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            linux&c++ R&D

            programing is a pleasure!

            Permission problem on a derived private method from public method

            problem:
            Is there a good explanation why compilation tags error for the call to
            AA.method(). It is public in the base class. If cast to baseclass,
            then compiler says it's ok.

            class A 


            public
                A() 
            {} 
                virtual 
            ~A() {} 
                virtual 
            void method(void{ std::cout << "A::method" <<std::endl; } 
            }
            ;// A 
            class AA : public A 
            public
                AA() 
            {} 
                virtual 
            ~AA() {} 
            private
                virtual 
            void method(void
                   A::method(); 
                   std::cout 
            << "AA::method" << std::endl; 
                }
             
            }
            ;// A 
            int main() 

                AA obj; 
                A
            * p = &obj; 
                p
            ->method(); 
                
            //((A&)obj).method(); 
                obj.method();                  // ERROR: 
             }


            gcc -o drvd drvd.cpp -lstdc++ -lm
            drvd.cpp: In function `int main()':
            drvd.cpp:26: error: `virtual void AA::method()' is private
            drvd.cpp:46: error: within this context
            http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/7ae23191ff4de6a9/4560e82948e9b91e#4560e82948e9b91e

            solution:
                 Above ,it seems as if it is a paradox !
                 why in devived class,function method() is declared as a private memeber,still in the way
                  A* p = &obj; 
                p
            ->method(); 
               it is right!
              The phenomena is explained in the Inside the C++ object model!
              In fact,p->method() is checked in two phases!
             At complie time,p->method()
             According to p type,check method()'s access level and method()'s validity,such as membership!
            so p->method() is right!
            At run time,
            p->method() is called in polymorphism way according to p's RTTI information!
            extending:
            Although it looks strange,maybe we can employ it!
            for example:
            class A{
            public:
            virtual void method()=0;
            }
            class AA:public A{
            private:
            virtual void method(){
            ...
            }
            };
            Obviously,
            A is interface and AA is a implemention!
            In the way,if you use AA directly,
            it will result in error,so prevent any subclass's use!
            A is the only entry!
            It may be helpful in some library's device!


            posted on 2007-04-21 12:44 丑石 閱讀(710) 評論(0)  編輯 收藏 引用 所屬分類: C++ problem and solution

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