Is it possible to change a C++ object's class after instantiation?

Question

I have a bunch of classes which all inherit the same attributes from a common base class. The base class implements some virtual functions that work in general cases, whilst eac

Answer 1

I suggest you use the Strategy Pattern, e.g.

#include 

class IAnnouncer {
public:
    virtual ~IAnnouncer() { }
    virtual void whoami() = 0;
};

class AnnouncerA : public IAnnouncer {
public:
    void whoami() override {
        std::cout << "I am A\n";
    }
};

class AnnouncerB : public IAnnouncer {
public:
    void whoami() override {
        std::cout << "I am B\n";
    }
};

class Foo
{
public:
    Foo(IAnnouncer *announcer) : announcer(announcer)
    {
    }
    void run()
    {
        // Do stuff
        if(nullptr != announcer)
        {
            announcer->whoami();
        }
        // Do other stuff
    }
    void expend(IAnnouncer* announcer)
    {
        this->announcer = announcer;
    }
private:
    IAnnouncer *announcer;
};


int main() {
    AnnouncerA a;
    Foo foo(&a);

    foo.run();

    // Ready to "expend"
    AnnouncerB b;
    foo.expend(&b);

    foo.run();

    return 0;
}

This is a very flexible pattern that has at least a few benefits over trying to deal with the issue through inheritance:

• You can easily change the behavior of Foo later on by implementing a new Announcer
• Your Announcers (and your Foos) are easily unit tested
• You can reuse your Announcers elsewhere int he code

I suggest you have a look at the age-old "Composition vs. Inheritance" debate (cf. https://www.thoughtworks.com/insights/blog/composition-vs-inheritance-how-choose)

ps. You've leaked a Derived in your original post! Have a look at std::unique_ptr if it is available.