Inheriting from built-in types
in [Learn C or C++]
|
Prev: A Question about ##
Next: Lesson 8, Structures, Unions, Fields, Pointers to Structures,Enums (Don't worry, Be happy: VS IDE 2008: WORSE CRAP than D2007 IDE :))
From: Usenet.9.OkianWarrior on 15 Apr 2008 23:18 I'd like to create a new type which is polymorphic to int but has some different characteristics, something like this: struct ModuloInt : int { // <-- Note: class derives from int ModuloInt(int Value) : int(Value) {}; }; int main(void) { ModuloInt i = 10; } Neither Microsoft C++ 2008 (Express) nor gcc version 4.1 thinks this is legal. I've looked all over the net, but no one has an example of this or explains why this type of inheritance won't work. I can of course make the new class by composition and operator overloading, but I always thought that the builtin types were considered classes in C++. Is there a rule somewhere which states that only user-defined classes can be inherited from?
From: Usenet.9.OkianWarrior on 15 Apr 2008 23:21 On Apr 15, 8:18 pm, Usenet.9.OkianWarr...(a)spamgourmet.com wrote: > I'd like to create a new type which is polymorphic to int but has some > different characteristics, something like this: > > struct ModuloInt : int { // <-- Note: class > derives from int > ModuloInt(int Value) : int(Value) {}; > }; > > int main(void) { > > ModuloInt i = 10; > } > > Neither Microsoft C++ 2008 (Express) nor gcc version 4.1 thinks this > is legal. I've looked all over the net, but no one has an example of > this or explains why this type of inheritance won't work. > > I can of course make the new class by composition and operator > overloading, but I always thought that the builtin types were > considered classes in C++. > > Is there a rule somewhere which states that only user-defined classes > can be inherited from? The "derives from int" in my post is part of the comment in the previous line - it's not part of the syntax of the class definition. Google word-wrapped it for me.
From: Ian Collins on 16 Apr 2008 00:18 Usenet.9.OkianWarrior(a)spamgourmet.com wrote: > I'd like to create a new type which is polymorphic to int but has some > different characteristics, something like this: > > struct ModuloInt : int { // <-- Note: class > derives from int > ModuloInt(int Value) : int(Value) {}; > }; > > int main(void) { > > ModuloInt i = 10; > } > No, you can't do this. > I can of course make the new class by composition and operator > overloading, but I always thought that the builtin types were > considered classes in C++. > No they are not. Maybe you are thinking of Java? > Is there a rule somewhere which states that only user-defined classes > can be inherited from? See section 10 of the standard - "[looking up a base class name] .. If the name found is not a class-name, the program is ill-formed." -- Ian Collins.
From: Andrey Tarasevich on 16 Apr 2008 19:27 Usenet.9.OkianWarrior(a)spamgourmet.com wrote: > I'd like to create a new type which is polymorphic to int but has some > different characteristics, something like this: > > struct ModuloInt : int { // <-- Note: class > derives from int > ModuloInt(int Value) : int(Value) {}; > }; > > int main(void) { > > ModuloInt i = 10; > } > > Neither Microsoft C++ 2008 (Express) nor gcc version 4.1 thinks this > is legal. Not surprisingly. This is illegal in C++. > I've looked all over the net, but no one has an example of > this or explains why this type of inheritance won't work. It depends on what you mean by "why". Short answer: 'int' is not a class type and in C++ one can inherit from class types only. > I can of course make the new class by composition and operator > overloading, but I always thought that the builtin types were > considered classes in C++. They have never been considered classes in C++. > Is there a rule somewhere which states that only user-defined classes > can be inherited from? Only classes can be inherited from, which is stated in the language standard. From the language point of view, all class types are user-defined. -- Best regards, Andrey Tarasevich
From: A. W. Dunstan on 21 Apr 2008 09:58
Ian Collins wrote: > Usenet.9.OkianWarrior(a)spamgourmet.com wrote: >> I'd like to create a new type which is polymorphic to int but has some >> different characteristics, something like this: >> >> struct ModuloInt : int { // <-- Note: class >> derives from int >> ModuloInt(int Value) : int(Value) {}; >> }; >> >> int main(void) { >> >> ModuloInt i = 10; >> } >> > No, you can't do this. > >> I can of course make the new class by composition and operator >> overloading, but I always thought that the builtin types were >> considered classes in C++. >> > No they are not. Maybe you are thinking of Java? > Even in Java you can't derive from int - that's a 'native data type', and not a class. "Integer" is a class, but since it's 'final' you can't derive from it, either. Ruby and C# (and other) will let you derive from int, as they treat int (and float, and double, etc) as full-fledged objects. -- Al Dunstan, Software Engineer OptiMetrics, Inc. 3115 Professional Drive Ann Arbor, MI 48104-5131 |