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c++inheritancepolymorphismvirtual

virtual function question

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

class Helper 
{
public:
    Helper() { init(); }
    virtual void print() {
        int nSize = m_vItems.size();
        std::cout << "Size : " << nSize << std::endl;
        std::cout << "Items: " << std::endl;
        for(int i=0; i<nSize; i++) {
            std::cout << m_vItems[i] << std::endl;
        }
    }
protected:
    virtual void init() { m_vItems.push_back("A"); }
    std::vector<std::string> m_vItems;
};

class ItemsHelper : public Helper
{
public:
    ItemsHelper() { }
protected:
    virtual void init() { 
        Helper::init();
        m_vItems.push_back("B");
    }
};

int _tmain(int argc, _TCHAR* argv[]) {
    ItemsHelper h;
    h.print();
}

This output's that the size of the vector is 1. I expected the size to be 2 because in the ItemsHelper::init function I called the base class Helper::init() function, then I add a second item to the vector. The problem is, the ItemsHelper::init doesn't get called, the base class init function gets called instead.

I want the ItemsHelper::init function to get called, and I can do that by calling the init function in the ItemsHelper ctor rather than in the base class. BUT, the question is, is there a better way to achieve that and still keep the call to the init() in the base class? Because what if I want to create a Helper object instead of a ItemsHelper, then the init function would never get called.

btw, this is a simplified version of a issue I'm seeing in a much larger object, I just made these objects up for example.

Solution

  • In a base class constructor, the derived class has not yet been constructed so the overriden function on the derived class is not yet available. There's a FAQ entry on this somewhere... which I can't find.

    The simplest solution is to just put the .push_back("A") part of init into the Helper constructor and the .push_back("B") into the ItemsHelper constructor. This seems to do what you are trying to do and cuts out the unnecessary init virtual function.