Thread: Solution to practice problem 1 from the book Jumping into C++

Hello,

I wrote a solution to the following practice problem.

Implement a sort function that takes a vector of pointers to an interface class, Comparable, that defines a method, compare(Comparable& other), and returns 0 if the objects are the same, 1 if the object is greater than other, and -1 if the object is less than other. Create a class that implements this interface, create several instances, and sort them. If you're looking for some inspiration for what to create—try a HighScoreElement class that has a name and a score, and sorts so that the top scores are first, but if two scores are the same, they are sorted next by name.

My code is below

Code:

#include <iostream>
#include <string>
#include <vector>
#include <cstdlib>
using namespace std;
class Comparable
{
    public:
        virtual int compare(Comparable& other)=0;
};

class HighScoreElement : public Comparable
{
    public:
        virtual int compare(HighScoreElement& other);
        void set_members(string user_name, int user_high_score)
        {
            name=user_name;
            high_score = user_high_score;
        }
        void print_members()
        {
            cout<<"Name:\t"<<name<<"\tHigh Score:\t"<<high_score<<endl;
        }
        string get_name()
            {
                return name;
            }
        int get_high_score()
        {
            return high_score;
        }
    private:
        string name;
        int high_score;
};
int HighScoreElement :: compare(HighScoreElement& other)
{
    if(this->high_score>other.high_score) return (1);
    if(this->high_score < other.high_score) return(-1);
    if(this->name>other.name) return(1);
    if(this->name<other.name) return(-1);else return(0);
}

void sort(vector<Comparable*> comparables)
{
    cout<<"\nEntering the Sort Function";
    cout<<"\nThis is the state of the vector at the beginning of the sort function  using comparables parameter\n";
    for(int i=0;i<comparables.size();i++)
    {
        ((HighScoreElement*)comparables[i])->print_members();
    }
     Comparable *p_temp_Comparable;
     for(int i=0;i<comparables.size()-i;i++)
    {
        for(int j=i+1; j<comparables.size();j++)
        {
            if(comparables[i]->compare(*comparables[j])<0)
            {
                   p_temp_Comparable = comparables[i];
                   comparables[i]=comparables[j];
                   comparables[j] = p_temp_Comparable;
            }
        }
    }
}
int main()
{
    cout << "Hello world!" << endl;
    HighScoreElement *p_HighScoreElement;
    vector<Comparable*> comparables;
    vector<HighScoreElement*> highscoreelements;
    for(int i =0;i<10;i++)
    {
        p_HighScoreElement = new HighScoreElement;
        p_HighScoreElement->set_members("Ateeque Mohammad", rand()%100);
        comparables.push_back(p_HighScoreElement);
        highscoreelements.push_back(p_HighScoreElement);
    }
    for(int i=0;i<10;i++)
    {
        p_HighScoreElement = new HighScoreElement();
        p_HighScoreElement->set_members("Abdullah Mohammad", highscoreelements[i]->get_high_score());
        comparables.push_back(p_HighScoreElement);
        highscoreelements.push_back(p_HighScoreElement);
    }

    for(int i=0;i<10;i+=3)
    {
        p_HighScoreElement = new HighScoreElement();
        p_HighScoreElement->set_members("Abdurrahman Mohammad", rand()%100);
        comparables.push_back(p_HighScoreElement);
        highscoreelements.push_back(p_HighScoreElement);
    }
    cout<<"\nThis is the state of the vector after the auto fill was run\n";
    /*for ( vector<HighScoreElement*>::iterator itr = highscoreelements.begin(), end = highscoreelements.end();
    itr != end; ++itr )
        (*itr)->print_members();
    */

    for(int i=0;i<highscoreelements.size();i++)
    {
        highscoreelements[i]->print_members();
    }
    cout<<"\nThis is the state of the vector after the auto fill was run using comparables\n";
    for(int i=0;i<comparables.size();i++)
    {
        ((HighScoreElement*)comparables[i])->print_members();
    }
    sort(comparables);

    cout<<"\nThis is the state of the vector after the sorting of the vector\n";
    for(int i=0;i<highscoreelements.size();i++)
    {
        highscoreelements[i]->print_members();
    }
    //auto_fill_vector();
    return 0;
}

When I build the code I get the below errors.

\Sort_Inheritance\main.cpp|87|error: invalid new-expression of abstract class type 'HighScoreElement'|
C:\Users\atmoha\OneDrive\CPLUSPLUS\Chapter 27\Sort_Inheritance\main.cpp|16|note: because the following virtual functions are pure within 'HighScoreElement':|
C:\Users\atmoha\OneDrive\CPLUSPLUS\Chapter 27\Sort_Inheritance\main.cpp|11|note: virtual int Comparable::compare(Comparable&)|
C:\Users\atmoha\OneDrive\CPLUSPLUS\Chapter 27\Sort_Inheritance\main.cpp|96|error: invalid new-expression of abstract class type 'HighScoreElement'|
C:\Users\atmoha\OneDrive\CPLUSPLUS\Chapter 27\Sort_Inheritance\main.cpp|105|error: invalid new-expression of abstract class type 'HighScoreElement'|

Looks like the Compare method in the class HighScoreElement is not overriding the Compare method of the Comparable class when I did declare it pure virtual in Comparable class and then made the declaration of the method as a virtual in HighScoreElement class and then defined it as well.

Can someone point out what am I doing wrong?

Since the tags took out the empty lines the error in line 87 appears on line 75 in the post here

Thanks very much for your response. Your comments are really helpful and the I now understand the concept of the override of a virtual function and retaining the function signature.

About the const-correct below

Code:

?

About the const-correct below

Code:

virtual int compare(const Comparable& other) const = 0;

What is the second const for before the =0?