C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  pre/post increment/decrement return type?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Angel Tsankov Guest Posted: Sun Feb 26, 2006 11:06 pm    Post subject: pre/post increment/decrement return type? Should pre/post increment/decrement return const or non-const? What about other functions? Back to top Ian Collins Guest Posted: Sun Feb 26, 2006 11:06 pm    Post subject: Re: pre/post increment/decrement return type? Angel Tsankov wrote: Quote: Should pre/post increment/decrement return const or non-const? Non-const, otherwise the expression can't be an lvalue. Quote: What about other functions? That depends what they return. These is no fixed rule. -- Ian Collins. Back to top Angel Tsankov Guest Posted: Sun Feb 26, 2006 11:06 pm    Post subject: Re: pre/post increment/decrement return type? "Ian Collins" <ian-news (AT) hotmail (DOT) com> wrote in message news:1140993572.791376@drone2-svc-skyt.qsi.net.nz... Quote: Angel Tsankov wrote: Should pre/post increment/decrement return const or non-const? Non-const, otherwise the expression can't be an lvalue. Why should it be an lvalue? int i = 0; i++; // returns rvalue Back to top Kai-Uwe Bux Guest Posted: Mon Feb 27, 2006 12:06 am    Post subject: Re: pre/post increment/decrement return type? Ian Collins wrote: Quote: Angel Tsankov wrote: Should pre/post increment/decrement return const or non-const? Non-const, otherwise the expression can't be an lvalue. Why would you want, e.g., iter++ to be an lvalue? Also, the standard specified the following requirements for iterators: ++r returns &X (X being the type of the iterator) r++ returns something convertible to X const & [snip] Best Kai-Uwe Bux Back to top Ian Collins Guest Posted: Mon Feb 27, 2006 12:06 am    Post subject: Re: pre/post increment/decrement return type? Kai-Uwe Bux wrote: Quote: Ian Collins wrote: Angel Tsankov wrote: Should pre/post increment/decrement return const or non-const? Non-const, otherwise the expression can't be an lvalue. Why would you want, e.g., iter++ to be an lvalue? Also, the standard specified the following requirements for iterators: You're right, I probably wouldn't, but ++iter could be if you wanted to assign something to the result, *(++i) = x; where i is some type with const and non-const operator*(). Then again, if iter were some form of smart pointer, one might want to write *(i++) = x; because it's valid for normal pointers. Make sense, or am I lacking caffeine? -- Ian Collins. Back to top Victor Bazarov Guest Posted: Mon Feb 27, 2006 12:06 am    Post subject: Re: pre/post increment/decrement return type? Angel Tsankov wrote: Quote: Should pre/post increment/decrement return const or non-const? What about other functions? Totally up to you. While the behaviour of pre- and post-increment is somewhat expected, based on the behaviour of the built-in types, 'other functions' are certaintly not. And even then, if you are the one who designs and implements pre- and post-increments for your UDTs, you're free to make them do what you please. For all we care, they can all return 'void'. Do as your model dictates. There is no good enough general rule. V -- Please remove capital As from my address when replying by mail Back to top benben Guest Posted: Mon Feb 27, 2006 1:06 am    Post subject: Re: pre/post increment/decrement return type? Quote: Totally up to you. While the behaviour of pre- and post-increment is somewhat expected, based on the behaviour of the built-in types, 'other functions' are certaintly not. And even then, if you are the one who designs and implements pre- and post-increments for your UDTs, you're free to make them do what you please. For all we care, they can all return 'void'. Do as your model dictates. There is no good enough general rule. However, if you want your overloaded operators to behave just like those of built-in types then you should consider making the return type non-const. Operators like ++ typically returns a self-reference and are themselves hardly a const operation. In order to use the operator the user must have a non-const reference to the object so it is reasonable to assume the user already being in power of legally modifying the object, which makes returning a const reference from the operator less convincing. Regards, Ben Back to top Kai-Uwe Bux Guest Posted: Mon Feb 27, 2006 1:06 am    Post subject: Re: pre/post increment/decrement return type? Ian Collins wrote: Quote: Kai-Uwe Bux wrote: Ian Collins wrote: Angel Tsankov wrote: Should pre/post increment/decrement return const or non-const? Non-const, otherwise the expression can't be an lvalue. Why would you want, e.g., iter++ to be an lvalue? Also, the standard specified the following requirements for iterators: You're right, I probably wouldn't, but ++iter could be if you wanted to assign something to the result, *(++i) = x; where i is some type with const and non-const operator*(). Then again, if iter were some form of smart pointer, one might want to write *(i++) = x; because it's valid for normal pointers. Make sense, or am I lacking caffeine? Maybe, I am missing something, but isn't that confusing: T const * <--> const_iterator and: T * const <--> iterator const The following compiles: class iter_dummy { int* ptr; public: iter_dummy ( int i ) : ptr ( new int ( i ) ) {} ~iter_dummy ( void ) { delete ptr; } iter_dummy const & operator++ ( int ) { return ( *this ); } int & operator* ( void ) const { return ( *ptr ); } }; // iter_dummy int main ( void ) { iter_dummy iter ( 4 ); *(iter++) = 5; } Note how operator*() is const as it does not change the iterator but only the pointee. Thus, you can have iter++ return a const reference, yet *(iter++) be and lvalue. Also, note that the standard, e.g., in the description of std::reverse_iterator, sets a corresponding precedence: the signature of the dereferencing operator is: reference operator*() const; Best Kai-Uwe Bux Back to top Ian Collins Guest Posted: Mon Feb 27, 2006 2:06 am    Post subject: Re: pre/post increment/decrement return type? Kai-Uwe Bux wrote: Quote: int & operator* ( void ) const { return ( *ptr ); } }; // iter_dummy int main ( void ) { iter_dummy iter ( 4 ); *(iter++) = 5; } Note how operator*() is const as it does not change the iterator but only the pointee. Thus, you can have iter++ return a const reference, yet *(iter++) be and lvalue. Good point, I was a little short on caffeine. -- Ian Collins. 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