C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Rvalue Reference question: Overloading on whether *this is          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Joe Gottman Guest Posted: Fri Sep 24, 2004 4:49 pm    Post subject: Rvalue Reference question: Overloading on whether *this is I have a question about that rvalue reference proposal in the pre-Redmond mailing ([url]http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html)[/url]. Is there any way to overload a member function on whether *this is an lvalue or an rvalue? Consider the following code: #include <vector> #include <string> vector<string> createVector(); int main() { vector<string> v(1, "Hello World"); std::string s1 = v[0]; std::string s2 = createVector()[0]; return 1; } Since v is an lvalue, s1 has to be created using a copy constructor. However, since the result of createVector() is an rvalue, theoretically s2 could be created using a move constructor, thus saving the cost of copying the string. Unfortunately, there seems to be no way to define a member function that is overloaded on whether *this is an lvalue or an rvalue. For a non-member function, we can define an overload set as follows: string const &getElement(vector<string> const &v, size_t index); //const lvalues string &getElement(vector<string> &v, size_t index); // non-const lvalues string &&getElement(vector<string> &&v, size_t index); //non-const rvalues It would be nice if we could do the same thing for member functions template <class T, class A> class vector { ... T const & operator[](size_t index) const; T & operator[](size_t index); T && operator[](size_t index) &&; //Use && at end to say *this is an rvalue reference, similar to const at end on the first function. }; The third function above would be illegal under the rvalue-reference proposal in its current form, but I think it should be possible to do something like this. Joe Gottman --- [ 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 David Abrahams Guest Posted: Fri Sep 24, 2004 5:55 pm    Post subject: Re: Rvalue Reference question: Overloading on whether *this [email]jgottman (AT) carolina (DOT) rr.com[/email] ("Joe Gottman") writes: Quote: I have a question about that rvalue reference proposal in the pre-Redmond mailing ([url]http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html)[/url]. Is there any way to overload a member function on whether *this is an lvalue or an rvalue? No. *some_pointer is always an lvalue. Quote: Consider the following code: #include <vector #include vector int main() { vector<string> v(1, "Hello World"); std::string s1 = v[0]; std::string s2 = createVector()[0]; return 1; } Since v is an lvalue, s1 has to be created using a copy constructor. However, since the result of createVector() is an rvalue, theoretically s2 could be created using a move constructor, thus saving the cost of copying the string. Unfortunately, there seems to be no way to define a member function that is overloaded on whether *this is an lvalue or an rvalue. For a non-member function, we can define an overload set as follows: string const &getElement(vector<string> const &v, size_t index); //const lvalues string &getElement(vector<string> &v, size_t index); // non-const lvalues string &&getElement(vector<string> &&v, size_t index); //non-const rvalues It would be nice if we could do the same thing for member functions template class vector { ... T const & operator[](size_t index) const; T & operator[](size_t index); T && operator[](size_t index) &&; //Use && at end to say *this is an rvalue reference, similar to const at end on the first function. }; The third function above would be illegal under the rvalue-reference proposal in its current form, but I think it should be possible to do something like this. Hmm. I don't love the syntax, though I agree for symmetry's sake something like that should be possible. I also think allowing operator[] to be a non-member is probably a good idea. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com --- [ 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 Howard Hinnant Guest Posted: Fri Sep 24, 2004 5:56 pm    Post subject: Re: Rvalue Reference question: Overloading on whether *this In article <JaK4d.7177$zA3.1500343 (AT) twister (DOT) southeast.rr.com>, [email]jgottman (AT) carolina (DOT) rr.com[/email] ("Joe Gottman") wrote: Quote: I have a question about that rvalue reference proposal in the pre-Redmond mailing ([url]http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html)[/url]. Is there any way to overload a member function on whether *this is an lvalue or an rvalue? Consider the following code: #include <vector #include vector int main() { vector<string> v(1, "Hello World"); std::string s1 = v[0]; std::string s2 = createVector()[0]; return 1; } Since v is an lvalue, s1 has to be created using a copy constructor. However, since the result of createVector() is an rvalue, theoretically s2 could be created using a move constructor, thus saving the cost of copying the string. Unfortunately, there seems to be no way to define a member function that is overloaded on whether *this is an lvalue or an rvalue. For a non-member function, we can define an overload set as follows: string const &getElement(vector<string> const &v, size_t index); //const lvalues string &getElement(vector<string> &v, size_t index); // non-const lvalues string &&getElement(vector<string> &&v, size_t index); //non-const rvalues It would be nice if we could do the same thing for member functions template class vector { ... T const & operator[](size_t index) const; T & operator[](size_t index); T && operator[](size_t index) &&; //Use && at end to say *this is an rvalue reference, similar to const at end on the first function. }; The third function above would be illegal under the rvalue-reference proposal in its current form, but I think it should be possible to do something like this. That's an interesting suggestion. Section 13.3.1, in describing overload resolution and member functions, describes member functions as having an implicit object parameter with type cv X&, except that it will also bind to rvalue X even if cv does not contain const. In other words, C++98, rewritten with the syntax, but not the capabilities of N1690, says that member functions are transformed into the following for the purpose of overload resolution: class X { R f(A1, A2, ...); }; R X::f(A1, A2, ...); becomes: R f(X&&, A1, A2, ...); // C++98 I.e. the X&& already exists in C++98, but is only available to the compiler, and only in the context of binding the implicit object parameter. Your suggestion would require a way to overload on the implicit object parameter with respect to lvalue/rvalue (we already can with respect to cv qualifiers). The ironic part is that in C++98, there is currently no way to specify an implicit object parameter with an lvalue reference! :-) R f(X&, A1, A2, ...); // not doable in C++98! I believe the trick in implementing your suggestion is to figure out how to allow the above pseudo signature without breaking backward compatibility. Getting back to your example: T & operator[](size_t index); // 1 T && operator[](size_t index) &&; // 2 We have 2 today (except of course for the return type), and 1 is really what you're requesting. Therefore in the interest of backwards compatibility the syntax would have to look something like: T & operator[](size_t index) &; // 1 T && operator[](size_t index); // 2 Although I suppose we could put the decoration on 2, but tell the compiler to vary the implicit object parameter on the undecorated 1 according to whether or not the decorated 2 actually existed in the overload set. Whether or not that is too complicated for the compiler, I'm not in a position to say. I'm stopping short of actually recommending such a proposal (nor do I oppose it). My post is merely meant to clarify what the situation is. If overloading on the implicit object parameter rvalue/lvalue-ness is not deemed practical, the client can still force the issue: std::string s2 = move(createVector()[0]); -Howard --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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