Re: Templated class not modified by parent member function?

From:
Victor Bazarov <v.bazarov@comcast.invalid>
Newsgroups:
comp.lang.c++
Date:
Mon, 21 Mar 2011 09:19:32 -0400
Message-ID:
<im7j96$mhv$1@news.eternal-september.org>
On 3/20/2011 5:55 PM, JPonens wrote:

Greetings& salutations everyone,
I'm struggling to wrap my brain around this one, because everything

 > compiles just fine (g++); the sortable_list Mylist is not modified
 > by insertion_sort() and has an apparent count == 0, even after
 > adding to the list. I think the problem is in how I create the
 > classes, but I don't see anything wrong. Can you see anything
 > obvious that might be causing this behavior?

Yes. See below.

===================The main test file:
#include "SortingAlgorithms.h"

using namespace std;

int main() {
     Sortable_list<int> Mylist;
     Mylist.insert(0,12);
     Mylist.insert(1,36);
     Mylist.insert(2,1);

     Mylist.traverse(printone); // prints 12\n36\n1\n
     Mylist.insertion_sort();
     cout<< "-------"<< endl;
     Mylist.traverse(printone); // prints 12\n36\n1\n ... these should be in ascending order!
     return 0;
}

======== SortingAlgorithms.h =========
const int max_list = 500000;

template<class T>
class List {
public:
// methods of the List ADT
    List();
    template<class U> friend class Sortable_list;
    int size() const;
    bool full() const;
    bool empty() const;
    void clear();
    void traverse(void (*visit)(T&));
    Error_code retrieve(int position, T&x) const;
    Error_code replace(int position, const T&x);
    Error_code remove(int position, T&x);
    Error_code insert(int position, const T&x);
protected:
// data members for a contiguous list implementation
    int count;
    T entry[max_list];
};

// I presume the problem lies here:
template<class T>
class Sortable_list : public List<T> {
public: // Add prototypes for sorting methods here.
     // void stl_sort();
     // etc..

     void insertion_sort(); // offending algorithm
protected:
     // Add prototypes for auxiliary functions here.
     int count;
     T entry[max_list];


Your base class has 'int count' and your derived class has its own 'int
count'. If you declare a member named "blah" in the derived class, it
*hides* the member named "blah" in the base class. Same with 'entry'
array. Why do you think you need to define two additional members in
the derived class if your base class already has them?

You need to read again about inheritance and object layout, about
members and access to them.

};
[...]

============
Thank you for any assistance!


V
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