C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Suggestion: Compile-time "typeof" Goto page 1, 2, 3  Next          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Ron House Guest Posted: Tue May 10, 2005 4:37 am    Post subject: Suggestion: Compile-time "typeof" It often happens that one want to propagate an argument or an object type as the result of a function. When the arg is a ref or ptr, the actual arg might not be the declared type, but a descendant. It would be very useful to be able to propagate that information into the result type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } Now, if the << operator is used as part of a larger expression, the compiler might be able to deduce stronger limits on the type of the entire expression than it can if the result type is merely ostream. I ran into this one while writing a parent class with methods that were designed to be 'strung together', each returning a ref to the owning object, *this. The classes would string together nicely, but when used in a descendant, the compiler thought the type was the ancestor, so I had to write inline 'shell' functions in every descendant to merely call the ancestor but give it the more descriptive type. It would have been great to be able to write something like: typeof(*this)& mymethod() { ... return *this; } Of course, I don't think it is beyond credibility that I have entirely overlooked some way that already exists to accomplish this... -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 Max T. Woodbury Guest Posted: Wed May 11, 2005 1:34 am    Post subject: Re: Suggestion: Compile-time "typeof" Ron House wrote: Quote: It often happens that one want to propagate an argument or an object type as the result of a function. When the arg is a ref or ptr, the actual arg might not be the declared type, but a descendant. It would be very useful to be able to propagate that information into the result type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } Now, if the << operator is used as part of a larger expression, the compiler might be able to deduce stronger limits on the type of the entire expression than it can if the result type is merely ostream. I ran into this one while writing a parent class with methods that were designed to be 'strung together', each returning a ref to the owning object, *this. The classes would string together nicely, but when used in a descendant, the compiler thought the type was the ancestor, so I had to write inline 'shell' functions in every descendant to merely call the ancestor but give it the more descriptive type. It would have been great to be able to write something like: typeof(*this)& mymethod() { ... return *this; } Of course, I don't think it is beyond credibility that I have entirely overlooked some way that already exists to accomplish this... GRIN! You really should search the archives. I proposed almost exactly this quite some time ago only to discover that the GNU compiler already had something like it. Further, you can apparently get this information with an appropriate specified template class. [email]max (AT) mtew (DOT) isa-geek.net[/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 Daniel Frey Guest Posted: Wed May 11, 2005 8:58 pm    Post subject: Re: Suggestion: Compile-time "typeof" Ron House wrote: Quote: type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } Now, if the << operator is used as part of a larger expression, the compiler might be able to deduce stronger limits on the type of the entire expression than it can if the result type is merely ostream. I ran into this one while writing a parent class with methods that were designed to be 'strung together', each returning a ref to the owning object, *this. The classes would string together nicely, but when used in a descendant, the compiler thought the type was the ancestor, so I had to write inline 'shell' functions in every descendant to merely call the ancestor but give it the more descriptive type. It would have been great to be able to write something like: typeof(*this)& mymethod() { ... return *this; } Of course, I don't think it is beyond credibility that I have entirely overlooked some way that already exists to accomplish this... You mean like: template< typename T > T& operator<<( T& out, const Person& p ) { ...; return out; } Obviously this doesn't solve the general case you addressed, but I guess your example is out ;) Regards, Daniel --- [ 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 James Dennett Guest Posted: Wed May 11, 2005 8:58 pm    Post subject: Re: Suggestion: Compile-time "typeof" Ron House wrote: Quote: It often happens that one want to propagate an argument or an object type as the result of a function. When the arg is a ref or ptr, the actual arg might not be the declared type, but a descendant. It would be very useful to be able to propagate that information into the result type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } Now, if the << operator is used as part of a larger expression, the compiler might be able to deduce stronger limits on the type of the entire expression than it can if the result type is merely ostream. I ran into this one while writing a parent class with methods that were designed to be 'strung together', each returning a ref to the owning object, *this. The classes would string together nicely, but when used in a descendant, the compiler thought the type was the ancestor, so I had to write inline 'shell' functions in every descendant to merely call the ancestor but give it the more descriptive type. It would have been great to be able to write something like: typeof(*this)& mymethod() { ... return *this; } Of course, I don't think it is beyond credibility that I have entirely overlooked some way that already exists to accomplish this... typeof has been implemented by some compilers; the proposal under consideration by the committee for the next C++ standard uses the keyword "decltype" for something along similar lines. -- James --- [ 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 Alberto Barbati Guest Posted: Wed May 11, 2005 8:59 pm    Post subject: Re: Suggestion: Compile-time "typeof" Max T. Woodbury wrote: Quote: Ron House wrote: GRIN! You really should search the archives. I proposed almost exactly this quite some time ago only to discover that the GNU compiler already had something like it. Further, you can apparently get this information with an appropriate specified template class. You really should search the archives, too. There is an ongoing formal proposal of a typeof-like core language feature, called decltype. You can have a read about it here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1705.pdf The paper also contains the rationale why the proposed name is not typeof and much more (a new use of the auto keyword, a new syntax for specifying function return values, etc.) Alberto --- [ 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 msalters Guest Posted: Thu May 12, 2005 2:33 am    Post subject: Re: Suggestion: Compile-time "typeof" Ron House schreef: Quote: It often happens that one want to propagate an argument or an object type as the result of a function. When the arg is a ref or ptr, the actual arg might not be the declared type, but a descendant. It would be very useful to be able to propagate that information into the result type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } That is of course impossible. The returned object has a static and dynamic type. Clearly, ostream& is the static type, and the dynamic type is whatever is passed in at runtime. You cannot make the static type identical to the dynamic type, because (a) there can be multiple dynamic types, for each call to operator<< while there's only one static type and (b) static types are determined before dynamic types are even known. HTH, Michiel Salters --- [ 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 Ron House Guest Posted: Thu May 12, 2005 6:06 am    Post subject: Re: Suggestion: Compile-time "typeof" Alberto Barbati wrote: Quote: Max T. Woodbury wrote: Ron House wrote: GRIN! You really should search the archives. I proposed almost exactly this quite some time ago only to discover that the GNU compiler already had something like it. Further, you can apparently get this information with an appropriate specified template class. You really should search the archives, too. There is an ongoing formal proposal of a typeof-like core language feature, called decltype. You can have a read about it here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1705.pdf The paper also contains the rationale why the proposed name is not typeof and much more (a new use of the auto keyword, a new syntax for specifying function return values, etc.) Thanks for that ref. Having read it, though, I see they are not actually solving the problem I am raising. Their proposal is largely syntactic sugar. Their decltype gives the type of the formal parameter, not the (compile-time) type of the actual parameter, and is therefore vastly less useful. What I want is this: class A { ... A& mymethod() { ... return *this; } }; class B: public A { ... B& bmethod() { ... } }; B b; .. b.mymethod() <--- at this point the compiler knows the expression result is a B&, not an A&. The code as written clearly tells the compiler that mymethod returns an A&, but in fact static examination of the code promises that mymethod is actually returning a B& at this point. My suggested rewriting of mymethod: typeof(*this)& mymethod() { ... return *this; } ensures that no other returns can be inserted returning supertypes of B, because, in order to write a return that satisfies typeof(*this)&, it more or less can't be anything but *this. Any actual typed expression (such as an A variable) _might_ be a supertype of *this at runtime. So consider: A& mymethod() { if (...) return somethingoftypeA; else return *this; } The above is syntactically correct, and we have no guarantee that mymethod will, for a B object, return a B. The compiler must treat the result as of type A, and the following will be illegal: b.mymethod().bmethod(); However, if we could write: typeof(*this)& mymethod() { if (...) return somethingoftypeA; else return *this; } then the compiler would give a syntax error because it cannot guarantee that somethingoftypeA will indeed have a type compatible with *this at runtime (because the method might be executed from a descendant). However, the simpler version: typeof(*this)& mymethod() { ... return *this; } will be correct, and can be guaranteed at compile time to actually return something equal or a subtype of its owning object at runtime. Then the call: b.mymethod().bmethod(); will be seen to be sane and will compile correctly. Aikens designed a simple language called Cool for his compiler design class, and he convinced me with it that he is a genius! Cool does all this right, despite being a 'toy' language. There is no reason at all C++ should get it wrong. (See http://www.cs.berkeley.edu/~aiken/cool) -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 Ron House Guest Posted: Thu May 12, 2005 6:06 am    Post subject: Re: Suggestion: Compile-time "typeof" Daniel Frey wrote: Quote: You mean like: template< typename T T& operator<<( T& out, const Person& p ) { ...; return out; } Obviously this doesn't solve the general case you addressed, but I guess your example is out Yes, that one does seem to solve the problem when there is an explicit argument, although it leaves the method return type out in the cold. And it sort of rankles to use a template for such a simple requirement. -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 Ron House Guest Posted: Mon May 16, 2005 4:19 pm    Post subject: Re: Suggestion: Compile-time "typeof" James Dennett wrote: Quote: typeof has been implemented by some compilers; the proposal under consideration by the committee for the next C++ standard uses the keyword "decltype" for something along similar lines. Unfortunately the similar lines are not similar enough to solve the major problem. See my other post today. -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 Ron House Guest Posted: Mon May 16, 2005 4:19 pm    Post subject: Re: Suggestion: Compile-time "typeof" msalters wrote: Quote: Ron House schreef: It often happens that one want to propagate an argument or an object type as the result of a function. When the arg is a ref or ptr, the actual arg might not be the declared type, but a descendant. It would be very useful to be able to propagate that information into the result type. For example, typeof(out) operator<<(ostream& out, const Person& p) { ... output the Person data ... return out; } That is of course impossible. The returned object has a static and dynamic type. Clearly, ostream& is the static type, and the dynamic type is whatever is passed in at runtime. I never mentioned the dynamic type, I mentioned the type of the actual parameter, meaning the declared type as known to the compiler. I think you misunderstand the requirement. I am not asking that the compiler use magic to know what the dynamic type will be at runtime. But the compiler very well will know the declared type of the actual parameter at compile time, and that (in order to allow at runtime descendants to be passed) might be a descendant of the type of the formal parameter. It will know statically that the method returns that descendant type. But C++ doesn't allow me to specify this to the compiler, entirely due a language shortcoming, nothing to do with runtime types at all. And the decltype suggestion does NOT meet that need. -- Ron House [email]house (AT) usq (DOT) edu.au[/email] http://www.sci.usq.edu.au/staff/house --- [ 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 Hyman Rosen Guest Posted: Mon May 16, 2005 4:19 pm    Post subject: Re: Suggestion: Compile-time "typeof" Ron House wrote: Quote: typeof(*this)& mymethod() { if (...) return somethingoftypeA; else return *this; } I doubt the utility of returning a reference to a different object of the same type as the caller, so I would refine this proposal to simply allow a method to be declared of type 'this', and have such a method implicitly return *this without needing it to be explicitly specified. It's the perfect return type for assignment operators, as well as the stream pass-throughs. struct mystream : ostream { template <typename T> this operator<<(T v) { (ostream &)*this << v; } }; struct point { double x, y, z; this operator=(const point &o) { x = o.x; y = o.y; z = o.z; } }; And the static type of a 'this' method invocation will of course be the static type of the class to which the initial invoking pointer, reference, or actual object refers. --- [ 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 Alberto Barbati Guest Posted: Mon May 16, 2005 4:21 pm    Post subject: Re: Suggestion: Compile-time "typeof" Ron House wrote: Quote: Alberto Barbati wrote: You really should search the archives, too. There is an ongoing formal proposal of a typeof-like core language feature, called decltype. You can have a read about it here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1705.pdf The paper also contains the rationale why the proposed name is not typeof and much more (a new use of the auto keyword, a new syntax for specifying function return values, etc.) Thanks for that ref. Having read it, though, I see they are not actually solving the problem I am raising. Their proposal is largely syntactic sugar. I'm not sure the authors of that proposal are very happy the hear that... ;-) Quote: sugar. Their decltype gives the type of the formal parameter, not the (compile-time) type of the actual parameter, and is therefore vastly less useful. What I want is this: class A { ... A& mymethod() { ... return *this; } }; class B: public A { ... B& bmethod() { ... } }; B b; . b.mymethod() <--- at this point the compiler knows the expression result is a B&, not an A&. Hmmmm... So you are telling me that you want the *same* function A::mymethod to return two different types and the compiler should choose which type according to type of one of the functions parameters (the implicit "this" parameter, in this case)? I'm not saying that it couldn't be done, but: 1) if it's limited to typeof(*this), then it's just some kind of limited static covariance. It might even be easy to implement, although requiring a new keyword for this feature looks a bit too much to me. Frankly I don't see a case where this feature could really be useful. Could you please enlighten me by posting a *real-life* use case where this idiom could effectively help? Moreover, consider this: class A { ... A& mymethod() { ... return *this; } }; template T& mymethod(T& x) { x.mymethod(); return x; } and just use mymethod(b) instead of b.mymethod(). 2) if this feature is taken in full generality (any expression in typeof depending on any parameter) I find it hard to believe that such thing can be implemented without troubles... Quote: Aikens designed a simple language called Cool for his compiler design class, and he convinced me with it that he is a genius! Cool does all this right, despite being a 'toy' language. There is no reason at all C++ should get it wrong. (See http://www.cs.berkeley.edu/~aiken/cool) I have no reason to doubt about the intelligence of this person, but the fact that you don't see a reason why C++ does not have such a feature does not imply that they don't exist, neither that C++ is "wrong" about it. You have to be more convincing than that. Alberto --- [ 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 Dave Harris Guest Posted: Mon May 16, 2005 10:04 pm    Post subject: Re: Suggestion: Compile-time "typeof" [email]hyrosen (AT) mail (DOT) com[/email] (Hyman Rosen) wrote (abridged): Quote: typeof(*this)& mymethod() { if (...) return somethingoftypeA; else return *this; } I doubt the utility of returning a reference to a different object of the same type as the caller, so I would refine this proposal to simply allow a method to be declared of type 'this', and have such a method implicitly return *this without needing it to be explicitly specified. Interesting digression, but I think the original proposal was supposed to work for non-member functions too. It would be handy if: typeof( str ) strchr( const char *str, char ch ); char str[10] = "test"; char *s = strchr( str, 's' ); *s = 'x'; compiled. This example doesn't use classes or inheritance at all. Returning an object of the same type as an argument, but not the same object, is useful sometimes. For example, the intersection of one rectangle with another rectangle is always a rectangle. It may or may not be equal to one or both argument rectangles, and if it is equal, it may or may not be the same object. However, if it is a different object, I doubt the compiler could statically prove it was the same type, in general. -- Dave Harris, Nottingham, UK. --- [ 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 Hyman Rosen Guest Posted: Mon May 16, 2005 11:15 pm    Post subject: Re: Suggestion: Compile-time "typeof" Dave Harris wrote: Quote: Interesting digression The sneaky part of my suggestion, if you looked at my code sample, was that such functions looked very much like they could be declared as returning void. And they could be, because all the compiler has to do is at the call site, make believe that the function has returned the call object (or in your example, some distinguished parameter). --- [ 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 Seungbeom Kim Guest Posted: Tue May 17, 2005 1:59 am    Post subject: Re: Suggestion: Compile-time "typeof" Dave Harris wrote: Quote: Interesting digression, but I think the original proposal was supposed to work for non-member functions too. It would be handy if: typeof( str ) strchr( const char *str, char ch ); char str[10] = "test"; char *s = strchr( str, 's' ); *s = 'x'; compiled. This example doesn't use classes or inheritance at all. Returning an object of the same type as an argument, but not the same object, is useful sometimes. For example, the intersection of one rectangle with another rectangle is always a rectangle. It may or may not be equal to one or both argument rectangles, and if it is equal, it may or may not be the same object. Why not use template? template <typename Char> Char* strchr(Char* str, char ch); -- Seungbeom Kim --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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