BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Comparing objects Goto page 1, 2  Next          BorlandTalk.com Forum Index -> C++ Builder (Non-Technical) View previous topic :: View next topic   Author Message Jennifer-Ashley Guest Posted: Fri Mar 30, 2007 7:30 pm    Post subject: Comparing objects What is the C++Builder equivalent again of the Delphi statement comparing two objects: if OneObject is AnotherObject then. Back to top OBones Guest Posted: Fri Mar 30, 2007 7:41 pm    Post subject: Re: Comparing objects Jennifer-Ashley wrote: Quote: What is the C++Builder equivalent again of the Delphi statement comparing two objects: if OneObject is AnotherObject then. if (dynamic_cast<TSomeType*>(OneObject)) { } Back to top Chris Uzdavinis (TeamB) Guest Posted: Fri Mar 30, 2007 9:57 pm    Post subject: Re: Comparing objects OBones <obones_gfd_ (AT) _gfz_altern (DOT) org> writes: Quote: Jennifer-Ashley wrote: What is the C++Builder equivalent again of the Delphi statement comparing two objects: if OneObject is AnotherObject then. if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. -- Chris (TeamB); Back to top Ed Mulroy Guest Posted: Sat Mar 31, 2007 12:00 am    Post subject: Re: Comparing objects Quote: if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. In the first example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL. In the second example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL and is then assigned to a temporary declared in the boolean expression area of an 'if' statement. Assuming a smart compiler the second example will be compiled as that of the first. If the compiler's optimization is less smart the second example will generate the same code plus more code and/or a storage location or register assignment. How is the second example better than the first example? Where does the issue of "confined to the scope where it could be valid" arise? .. Ed Quote: Chris Uzdavinis wrote in message news:86zm5u7jpz.fsf (AT) explicit (DOT) atdesk.com... What is the C++Builder equivalent again of the Delphi statement comparing two objects: if OneObject is AnotherObject then. if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. Back to top Duane Hebert Guest Posted: Sat Mar 31, 2007 12:21 am    Post subject: Re: Comparing objects "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> wrote in message news:460d5eac$1 (AT) newsgroups (DOT) borland.com... Quote: if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. In the first example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL. In the second example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL and is then assigned to a temporary declared in the boolean expression area of an 'if' statement. Assuming a smart compiler the second example will be compiled as that of the first. If the compiler's optimization is less smart the second example will generate the same code plus more code and/or a storage location or register assignment. How is the second example better than the first example? Where does the issue of "confined to the scope where it could be valid" arise? In the first example, how would you use the result of the dynamic cast? Back to top Alan Bellingham Guest Posted: Sat Mar 31, 2007 12:24 am    Post subject: Re: Comparing objects "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> wrote: Quote: if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. In the first example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL. Yes Quote: In the second example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL and is then assigned to a temporary declared in the boolean expression area of an 'if' statement. Yes Quote: Assuming a smart compiler the second example will be compiled as that of the first. If the compiler's optimization is less smart the second example will generate the same code plus more code and/or a storage location or register assignment. An extra assignment ought to be all. For it to do more than store a register would be abysmal code quality, and there's a pretty good chance it won't need to do even that. Quote: How is the second example better than the first example? Where does the issue of "confined to the scope where it could be valid" arise? Because the storage location will be used anyway. The first form will be something like if (dynamic_cast<TSomeType*>(OneObject)) { TSomeType * obj = static_cast<TSomeType*>(OneObject); obj->... } (I'm assuming that the downcast can be done statically.), whereas the second is if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } which is simpler. (The variable may conceivably not be used, but if so, why downcast at all, then? It'd not pass my code reviews, on the basis of being pointless.) Quote: Where does the issue of "confined to the scope where it could be valid" arise? Imagine the user leaving the pointer declaration outside the scope TSomeType * obj; if (obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } obj->... // ooops, it may be null as opposed to if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } obj->... // compilation error It's good coding practice not to have a variable visible outside its scope of validity. That's not always possible, but it's still a good idea when it is. Alan Bellingham -- Team Thai Kingdom <url:http://www.borland.com/newsgroups/> Borland newsgroup descriptions <url:http://www.borland.com/newsgroups/netiquette.html> netiquette Back to top Ed Mulroy Guest Posted: Sat Mar 31, 2007 12:41 am    Post subject: Re: Comparing objects Quote: In the first example, how would you use the result of the dynamic cast? The question was how one tests that an object is of a given type. The replies answer not this question but rather an assumed question. The answer tells not of how one tests if an object is the same as another but rather how one can test if the object is either of the other's type or is of a derived type which can be interpreted as the other's because the other is of a type in the chain of inheritance. Nothing in the question mentioned using or creating another pointer to object and in fact the existence of the initial object to be tested implies that for many operations, once one verifies that it is a compatible type in the inheritance chain, it may well be usable for the desired task without a cast. .. Ed Quote: Duane Hebert wrote in message news:460d6327$1 (AT) newsgroups (DOT) borland.com... Back to top Ed Mulroy Guest Posted: Sat Mar 31, 2007 12:47 am    Post subject: Re: Comparing objects I saw it differently. Restating the original question: I have a variable, call it 'var', and I want to test if it is of a given type. No answer was given on how to do that and I am unsure of in C++ once can do it. The answer given as was to if it is either of that given type or of a type that can be polymorphically (sp?) used as that type. The implication is that the questioner already has a variable name that is an object or a pointer to an object. He wishes to test if it can be used as a certain type. If it can be, then he needs no more storage for it as he already has it, it appeared complete with storage allocation before the code associated with the question was encountered. To use your phrase, the "scope of visibility" of the name began long before the requested code performing the test occurs. .. Ed Quote: Alan Bellingham wrote in message news:3coq03prbk3le1g0no3ogtflubtqnn158v (AT) 4ax (DOT) com... if (dynamic_cast<TSomeType*>(OneObject)) { } Better: if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { // use obj } This keeps obj confined to the scope where it could be valid, and avoids having to do a 2nd cast. In the first example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL. Yes In the second example the dynamic_cast return value is a pointer to a type of TSomeType or is NULL and is then assigned to a temporary declared in the boolean expression area of an 'if' statement. Yes Assuming a smart compiler the second example will be compiled as that of the first. If the compiler's optimization is less smart the second example will generate the same code plus more code and/or a storage location or register assignment. An extra assignment ought to be all. For it to do more than store a register would be abysmal code quality, and there's a pretty good chance it won't need to do even that. How is the second example better than the first example? Where does the issue of "confined to the scope where it could be valid" arise? Because the storage location will be used anyway. The first form will be something like if (dynamic_cast<TSomeType*>(OneObject)) { TSomeType * obj = static_cast<TSomeType*>(OneObject); obj->... } (I'm assuming that the downcast can be done statically.), whereas the second is if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } which is simpler. (The variable may conceivably not be used, but if so, why downcast at all, then? It'd not pass my code reviews, on the basis of being pointless.) Where does the issue of "confined to the scope where it could be valid" arise? Imagine the user leaving the pointer declaration outside the scope TSomeType * obj; if (obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } obj->... // ooops, it may be null as opposed to if (TSomeType * obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } obj->... // compilation error It's good coding practice not to have a variable visible outside its scope of validity. That's not always possible, but it's still a good idea when it is. Back to top Chris Uzdavinis (TeamB) Guest Posted: Sat Mar 31, 2007 1:01 am    Post subject: Re: Comparing objects "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> writes: Quote: In the first example, how would you use the result of the dynamic cast? The question was how one tests that an object is of a given type. The All right, I'll concede that technically the author didn't ask how to USE the result as the target type. If answering the question was all that was needed, then remembering the result is an unnessary instruction. But most of the time, code only does a type query becuase it actually intends to use the object as the other type. Sure, cases exist where the function's behavior depends on the dynamic type of an object but doesn't actually use the dynamic type of the object, but those cases are unusual. Quote: replies answer not this question but rather an assumed question. The answer Yep. Guilty as charged. Let's ask the original poster fill us in on what the program will do with the result of the question. If the code uses the dynamic_cast's result, you owe me a beer. If the code purely wants to know the answer to the type query without actually using the resulting type, I owe you a beer. Fair enough? Original Poster: Please let us know how you plan to use the result of this type query. Thanks. (Please answer quickly, I'm thirsty.) <g> -- Chris (TeamB); Back to top Duane Hebert Guest Posted: Sat Mar 31, 2007 1:26 am    Post subject: Re: Comparing objects "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> wrote in message news:460d69a5 (AT) newsgroups (DOT) borland.com... Quote: In the first example, how would you use the result of the dynamic cast? The question was how one tests that an object is of a given type. The replies answer not this question but rather an assumed question. The answer tells not of how one tests if an object is the same as another but rather how one can test if the object is either of the other's type or is of a derived type which can be interpreted as the other's because the other is of a type in the chain of inheritance. Well I was answering the part of your question which was basically Quote: How is the second example better than the first example? Where does the issue of "confined to the scope where it could be valid" arise? especially about the scope bit. Quote: Nothing in the question mentioned using or creating another pointer to object and in fact the existence of the initial object to be tested implies that for many operations, once one verifies that it is a compatible type in the inheritance chain, it may well be usable for the desired task without a cast. Not sure why you would use a dynamic cast to determine whether you can use a type without a cast but I imagine that it could happen. As for dynamic cast testing that an object is of a given type, we could argue about that as well but here in Montreal it's 30 minutes to pub time. Back to top Alan Bellingham Guest Posted: Sat Mar 31, 2007 3:26 am    Post subject: Re: Comparing objects "Ed Mulroy" <dont_email_me (AT) bitbuc (DOT) ket> wrote: Quote: I saw it differently. Restating the original question: I have a variable, call it 'var', and I want to test if it is of a given type. You failed the underlying question. The unwritten one. *WHY* does he want to know? Just answering the written question isn't always the right thing to do. Often we need to tease out what the underlying question actually is. Yes, we could just answer the explicit question, but if we fail to address the implicit question, then we fail to properly teach. I'm not here to answer questions on a points per answer basis, where by making a minimal answer I get more questions which mean I make more answers - I prefer to try to understand why someone is doing something and, if there's a higher level matter going on, to address that. You usually do that too, I'm pretty sure. And I'm sure, at least as often as we do. So my question to you is, why did you really object to CU's initial answer? Because I'm not sure. Quote: The implication is that the questioner already has a variable name that is an object or a pointer to an object. Pointer (or reference) - you can't dynamically cast actual objects. Quote: He wishes to test if it can be used as a certain type. If it can be, then he needs no more storage for it as he already has it, it appeared complete with storage allocation before the code associated with the question was encountered. The point there is, how *is* he going to use it as that particular type? You're objecting to storing the result of the cast, and yet, without storing it, how is he to know what the result is? Setting a bool? Recalculating later? Doing a static cast? I can't see anything which isn't actually *worse*. Alan Bellingham -- Team Thai Kingdom <url:http://www.borland.com/newsgroups/> Borland newsgroup descriptions <url:http://www.borland.com/newsgroups/netiquette.html> netiquette Back to top Ed Mulroy Guest Posted: Sat Mar 31, 2007 8:10 am    Post subject: Re: Comparing objects Quote: The point there is, how *is* he going to use it as that particular type? You're objecting to storing the result of the cast, and yet, without storing it, how is he to know what the result is? Setting a bool? The result is used because the code which uses it as that type is the target of the 'if' statement, the code between the curly braces which follows it. No need to store the result of the cast because if the test passes, the item is usable as the desired object type and if not, that code never executes. The item which is tested is already an instance name, reference or pointer (allowing for trivial adjustment of the test line for whichever it is). There need not be a new element declared to hold that which he already has. The code between the curly braces need not have any effect on the return value of the function or on the rest of the code in the function. It quite easily could do something outside of the function such as make a native API call to cause something to happen in the machine or such as posting a message to somewhere else. class SomeClass { public: virtual int DoWhatever(); ----blah blah blah---- }; : class DerivedClass : public SomeClass { public: ----blah blah blah---- }; : if (dynamic_cast<SomeType*>(&instance_or_reference_name)) { instance_or_reference_name.DoWhatever(); } else { DoSomethingElse(); } .. Ed Quote: Alan Bellingham wrote in message news:9t2r03hnqgdl43chmiu1kugk81va0rmm74 (AT) 4ax (DOT) com... I saw it differently. Restating the original question: I have a variable, call it 'var', and I want to test if it is of a given type. You failed the underlying question. The unwritten one. *WHY* does he want to know? Just answering the written question isn't always the right thing to do. Often we need to tease out what the underlying question actually is. Yes, we could just answer the explicit question, but if we fail to address the implicit question, then we fail to properly teach. I'm not here to answer questions on a points per answer basis, where by making a minimal answer I get more questions which mean I make more answers - I prefer to try to understand why someone is doing something and, if there's a higher level matter going on, to address that. You usually do that too, I'm pretty sure. And I'm sure, at least as often as we do. So my question to you is, why did you really object to CU's initial answer? Because I'm not sure. The implication is that the questioner already has a variable name that is an object or a pointer to an object. Pointer (or reference) - you can't dynamically cast actual objects. He wishes to test if it can be used as a certain type. If it can be, then he needs no more storage for it as he already has it, it appeared complete with storage allocation before the code associated with the question was encountered. The point there is, how *is* he going to use it as that particular type? You're objecting to storing the result of the cast, and yet, without storing it, how is he to know what the result is? Setting a bool? Recalculating later? Doing a static cast? I can't see anything which isn't actually *worse*. Back to top Ed Mulroy Guest Posted: Sat Mar 31, 2007 8:10 am    Post subject: Re: Comparing objects Ok Chris, a beer it is! <g> .. Ed Quote: Chris Uzdavinis wrote in message news:86vegi7b65.fsf (AT) explicit (DOT) atdesk.com... In the first example, how would you use the result of the dynamic cast? The question was how one tests that an object is of a given type. The All right, I'll concede that technically the author didn't ask how to USE the result as the target type. If answering the question was all that was needed, then remembering the result is an unnessary instruction. But most of the time, code only does a type query becuase it actually intends to use the object as the other type. Sure, cases exist where the function's behavior depends on the dynamic type of an object but doesn't actually use the dynamic type of the object, but those cases are unusual. replies answer not this question but rather an assumed question. The answer Yep. Guilty as charged. Let's ask the original poster fill us in on what the program will do with the result of the question. If the code uses the dynamic_cast's result, you owe me a beer. If the code purely wants to know the answer to the type query without actually using the resulting type, I owe you a beer. Fair enough? Original Poster: Please let us know how you plan to use the result of this type query. Thanks. (Please answer quickly, I'm thirsty.) g Back to top Hendrik Schober Guest Posted: Mon Apr 02, 2007 6:53 pm    Post subject: Re: Comparing objects Duane Hebert <spoo (AT) flarn (DOT) com> wrote: Quote: [...] Not sure why you would use a dynamic cast to determine whether you can use a type without a cast but I imagine that it could happen. <nitpicking> You could then 'static_cast'. </nitpicking> Quote: As for dynamic cast testing that an object is of a given type, we could argue about that as well but here in Montreal it's 30 minutes to pub time. <sigh style="Yossarian"/> Schobi -- SpamTrap (AT) gmx (DOT) de is never read I'm HSchober at gmx dot org "My hope is that if more people start reading books, the world will become a better place." froarulv in afp Back to top Torsten Franz Guest Posted: Mon Apr 02, 2007 7:11 pm    Post subject: Re: Comparing objects Quote: Imagine the user leaving the pointer declaration outside the scope TSomeType * obj; if (obj = dynamic_cast<TSomeType*>(OneObject)) { obj->... } obj->... // ooops, it may be null or even worse, a random memory location that is coincidentally valid... 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