C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  UB?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Al Guest Posted: Fri Dec 15, 2006 10:10 am    Post subject: UB? Hi, Does the following invoke undefined behavior? int main() { int i = 0; while ((++i)--); return 0; } Do parentheses affect sequence points? Thanks, -Al. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matthew T. Staebler Guest Posted: Sat Dec 16, 2006 10:10 am    Post subject: Re: UB? While the statement (--i)++ results in undefined behavior when the variable i is an integer, is the same true if the variable i is a class for which there are overloaded prefix decrement and postfix increment operators? struct Integer { int x; Integer( int x_ ) : x( x_ ) {} operator int( void ) const { return x; } }; Integer& operator --( Integer& i ) { --i.x; return i; } Integer operator ++( Integer& i, int ) { Integer original( i ); i.x++; return original; } int A( void ) { Integer i = 0; while( operator ++( operator --( i ), 0 ) ); return i; } I assume that, in the execution of function A, there is a sequence point before the execution of the postfix increment operation. This sequence points necessitates that the prefix decrement operation be complete before the beginning execution of the postfix increment operation. Consequently, the behavior of function A is well-defined. int B( void ) { Integer i = 0; while( (--i)++ ); return i; } Does the statement (--i)++ in function B break down into two function calls akin to what is in function A? If so, does this insert a "hidden" sequence point in the execution of function B that makes its behavior well-defined? ----- Matthew T. Staebler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carlos Moreno Guest Posted: Sun Dec 17, 2006 12:22 am    Post subject: Re: UB? Matthew T. Staebler wrote: Quote: While the statement (--i)++ results in undefined behavior when the variable i is an integer, is the same true if the variable i is a class for which there are overloaded prefix decrement and postfix increment operators? No. The behaviour is perfectly defined in that case. In some cases, however, you get unspecified behaviour, when the order of evaluation of parameters to functions can affect the outcome --- but I don't think this is the case, since there is no way to call the postfix ++ operator without first having the result from the prefix -- operator. The order of evaluation is forced by the expression, in this particular case. Carlos -- -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matthew T. Staebler Guest Posted: Sun Dec 17, 2006 6:17 pm    Post subject: Re: UB? Carlos Moreno wrote: Quote: Matthew T. Staebler wrote: While the statement (--i)++ results in undefined behavior when the variable i is an integer, is the same true if the variable i is a class for which there are overloaded prefix decrement and postfix increment operators? No. The behaviour is perfectly defined in that case. In some cases, however, you get unspecified behaviour, when the order of evaluation of parameters to functions can affect the outcome --- but I don't think this is the case, since there is no way to call the postfix ++ operator without first having the result from the prefix -- operator. The order of evaluation is forced by the expression, in this particular case. What would the ramifications be of treating all operators as if they involved an implicit function call regardless of whether the operator is a built-in operator or a user-defined operator? Would such a modification impractically hamper a compiler's ability to optimize code? ----- Matthew T. Staebler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Mon Dec 18, 2006 12:32 am    Post subject: Re: UB? Matthew T. Staebler wrote: Quote: Carlos Moreno wrote: Matthew T. Staebler wrote: While the statement (--i)++ results in undefined behavior when the variable i is an integer, is the same true if the variable i is a class for which there are overloaded prefix decrement and postfix increment operators? No. The behaviour is perfectly defined in that case. In some cases, however, you get unspecified behaviour, when the order of evaluation of parameters to functions can affect the outcome --- but I don't think this is the case, since there is no way to call the postfix ++ operator without first having the result from the prefix -- operator. The order of evaluation is forced by the expression, in this particular case. What would the ramifications be of treating all operators as if they involved an implicit function call regardless of whether the operator is a built-in operator or a user-defined operator? Would such a modification impractically hamper a compiler's ability to optimize code? It depends on who you ask. I've been told by some of the best specialists in optimization technology that imposing a rigorous order, as in Java, would have no noticeable effect. I've also been told, by other people actually working on production compilers, that imposing such total ordering would have a significant effect. My own knowledge of optimizing techniques suggest that the first is true, but it does require more work on the part of the optimizer to obtain equivalent results. Just imposing a sequence point after each operator would probably have similar effects on the optimization possibilities, without offering nearly as many benefits for the programmer as full ordering. It would not, for example, be sufficient to make: "f( auto_ptr< T >( new T ), auto_ptr< U >( new U ) )" work. -- James Kanze (Gabi Software) email: james.kanze (AT) gmail (DOT) com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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