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Editorial Team
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Asked: 2026-05-26T07:45:19+00:00

This is a follow up of this question. I need to implement a conversion

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This is a follow up of this question.

I need to implement a conversion operation to pass responsibility for a member object from one object to another. The conversion operation takes place in a class hierarchy. There is a Base class and two deriving classes, say Child1 and Child2. In the Base class, there is a dynamically created object I need to pass from Child1 to Child2 (along with responsibility) while the conversion happens and not let Child1‘s destructor destroy it. I’ve written a simple example to illustrate what I’m trying to achieve:

#include <iostream>
using namespace std;

class Base {
public:
    Base() {
        p_int = new int; *p_int = 0;
        cout << "In Base constructor, reserving memory." << endl;
    }
    Base(const Base& other) : p_int(other.p_int), a(other.a) {
        cout << "In Base copy-constructor." << endl;
    }
    virtual ~Base() { delete p_int; p_int = NULL; cout << "Freeing memory." << endl; }

    void setpInt(int val) { *p_int = val; }
    void setInt(int val) { a = val; }
    virtual void print() {
        cout << "Base: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
protected:
    int* p_int;
    int a;
};

class Child1 : public Base {
public:
    Child1() : Base() {};
    Child1(const Base& base) : Base(base) {}

    void print() {
        cout << "Child1: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

class Child2 : public Base {
public:
    Child2() : Base() {};
    Child2(const Base& base) : Base(base) {}

    void print() {
        cout << "Child2: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

int main() {
    Child1* c1 = new Child1();
    c1->setpInt(3);
    c1->setInt(2);
    c1->print();

    Child2* c2 = new Child2(*c1);
    c2->print();

    delete c1;      //Obviously c1's destructor is called here.
    c2->print();

    delete c2;

    return 0;
}

And the outcome is:

In Base constructor, reserving memory.
Child1: 158711832:3 2
In Base copy-constructor.
Child2: 158711832:3 2
Freeing memory.
Child2: 158711832:0 2
Freeing memory.

Is there a way to do what I’m trying to do in a clean way? I can’t copy-construct p_int because it’s very heavy. The project is an embedded, AVR-project, so smart-pointers or boost library might not be available (I don’t know, though) – I just recall this might be a solution, but I’ve never used them yet.

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1 Answer

  1. Editorial Team

    I think you need to look at reference-counted objects. Essentially, the object itself tracks it’s usage itself, and instead of storing a raw pointer to the object, you use a reference-counted pointer.

    Scott Meyers talks about this here:

    http://www.aristeia.com/BookErrata/M29Source.html

    As you’re on an embedded system, I’m not sure you can use boost::shared_ptr<> but there’s nothing stopping you implementing this as Meyers outlines.

    So, instead of having a raw pointer to the object in question in the base class, have a shared pointer/reference-counted pointer that will prevent the deletion of the object once it gets copied in to Child2. By the construction of Child2, it will have 2 references so won’t die when Child1 gets deleted. It will, however, when Child2 is deleted.

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