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.NET Forum / Languages / Managed C++ / October 2007derived class can not access base class protected member?
Hello everyone, I met with a strange issue that derived class function can not access base class's protected member. Do you know why? Here is the error message and code. [Code] error C2248: 'base::~base' : cannot access protected member declared in class 'base' [/Code] [Code] class base { protected: ~base() {} private: void foo() { base* b = new base; delete b; } }; class derived : public base { public: ~derived() {} private: void goo() { base* b = new derived; delete b; // error in this line } }; [/Code] thanks in advance, George >I met with a strange issue that derived class function can not access base >class's protected member. Do you know why? George, The code you have isn't using b as a base class, it's just the same as though the classes were unrelated. If you add: friend class derived; to class base, it will then compile, but whether that's what you really want is another question. Dave Hi Dave, What do you mean > The code you have isn't using b as a base class, it's just the same as > though the classes were unrelated. I think I use the code base* b = new derived; delete b; // error in this line in function goo, which is in derived class right? regards, George > >I met with a strange issue that derived class function can not access base > >class's protected member. Do you know why? [quoted text clipped - 12 lines] > > Dave >I think I use the code > >base* b = new derived; >delete b; // error in this line > >in function goo, which is in derived class right? The class is a derived class, but your usage isn't. I'm not sure what you're really trying to do, but since "derived" is derived from "base" it already is a base class, there's no need to create one. Dave Hi Dave, What I want to do is, 1. in derived class member function goo, create a new instance of base class object; 2. call protected method of the base class object instance. But I do not know why there is access violation error in step 2, since I think we can access protected member from derived class, right? regards, George > >I think I use the code > > [quoted text clipped - 10 lines] > > Dave >1. in derived class member function goo, create a new instance of base class >object; [quoted text clipped - 3 lines] >But I do not know why there is access violation error in step 2, since I >think we can access protected member from derived class, right? A derived class can access *its* base class protected members, but clearly from the error you're getting, it can't do it for an arbitrary instance of the base class. I think you need to use "friend" to do that. Dave So, Dave, as you mentioned below, > A derived class can access *its* base class protected members, but > clearly from the error you're getting, it can't do it for an arbitrary > instance of the base class. I think we can understand that C++ access module is based on instance level, not class level. Right? regards, George > >1. in derived class member function goo, create a new instance of base class > >object; [quoted text clipped - 10 lines] > > Dave >> A derived class can access *its* base class protected members, but >> clearly from the error you're getting, it can't do it for an arbitrary >> instance of the base class. > >I think we can understand that C++ access module is based on instance level, >not class level. Right? I'm not sure what you'd call it (I'm not a language expert, I just use it). Dave Cool, Dave. I appreciate all of your help on this topic. regards, George > >> A derived class can access *its* base class protected members, but > >> clearly from the error you're getting, it can't do it for an arbitrary [quoted text clipped - 7 lines] > > Dave So, Dave, as you mentioned below, > A derived class can access *its* base class protected members, but > clearly from the error you're getting, it can't do it for an arbitrary > instance of the base class. I think we can understand that C++ access module is based on instance level, not class level. Right? regards, George > >1. in derived class member function goo, create a new instance of base class > >object; [quoted text clipped - 10 lines] > > Dave > So, Dave, as you mentioned below, > [quoted text clipped - 5 lines] > level, > not class level. Right? Based on the compile-time type of the instance. It doesn't matter that the object really is a "derived" -- if it's being accessed through a base pointer, you get the same kind of access as to other objects subtyped from base. > regards, > George [quoted text clipped - 14 lines] >> >> Dave Hi Ben, I found my previous conclusion that then entry point of an instance must be public and C++ provides instance level (not class level) access model is not correct. I have developed the following sample, in my sample, calling private member instance2.goo2() is correct in Visual Studio 2005, even if instance2 is not *this*. So I do not think C++ provides instance level access model. But from my original question, it seems that the access model is instance level -- derived class can not access protected member of base class. So, what is the access model? Any comments? class base { protected: ~base() {} private: void foo() { base* b = new base; delete b; } }; class derived : public base { public: ~derived() {} private: void goo (derived& instance2) //derived class object { instance2.goo2(); } void goo2() { } }; regards, George > > So, Dave, as you mentioned below, > > [quoted text clipped - 29 lines] > >> > >> Dave > Hi Ben, > [quoted text clipped - 13 lines] > > So, what is the access model? Any comments? As I explained, it is based on the compile-time type. Members and friends of class derived can access private and protected members of derived through any derived* (this includes references, local variables, etc, as long as the compiler can get a "this" pointer of type derived* using the normal casting rules). I'm ignoring const for this discussion. > class base > { [quoted text clipped - 64 lines] >> >> >> >> Dave Thanks Ben, I am clear now. regards, George > > Hi Ben, > > [quoted text clipped - 88 lines] > >> >> > >> >> Dave > private: > void foo() Because foo( ) is defined as PRIVATE (not protected) in the base class (typo?)... ;) [==Peter==] > Hello everyone, > [quoted text clipped - 36 lines] > thanks in advance, > George Hi Peter, I am not invoking foo in derived class, the error occurs when I invoke destructor (protected, by using the statement delete b below). I am confused why I can not access the protected method of base class in derived class? void goo() { base* b = new derived; delete b; // error in this line } have a good weekend, George > > private: > > void foo() [quoted text clipped - 44 lines] > > thanks in advance, > > George Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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