C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  simple compile-time error problem...          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message mark.van.dijk@perform-sol Guest Posted: Fri Mar 11, 2005 11:31 am    Post subject: simple compile-time error problem... Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } :-) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Vinzenz Feenstra Guest Posted: Fri Mar 11, 2005 3:14 pm    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote: Quote: Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } operator[]()s aren't usable with pointers. Quote: int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok or int i4 = (*tmp).at(3); // will be ok but int i4 = tmp->[3]; <- will never work in C++ BR [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Sat Mar 12, 2005 1:50 am    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote: Quote: Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } The syntax ->[3] is not part of the language. -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Falk Tannhäuser Guest Posted: Sat Mar 12, 2005 1:53 am    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote: Quote: Hi, whats wrong with the line marked below??? I don't get it... std::vector<int>* tmp = new std::vector<int>(10); [...] int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error Why do you expect 'i3' and 'i4' to be equivalent? Note that generally foo->bar is equivalent to (*foo).bar but not to (*foo)bar The right-hand side of '->' must designate a member of the object pointed to by its left-hand side, and 'std::vector<int>' simply has no member called '[3]' - it just has two member functions int& std::vector<int>::operator[](std::vector<int>::size_type n); int const& std::vector<int>::operator[](std::vector<int>::size_type n) const; which can be designated by the name 'operator[]' and called using function syntax (as in your initialisation of 'i2') or operator syntax (as in your initialisation of 'i3'). When you check the list of possible postfix expressions in § 5.2/1, there is just no valid match for 'tmp->[3]'. In particular, '[3]' is not an id-expression. Another question is why the vector was dynamically allocated in the first place. C++ is not Java where class objects must be put on the heap, and std::vector<int> tmp(10); or for a vector being a class member struct foo { std::vector<int> tmp; foo() : tmp(10) {} }; are just fine! Falk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hiroaki Ramone Guest Posted: Sat Mar 12, 2005 1:58 am    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote in message news:<1110504289.119543.155400 (AT) g14g2000cwa (DOT) googlegroups.com>... Quote: Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } :-) tmp->[3] <=> (*tmp).[3] <=> (*tmp)..operator[](3) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top johnchx2@yahoo.com Guest Posted: Sat Mar 12, 2005 2:05 am    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote: Quote: Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } 5.2.5/1 says that the arrow operator must be followed by an id-expression. "[3]" is not an id-expression. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Randy Guest Posted: Sat Mar 12, 2005 2:09 am    Post subject: Re: simple compile-time error problem... [email]mark.van.dijk (AT) perform-sol (DOT) com[/email] wrote: Quote: Hi, whats wrong with the line marked below??? I don't get it... { std::vector<int>* tmp = new std::vector<int>(10); //access the third element of the vector in various ways int i1 = tmp->at(3); //ok int i2 = tmp->operator[](3); //ok int i3 = (*tmp)[3]; //ok int i4 = tmp->[3]; //compile-time error } :-) Try: int i4 = tmp->operator[](3); You have a pointer to a std::vector and need to use it as such. Randy. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Seungbeom Kim Guest Posted: Tue Mar 15, 2005 3:56 pm    Post subject: Re: simple compile-time error problem... Falk Tannhäuser wrote: Quote: Another question is why the vector was dynamically allocated in the first place. C++ is not Java where class objects must be put on the heap, and std::vector<int> tmp(10); or for a vector being a class member struct foo { std::vector<int> tmp; foo() : tmp(10) {} }; are just fine! Good point. I don't think I've ever had to dynamically allocate a container object, and such cases I've seen are almost always from those who had a Java background. -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mark van Dijk Guest Posted: Thu Mar 17, 2005 9:53 pm    Post subject: Re: simple compile-time error problem... Seungbeom Kim wrote: Quote: Falk Tannhäuser wrote: Another question is why the vector was dynamically allocated in the first place. C++ is not Java where class objects must be put on the heap, and std::vector<int> tmp(10); or for a vector being a class member struct foo { std::vector<int> tmp; foo() : tmp(10) {} }; are just fine! Good point. I don't think I've ever had to dynamically allocate a container object, and such cases I've seen are almost always from those who had a Java background. Hmmm, i certainly don't have a Java background... I have a low-level class XYZ that represents binary data stored in large files on disk. Clients of XYZ can request various sized blocks of data from this file. These blocks may exist im memory after the destruction of the original XYZ object. My first attempt has XYZ return std::vector<char> blocks, however the rest of the application ended up copying these vectors multiple times as this block was passed around my app. As the original XYZ object may no longer exist, using references to std::vector<>'s is not acceptable. The solution was to dynamically allocate these vectors and pass pointers around instead. Of course, for safety, I don't use raw pointers, I actually pass boost::shared_ptr<std::vector objects around instead... The boost library documentation also suggests that this is an acceptable C++ idiom - see documemtation for boost::shared_array (paragraph 5), where they suggest that boost::shared_ptr<std::vector<> > is a flexible alternative to boost::shared_array's. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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