casting error

This is a discussion on casting error within the C++ Programming forums, part of the General Programming Boards category; Hi, i got a base class,and 2 derieved classes.derieved1 and derieved 2. In the main class,i create a derieved1 and ...

  1. #1
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    Join Date
    Feb 2006
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    58

    casting error

    Hi,
    i got a base class,and 2 derieved classes.derieved1 and derieved 2.

    In the main class,i create a derieved1 and derieve2 object. add it to vector vector<base*> list .

    Then i try casting out the first vector item to see if dereived 1 or derieved2. the tests to check if derieved 1 or 2 work for both.

    The classes:

    Code:
    class base{
        protected:
            base(int val){
                value=val;
            }
        public:
    	int value;
    };
    
    class derieved1:public base {
        private:
    	string data;
             
            
        public:
    	derieved1(int val,string x):base(val),data(x){ }
            void print(){
                 cout<<"\ndereieved 1";
            }
    };
    
    class derieved2:public base {
        private:
            string data;
        public:
            derieved2(int val,string x):base(val),data(x){
            }
            void print(){
                 cout<<"\ndereieved 2";
            }
    };
    the main function:
    Code:
    int main(int argc, char *argv[])
    {
        vector<base*> list;
        derieved1* a=new derieved1(1,"derieved1");
        derieved2* b=new derieved2(2,"derieved2");
        
        list.push_back(a);
        list.push_back(b);
        
        base* bbbb=list[0];
        
        //cast it to both derieved objects.    
        derieved1*sb=static_cast<derieved1*>(bbbb);
        
        derieved2*sb1=static_cast<derieved2*>(bbbb);
        
        //check which one is valid.
        
        if(sb){
               cout<<"object of type derieved1"<<endl;
        }
        if(sb1){
               cout<<"object of type derieved2"<<endl; //should not display
        }
        
        system("PAUSE");
        return EXIT_SUCCESS;
    }
    The program prints outs :

    object of type derieved1
    object of type derieved2
    Last edited by rahulsk1947; 06-08-2009 at 06:36 PM.

  2. #2
    Registered User
    Join Date
    Jan 2008
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    28
    1. Don't use system(). Very bad habit. Use getchar() or something...
    2. If I remember, sb1 points to SOMETHING so it returns true. Also not something you should do everyday. You may check and see if the information it points to is valid. I think....
    3. Don't forget to dereference the pointers using the delete operator.

  3. #3
    and the hat of sweating
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    You should be using dynamic_cast, not static_cast.

    dynamic_cast will return NULL for a bad pointer cast or throw an exception for a bad reference cast.
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

    "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010

  4. #4
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    i convereted it to:

    derieved1*sb=dynamic_cast<derieved1*>(bbbb);
    derieved2*sb1=dynamic_cast<derieved2*>(bbbb);


    It gives me a error.

    'dynamic_cast' : 'base' is not a polymorphic type



    what should i modify in the base and derieved classes so that it becomes polymorphic?

  5. #5
    C++ Witch laserlight's Avatar
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    The base class should have at least one virtual function. The destructor should be virtual.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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