C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why Does an Object of an Empty Classs Require a User-declare          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Greg Herlihy Guest Posted: Tue Sep 26, 2006 3:39 pm    Post subject: Why Does an Object of an Empty Classs Require a User-declare Is there a particular reason why C++ requires that the empty class of a const object have a user-declared, default constructor? For example, this code will not compile: struct Sign { int operator()( int i ) const { return -(i < 0) + (i > 0); } }; const Sign SignFunction; // Error: uninitialized const object Instead, Sign must declare a default constructor: struct Sign { Sign() {} int operator()( int i ) const { return -(i < 0) + (i > 0); } }; Now, (according to a rather informal definition) a function object that maintains no state should not declare a constructor. Especially one whose declaration makes absolutely no difference to the program. In contrast, declaring a const function object variable does make sense. In fact it is not possible to define a function object in a shared header file unless it is declared const. A non-const object definition in a shared header file would violate the One Definition Rule (ODR). And the gist of these observations is simply that the error illustrated in the sample code above is not contrived, or even all that unlikely, in light of these factors. Now my usual response to an error compiling is to fix the error. But there are two problems that I have with this particular compiler error - first I don't see any error. Specifically, I don't see anything in Sign that needs to be initialized. So I have trouble identifying exactly what unitialized object the compiler is complaining about. Almost as baffling to me, is the whereabouts of the implictly-generated, default constructor that the compiiler is supposed to generate - the one used to construct non-const Sign objects. Did it disappear? As far as I can tell, the compiler-generated should still exist - but for some inscrutable reason the Standard has its invisibility prevent it from being called for a non-const instance. The other issue that I have is that I can't that the change that fixes this problem is any kind of change at all. How does the user declaring the same constructor that would have been generated had the constructor not been declared - do anything to fix this problem? As far as i can tell, nothing should have been affected by such a change, yet somehow the problem gets fixed. I guess I have a third problem as well. And that is a problem of perception: C++ seems to me to to be unique among computing languages. In particular, it is the only language in which successfully compiling a program may require that the programmer make a pointless change to fix a problem that did not exist in the first place. Greg --- [ 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 Gennaro Prota Guest Posted: Tue Sep 26, 2006 8:24 pm    Post subject: Re: Why Does an Object of an Empty Classs Require a User-dec On Tue, 26 Sep 2006 09:39:04 CST, "Greg Herlihy" <greghe (AT) pacbell (DOT) net> wrote: Quote: Is there a particular reason why C++ requires that the empty class of a const object have a user-declared, default constructor? For example, this code will not compile: struct Sign { int operator()( int i ) const { return -(i < 0) + (i > 0); } }; const Sign SignFunction; // Error: uninitialized const object Instead, Sign must declare a default constructor: It doesn't need to. You may just provide an initializer for the const instance: const Sign SignFunction = {}; I guess this problem was "popularized" by Scott Meyer's null (same problem there: either you gave it a default constructor or provided an initializer) -- Gennaro Prota --- [ 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 kanze Guest Posted: Wed Sep 27, 2006 3:38 pm    Post subject: Re: Why Does an Object of an Empty Classs Require a User-dec Gennaro Prota wrote: Quote: On Tue, 26 Sep 2006 09:39:04 CST, "Greg Herlihy" <greghe (AT) pacbell (DOT) net wrote: Is there a particular reason why C++ requires that the empty class of a const object have a user-declared, default constructor? For example, this code will not compile: struct Sign { int operator()( int i ) const { return -(i < 0) + (i > 0); } }; const Sign SignFunction; // Error: uninitialized const object Instead, Sign must declare a default constructor: It doesn't need to. You may just provide an initializer for the const instance: const Sign SignFunction = {}; But you've missed his point: you need a user specified initialization in order to initialize nothing. I suspect that when the rule was written, the authors intent was to prevent you from declaring uninitialized C-style structs. If I have a typical C style struct, I have to provide initialization (using the braces) if the object is const. (This would explain, for example, why the compiler doesn't take into account the compiler generated default constructor.) If I provide an explicit constructor, of course, the langage supposes that that constructor does the right thing, and that there is no problem. I agree with Greg that this should probably be fixed. I can see two simple fixes: -- In §8.5/9, change "if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor." to "if the object is of const-qualified type, and contains one or more non-static data members, the underlying class type shall have a user-declared default constructor." -- Or more simply, drop the above clause completely. After all, if the intent is mainly to require that C style const objects be initialized, C doesn't have the rule, and it is, in fact, not a rare practice to declare such a const object in C, to have an initialize for auto objects, e.g.: struct S { int i ; int j ; } ; S const sInit ; void f() { S someS = sInit ; // ... } The idea being to simulate for auto objects the same initialization we get for static objects. -- James Kanze GABI Software 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 --- [ 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 SuperKoko Guest Posted: Wed Sep 27, 2006 6:15 pm    Post subject: Re: Why Does an Object of an Empty Classs Require a User-dec kanze wrote: Quote: -- Or more simply, drop the above clause completely. After all, if the intent is mainly to require that C style const objects be initialized, C doesn't have the rule, C does require initializers for const objects! and it is, in fact, not a rare practice to declare such a const object in C, to have an initialize for auto objects, e.g.: struct S { int i ; int j ; } ; S const sInit ; void f() { S someS = sInit ; // ... } The code snippet below is flawed because "S const sInit;" is a declaration of an external const object while it's an ill-formed definition in C++ (because const variables with namespace scope are "static" by default in C++ but not in C). Replace "S const sInit;" with "static S const sInit;" and you'll see. Or simply compile your code and have a look at "undefined symbol" linker errors. Quote: The idea being to simulate for auto objects the same initialization we get for static objects. But in that case, the correct *definition* for sInit is: struct S const sInit={0}; Seriously, I don't see any benefit (e.g. it is not more convenient nor more simple) in specifying that an empty class can be initialized in such way. It breaks the easy to remember (but not exhaustive) rule that const aggregates require aggregate initializers. --- [ 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 kanze Guest Posted: Thu Sep 28, 2006 4:03 pm    Post subject: Re: Why Does an Object of an Empty Classs Require a User-dec SuperKoko wrote: Quote: kanze wrote: -- Or more simply, drop the above clause completely. After all, if the intent is mainly to require that C style const objects be initialized, C doesn't have the rule, C does require initializers for const objects! and it is, Where. I couldn't find it in my copy of C99. And gcc doesn't require it. Quote: in fact, not a rare practice to declare such a const object in C, to have an initialize for auto objects, e.g.: struct S { int i ; int j ; } ; S const sInit ; void f() { S someS = sInit ; // ... } The code snippet below is flawed because "S const sInit;" is a declaration of an external const object while it's an ill-formed definition in C++ (because const variables with namespace scope are "static" by default in C++ but not in C). It's illegal in C++, because there is no initializer for sInit. It is quite common practice in C. Except, of course, that I miscopied from my C++ source, rather than the C source; in C, I would have to write: struct S const sInit ; It's true that there are several differences here between C and C++: to begin with, as you say, in C++, the name sInit has internal linkage, where as in C, it has external. And while it is a definition in both languages (the const plays no role here), the way C and C++ define external linkage is slightly different. Quote: Replace "S const sInit;" with "static S const sInit;" and you'll see. See what? It doesn't change anything fundamental. Quote: Or simply compile your code and have a look at "undefined symbol" linker errors. There aren't any. There's no doubt that what I have written is a definition, and is sufficient. (I might add that gcc accepts it---that's the only C compiler I currently have access to.) Quote: The idea being to simulate for auto objects the same initialization we get for static objects. But in that case, the correct *definition* for sInit is: struct S const sInit={0}; That will also work, but it wasn't the idiomatic form when I was using C. (Admittedly, we didn't have the const back then, either, so it was simply "struct S sInit;". But adding the const, now that it has been added to C, is a logical progression, and it works fine with gcc.) Quote: Seriously, I don't see any benefit (e.g. it is not more convenient nor more simple) in specifying that an empty class can be initialized in such way. Supporting existing practice in C. Personally, it doesn't make much difference to me today, because about the only time I use an aggregate class is because I want to be able to use the {...} initialization. If we want to pretend that C++ POD are in some way compatible with C objects, then not having the rule makes sense. But I'll admit that's the only justification for using it rather than my first suggestion (only requiring initialization if there is one or more non-static data members). Quote: It breaks the easy to remember (but not exhaustive) rule that const aggregates require aggregate initializers. Yes, but... When writing C++ code, you don't have aggragate types except in order to use the {...} initialization; the simply problem doesn't occur. So the rule doesn't jurt, but it isn't necessary. When trying to compile C code with a C++ compiler, on the other hand, it makes one less thing you might have to change. -- James Kanze GABI Software 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 --- [ 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 Gennaro Prota Guest Posted: Thu Sep 28, 2006 7:41 pm    Post subject: Re: Why Does an Object of an Empty Classs Require a User-dec On Wed, 27 Sep 2006 09:38:30 CST, "kanze" <kanze@gabi-soft.fr> wrote: Quote: const Sign SignFunction; // Error: uninitialized const object Instead, Sign must declare a default constructor: It doesn't need to. You may just provide an initializer for the const instance: const Sign SignFunction = {}; But you've missed his point: you need a user specified initialization in order to initialize nothing. No no, I didn't miss it I was just replying to the "you must declare a default constructor part". I agree that this should be fixed (though admittedly it's not a big problem), and from what I remember, the "why should I specify an initialization to initialize nothing" was exactly the point raised by Scott Meyers. In Scott's code, by the way, the class was unnamed so defining a user constructor wasn't quite a choice :-) -- Gennaro Prota --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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