C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Type of "new" expression          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Tomás Guest Posted: Fri Mar 24, 2006 5:06 pm    Post subject: Type of "new" expression For argument's sake, let's say we have the following function which we want to invoke: void ProcessFiveMemberships( unsigned (&memberships)[5] ); If "new" is used to allocate an array, as follows: new unsigned[5]; Then the type of this expression is not "unsigned (*)[5]", but "unsigned *". My first question is, Why was this made so? It would make more sense to me to return a pointer to an array of five objects, rather than a pointer to one sole object. We want to invoke "ProcessFiveMemberships". The following is ill-formed: void ProcessFiveMemberships( unsigned (&memberships)[5] ) { //Empty function body for (; { break; } } int main() { unsigned* p_first_membership = new unsigned[5]; ProcessFiveMemberships( *p_first_membership ); // type mismatch delete [] p_first_membership; } Is there any perfectly legal, fully portable way of turning our "unsigned *" into an "unsigned (*)[5]"? How about the two following methods: ProcessFiveMemberships( *reinterpret_cast<unsigned (*)[5]>(p_first_membership) ); or: ProcessFiveMemberships( *static_cast< unsigned(*)[5] > ( static_cast<void*>(p_first_membership) ) ); -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Tomás Guest Posted: Sat Mar 25, 2006 1:56 am    Post subject: Re: Type of "new" expression Quote: Then the type of this expression is not "unsigned (*)[5]", but "unsigned *". I found a solution. Use: new unsigned[1][5] instead of: new unsigned[5] Now the following is okay: void Array() { unsigned (*p_numbers)[5] = new unsigned[1][5]; (*p_numbers)[0] = 5; (*p_numbers)[1] = 6; delete [] p_numbers; } --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Alf P. Steinbach Guest Posted: Sat Mar 25, 2006 2:06 am    Post subject: Re: Type of "new" expression * NULL (AT) NULL (DOT) NULL: Quote: For argument's sake, let's say we have the following function which we want to invoke: void ProcessFiveMemberships( unsigned (&memberships)[5] ); If "new" is used to allocate an array, as follows: new unsigned[5]; Then the type of this expression is not "unsigned (*)[5]", but "unsigned *". My first question is, Why was this made so? Most practical; usually the array size is determined at run-time. Quote: It would make more sense to me to return a pointer to an array of five objects, rather than a pointer to one sole object. Then J&W Pascal is the language for you. ;-) Quote: We want to invoke "ProcessFiveMemberships". The following is ill-formed: void ProcessFiveMemberships( unsigned (&memberships)[5] ) { //Empty function body for (; { break; } } int main() { unsigned* p_first_membership = new unsigned[5]; ProcessFiveMemberships( *p_first_membership ); // type mismatch delete [] p_first_membership; } Is there any perfectly legal, fully portable way of turning our "unsigned *" into an "unsigned (*)[5]"? void ProcessFiveMemberships( unsigned (&memberships)[5] ) { for (; { memberships; break; } } int main() { typedef unsigned UnsignedArray5[5]; UnsignedArray5* ua5 = new UnsignedArray5[1]; ProcessFiveMemberships( *ua5 ); delete[] ua5; } Quote: How about the two following methods: ProcessFiveMemberships( *reinterpret_cast<unsigned (*)[5]>(p_first_membership) ); Formally UB. Quote: or: ProcessFiveMemberships( *static_cast< unsigned(*)[5] > ( static_cast<void*>(p_first_membership) ) ); Not sure whether it's formally valid or not, but while it's in-practice technically OK (except perhaps on AS400 or original ENIAC...) it's ungood as software engineering practice. If you want a fixed-size array, just wrap it in a struct. You might also find it beneficial to check out the Boost array facilities. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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