Creating derived class instance using base class instance

Question

I have a base class instance, there is a derived class that inherits from the base class, I want to transform the base instance into derived instance, (if possible without copying anything (maybe sending to the derived class a reference of the base class)) how can I achieve that?

Note: I need this because I'm using factory design pattern which identify the derived class needed to be created using a parameter located in the base instance.

//class A
//class B: public A (pure virtual)
//class C: public B

B BFactory::makeB(A &a) {
    int n=a.getN();
    if(n==1){
        return new C();
    }
}

Thanks.

Answer 1

Consider the case of the car.

You can treat a Lamborghini as a car.

You can treat a Yugo as a car.

You can treat a car as a Lamborghini if it is a Lamborghini. In C++ this means a pointer to car that really points to a Lamborghini. In order to get a Lamborghini pointer back out of the car pointer you should use dynamic_cast. If the car does not point to a Lamborghini, dynamic_cast will return NULL. This keeps you from trying to pass off a Yugo as a Lamborghini and blowing the Yugo's engine.

But when the Lamborghini is being treated as a car, it can only do car things. If you copy a Lamborghini into a car, you strip out all Lamborghini-ness forever. It's gone.

Code time!

This, I'm afraid cannot be done:

//class A
//class B: public A (pure virtual)
//class C: public B

B BFactory::makeB(A &a) {
    int n=a.getN();
    if(n==1){
        return new C();
    }
}

C is being copied into a B and the B is being returned. B would need a constructor that took a C, but the point is moot. B cannot be instantiated if it's pure virtual. For now we'll ignore the leak that would be new C()

Also can't use a reference for this job, pretty much the same problem, so you're trapped into returning a pointer

B * BFactory::makeB(A &a) {
    int n=a.getN();
    if(n==1){
        return new C();
    }
}

Now I'm going to make a suggestion: Build the make function into B and handle the case where A doesn't map to anything recognized by B.

class B: public A
{
public:
    virtual ~B(){}
    static B * makeB(A & a)
    {
        switch(a.getN())
        {
            case 1:
                return new C();
        }
        return NULL;
    }
};

But this leads to another recommendation: Why should B know anything? And What is the point of A at this level? Why is A storing build codes for classes two or more steps down the hierarchy? Bad from a maintenance point of view. The point of objects is they know who they are and how to manipulate themselves. Short-circuiting this leads to pain.

class B: public A
{
public:
    virtual ~B(){}
    virtual B* makeB() = 0;
};

Now B only makes Bs, needs no help from A, and those who extend B are stuck with figuring out how to make themselves--a task they should know better than anyone else. Much safer because there is never any possibility of a code unrecognised by B for a new class.

class C: public B
{
public:
    B* makeB()
    {
        return new C();
    }
};

class D: public B
{
public:
    B* makeB()
    {
        return new D();
    }
};

Edit: Traditional factory

You're asking for an abstract factory. For that you need nothing. You don't even need a class. You certainly don't need a class A. The goal of this sort of factory is the caller knows nothing about the class. By providing an A, the caller needs to know how to make an A or have another factory that makes an A.

First a bit of set-up in a header file BFactory.h:

#ifndef BFACTORY_H_
#define BFACTORY_H_

#include <exception>
class B
{
public:
    virtual ~B(){}
    virtual std::string whatAmI() = 0;
protected:
    // data members common to all B subclasses
};

enum bType
{
    gimmie_a_C,
    gimmie_a_D,
    gimmie_an_E
};

class BadTypeException: public std::exception
{
public:
    const char* what() const noexcept
    {
        return "Dude! WTF?!?";
    }
};

B* BFactory(enum bType type);

#endif /* BFACTORY_H_ */

Here I'm going to deviate from the book way a little. Rather than using an integer to identify the type to be built, I'm going to use an enum. Two reasons: Easier to read and understand gimme_a_C than 1 and generates a compiler error if you try to provide a value that is not enumerated.

enum bType
{
    gimmie_a_C,
    gimmie_a_D,
    gimmie_an_E
};

And an exception to flag stupidity if the enum is updated with new types (gimmie_an_E) but the factory is not.

class BadTypeException: public std::exception
{
public:
    const char* what() const noexcept
    {
        return "Dude! WTF?!?";
    }
};

This is all the Factory client needs to see. They don't see C. They don't see D. They have no clue that C and D exist in any way other than the names listed in enum bType. All they ever see is pointers to B.

Now for the implementation BFactory.cpp:

#include "BFactory.h"

class C:public B
{
    std::string whatAmI()
    {
        return "C";
    }
};

class D:public B
{
    std::string whatAmI()
    {
        return "D";
    }
};

B* BFactory(enum bType type)
{
    switch(type)
    {
        case gimmie_a_C:
            return new C();
        case gimmie_a_D:
            return new C();
        default:
            throw BadTypeException();
    }
}

I'll leave it up to the reader to spot the stupid bug in the above code that makes these error prone and why I don't like them.

And usage, main.cpp:

#include "BFactory.h"

int main()
{
    B * temp;
    temp = BFactory(gimmie_a_C);
    std::cout << temp->whatAmI() << std::endl;
    delete temp;
    temp = BFactory(gimmie_a_D);
    std::cout << temp->whatAmI() << std::endl;
    delete temp;
    //temp = BFactory(1001); // won't compile
    try
    {
        temp = BFactory(gimmie_an_E); // will compile, throws exception 
        std::cout << temp->whatAmI() << std::endl;
    }
    catch(BadTypeException& wtf)
    {
        std::cerr << wtf.what() << std::endl;
    }
}

There is still absolutely no use for or involvement of A. A if it exists, should no nothing about B or the children of B.

These days there is a little improvement we can make so that the pointers are a little safer. unique_ptr allows us to maintain the polymporphic advantages of a pointer to B without the memory management woes.

std::unique_ptr<B> BFactory(enum bType type)
{
    switch(type)
    {
        case gimmie_a_C:
            return std::unique_ptr<B>(new C());
        case gimmie_a_D:
            return std::unique_ptr<B>(new D());
        default:
            throw BadTypeException();
    }
}

and the new main:

int main()
{
    std::unique_ptr<B> temp;
    temp = BFactory(gimmie_a_C);
    std::cout << temp->whatAmI() << std::endl;
    temp = BFactory(gimmie_a_D);
    std::cout << temp->whatAmI() << std::endl;
}