the static_cast was equivalent to the C-style cast. yes, u r right, see the above. but ideally u shud use dynamic_cast for polymorphic classes.. Why? You need dynamic_cast to cast from base to derived, not the other way round. Back to top Ron Natalie Guest Posted: Wed Jun 25, 2003 2:57 pm    Post subject: Re: static_cast confusion "buds" <buds (AT) ziplip (DOT) com> wrote Quote: static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. I thought the static_cast was equivalent to the C-style cast. You are wrong. Some C style casts are NOT the same as static_cast (but it's immaterial here). Quote: Then why this behavior. Can anyone please help me with this The base class copy constructor is called because in order to cast Derived to Base, a temporary Base object is created and that is what is assigned into. Back to top Ron Natalie Guest Posted: Wed Jun 25, 2003 3:00 pm    Post subject: Re: static_cast confusion "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote Quote: With this, you tell the compiler "hey, I know that 'this' is a pointer to Derived, but that's not really true. Actually it's a Base, so treat the pointer as if it were a Base.". It's equivalent to: *reinterpret_cast<Base*>(this) = inDerived;//works fine Don't expect that to work generally. Huh? The C cast should do a static cast here. The cast works generally provided that Base is a public base class of Derived. Back to top Rolf Magnus Guest Posted: Wed Jun 25, 2003 3:21 pm    Post subject: Re: static_cast confusion Ron Natalie wrote: Quote: "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote in message news:bdbuec$lod$04$1 (AT) news (DOT) t-online.com... With this, you tell the compiler "hey, I know that 'this' is a pointer to Derived, but that's not really true. Actually it's a Base, so treat the pointer as if it were a Base.". It's equivalent to: *reinterpret_cast<Base*>(this) = inDerived;//works fine Don't expect that to work generally. Huh? The C cast should do a static cast here. The cast works generally provided that Base is a public base class of Derived. Yes, I think you're right. Sorry. Back to top Mirek Fidler Guest Posted: Fri Jun 27, 2003 2:54 pm    Post subject: Re: static_cast confusion "buds" <buds (AT) ziplip (DOT) com> píse v diskusním príspevku news:31a7f443.0306250236.14381a92 (AT) posting (DOT) google.com... Quote: Hi all, Following is the assigment operator of a derived class Derived& Derived::operator=(const Derived& inDerived) { //to assign to the base class object the following statement static_cast<Base&>(*this)=inDerived;//works fine static_cast<Base> (*this)=inDerived;//calls the base class copy constructor. *((Base*)this) = inDerived;//works fine return (*this); } I thought the static_cast was equivalent to the C-style cast. Then why this behavior. Can anyone please help me with this It is. Problem is that second static_cast transforms into (Base)(*this) = inDerived; which, following rules for casting is equivalent for Base(*this) = inDerived; which means "create temporary object of Base type using Base(const Base&) copy constructor and use its operator= . 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