c++ inheritance design issue

Question

In c++ I want to have an array of the abstract type Query which has the function calcScore() which is a pure virtual function.
And I have two classes which are non-abstract: ConQuery and DisQuery that implements the calcScore function.

In order to do I defined the array like this:

vector<Query*> m;

and I iterate and call the function like this:

for (vector<Query*>::const_iterator it1 = index.begin() ;it1 != index.end() ; it1++)
{
     cout << (*it1)->CalcScore() << endl; 
}

I get an error for calling a pure virtual function of Query. How do I make it call to the function of ConQuery or the one of DisQuery by the polymorphic type? thanks.

Answer 1

The error can only occur in the language if you try to call a pure virtual function from the constructor or destructor of the type (where the most derived type is not yet built/has already been destroyed):

struct Query {
   virtual void f() = 0;
   Query() {
      f();                // !!
   }
   ~Query() {
      f();                // !!
   }
};

Note that compilers usually flag the code above as an error, but will fail to detect it if the call is not directly in the constructor/destructor, if for example you pass a reference to the object to a different function that performs the call.

Answer 2

Are you sure CalcScore is implemented in ConQuery and DisQuery ? I tried this:

#include <iostream>
#include <vector>

class Query{
public:
    virtual int CalcScore() = 0;
};

class Query2 : public Query
{
public:
    virtual int CalcScore()
    {

        return 2;
    }
};

class Query3 : public Query
{
public:
    virtual int CalcScore()
    {

        return 3;
    }
};

int main(int argc, char* argv[])
{

    std::vector<Query*> m;
    m.push_back(new Query2());
        m.push_back(new Query3());
    for (std::vector<Query*>::const_iterator it1 =  m.begin() ;it1 !=  m.end() ; it1++)
    {
        std::cout  << (*it1)->CalcScore();
    }
}

and it works fine under my VS2012.

Also I used it tons of times in some of my projects.

Maybe you try to push_back a Query item ( and not a con/dis ) ?

Answer 3

Change

for (vector<Query>::const_iterator it1 = index.begin() ;it1 != index.end() ; it1++)

to

for (vector<Query *>::const_iterator it1 = index.begin() ;it1 != index.end() ; it1++)

Answer 4

I might have missed something but this works for me: I already know that i is not initialised etc ;-)

#include "stdafx.h"
#include <vector>


class Query
{
public:
    int i;
    void virtual CalcScore() = 0;
};

class ConQuery :public Query
{
public:
    int i;
    void virtual CalcScore() {i++;}

};


int _tmain(int argc, _TCHAR* argv[])
{
    std::vector<Query*> index;
    ConQuery b;

    index.push_back(&b);

    for (std::vector<Query*>::const_iterator it1 = index.begin() ;it1 != index.end() ; it1++) 
    {      
        (*it1)->CalcScore();
    } 

    return 0;
}