C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  r-value, but non-const          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message JKop Guest Posted: Tue Oct 26, 2004 9:22 pm    Post subject: r-value, but non-const Given that the following is perfectly legal: void Blah( int const& k ) { const_cast< int& > ( k ) = 2; } int main() { int r; Blah( r ); } because the original object was non-const; would it not be fair to say that the following is also legal: void Blah( int const& k ) { const_cast< int& > (k) = 2; } int main() { Blah( int() ); } My reasoning behind this is that the temporary object passed by reference to "Blah" is in fact non-const (even though it is an r-value). Is there any official stance on this? While I'm at it, the only argument I've ever heard in favour of references being r-value's is the following code: void Blah(int &r) { r += 5; } int main() { float k = 78.2; Blah( k ); } Obviously, one wants the actual "k" object to be altered, but what's happening is that a temporary of type "int" must be created. Although this temporary object is not const, it is still and r-value and so it cannot be bound to a non-const reference. -JKop --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Bob Hairgrove Guest Posted: Tue Oct 26, 2004 10:22 pm    Post subject: Re: r-value, but non-const On Tue, 26 Oct 2004 21:22:30 GMT, [email]NULL (AT) NULL (DOT) NULL[/email] (JKop) wrote: Quote: Given that the following is perfectly legal: void Blah( int const& k ) { const_cast< int& > ( k ) = 2; } int main() { int r; Blah( r ); } [snip] Yes, but read 5.2.11.7. In general, we don't outlaw knives just because there is the danger that you might cut yourself with one. -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top johnchx Guest Posted: Wed Oct 27, 2004 4:34 pm    Post subject: Re: r-value, but non-const [email]NULL (AT) NULL (DOT) NULL[/email] (JKop) wrote Quote: because the original object was non-const; would it not be fair to say that the following is also legal: void Blah( int const& k ) { const_cast< int& > (k) = 2; } int main() { Blah( int() ); } Perfectly legal. Your reasoning -- that the fact that an expression is an rvalue does not mean that the object it denotes is const -- is correct. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top John Potter Guest Posted: Wed Oct 27, 2004 7:59 pm    Post subject: Re: r-value, but non-const On Wed, 27 Oct 2004 16:34:47 GMT, [email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote: Quote: NULL (AT) NULL (DOT) NULL (JKop) wrote because the original object was non-const; would it not be fair to say that the following is also legal: void Blah( int const& k ) { const_cast< int& > (k) = 2; } int main() { Blah( int() ); } Perfectly legal. Your reasoning -- that the fact that an expression is an rvalue does not mean that the object it denotes is const -- is correct. However, you missed the subtleties of 8.5.3/5. We reach the final otherwise which requires construction of a temporary CONST int for binding with the reference. Modifying this constant is undefined behavior. John --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top johnchx Guest Posted: Thu Oct 28, 2004 4:44 am    Post subject: Re: r-value, but non-const [email]jpotter (AT) lhup (DOT) edu[/email] (John Potter) wrote Quote: johnchx2 (AT) yahoo (DOT) com (johnchx) wrote: Perfectly legal. Your reasoning -- that the fact that an expression is an rvalue does not mean that the object it denotes is const -- is correct. However, you missed the subtleties of 8.5.3/5. We reach the final otherwise which requires construction of a temporary CONST int for binding with the reference. Modifying this constant is undefined behavior. Egad. So it would seem. The rvalue isn't const, but the copy is. Even more interesting is the case of an rvalue of class type, in which the object bound to the reference may or may not be const, depending on whether the implementation performs a copy. Subtle indeed. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top JKop Guest Posted: Fri Oct 29, 2004 4:51 am    Post subject: Re: r-value, but non-const johnchx posted: Quote: jpotter (AT) lhup (DOT) edu (John Potter) wrote [email]johnchx2 (AT) yahoo (DOT) com[/email] (johnchx) wrote: Perfectly legal. Your reasoning -- that the fact that an expression is an rvalue does not mean that the object it denotes is const -- is correct. However, you missed the subtleties of 8.5.3/5. We reach the final otherwise which requires construction of a temporary CONST int for binding with the reference. Modifying this constant is undefined behavior. Egad. So it would seem. The rvalue isn't const, but the copy is. Even more interesting is the case of an rvalue of class type, in which the object bound to the reference may or may not be const, depending on whether the implementation performs a copy. Subtle indeed. Well if the temporary is copied, then is there any advantage at all in using: AnyClass const &blah = FunctionThatReturnsAnyClass(); over: AnyClass blah = FunctionThatReturnsAnyClass(); I was under the impression that the former was preferable as it avoids a posssible copy... ? -JKop --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top johnchx Guest Posted: Fri Oct 29, 2004 5:20 pm    Post subject: Re: r-value, but non-const [email]NULL (AT) NULL (DOT) NULL[/email] (JKop) wrote Quote: Well if the temporary is copied, then is there any advantage at all in using: AnyClass const &blah = FunctionThatReturnsAnyClass(); over: AnyClass blah = FunctionThatReturnsAnyClass(); I was under the impression that the former was preferable as it avoids a posssible copy... ? The compiler is free to make (or not make) a copy in either case. There is an advantage to the reference form under certain special circumstances: if the function returns (by value) an object of a class derived from AnyClass, you can bind a reference-to-base to it without slicing. For an example of where this might come in handy, see: http://www.cuj.com/documents/s=8000/cujcexp1812alexandr/alexandr.htm The key condition is that the function you're calling is an implicit instantiation of a function template whose actual return type depends upon the types of its parameters. The advantage of the non-reference form is that the resulting object is guaranteed non-const, and since the name is also declared non-const, you don't even need the highly suspicious cast. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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