Base class method that returns a pointer to a derived class?

[Teis Draiby]

I want to write a base class that includes a member function that creates an
instance of a derrived class and returns a pointer to it.

Problem: The derived class definition has to follow the base class
definition. Therefore I can't specify the return type of the function, that
returns a pointer to the derived class, because the derived class is not yet
known at that point.
Hov can I solve this problem? In a nice manner :)

-- Source Code ------------------------------------------------

class myMainClass
{
public:
mySubClass createSubClassInstance(void);
};
class mySubClass: public myMainClass
{
};

----------------------------------------------------------------------
I'm am looking forward for your clever answer
regards, Teis

[Nils Petter Vaskinn]

On Fri, 02 Apr 2004 12:41:56 +0200, Teis Draiby wrote:

I want to write a base class that includes a member function that creates an
instance of a derrived class and returns a pointer to it.
Why? Ideally the base class shouldn't need to know about the derived
class(es)
Problem: The derived class definition has to follow the base class
definition. Therefore I can't specify the return type of the function, that
returns a pointer to the derived class, because the derived class is not yet
known at that point.
Hov can I solve this problem? In a nice manner :)
With a class declaration like this:

class mySubClass;
class myMainClass
{
public:
/* you did say you wanted it to return a pointer */
mySybClass *createSubClassInstance(void)
};
class mySubClass: public myMainClass
{
};

--
NPV

"the large print giveth, and the small print taketh away"
Tom Waits - Step right up

[Teis Draiby]

> Why? Ideally the base class shouldn't need to know about the derived

class(es)

Maybe you're right, but here's the reason (don't read it - it is not
important):

It's for a console window in a Win32 application.
The base class creates the console window and has methods like
'putMessage(char *message)'.

Different classes or other logical sections of my program should have the
option to create its own message class instance. These instances output to
the same console window and don't want to initialize any new console
windows. The main reason is to put a prefix to each message indicating who
send the message:

// the message class instance is created as the first thing:
myApp> Welcome to myApp!
// later the fileReader class makes its own instance:
myApp:fileReader> File "myFile.xml" loaded succesfully!

thus I want the base class to create these instances. Since the instances
would do almost the same as the base class except initializing the window
they are implemented as a derived class. It will contain additional methods
like toggleOnOff().

Maybe it's a better idea to make both classes as derrived classes of a new
base class.

regards, Teis

[Pete Vidler]

Teis Draiby wrote:

I want to write a base class that includes a member function that creates an
instance of a derrived class and returns a pointer to it.

I posted a very similar problem to the moderated newsgroup and got an
excellent reply. Firstly, it's best to note that the abstract factory
pattern might be more appropriate for what you want (I don't know your
situation). It's something to look into.

This is the trick, originally posted to the moderated group by Maxim
Yegorushkin (altered a little by me):

class Base
{
public:
Base( Derivation ) { ... }

template< class T >
T Create() { return T( Derivation() ); }

// To return a boost::shared_ptr, replace the above with:
// template< class T >
// boost::shared_ptr< T > Create() { return new T( Derivation ); }

protected:
Base() { ... }
struct Derivation {};

private:
// You might want copy ctors and copy assignment operators here.
};

class Derived : public Base
{
public:
Derived( Base::Derivation ) { ... }

protected:
Derived() { ... }
};

Derived can now only be created like this:

Derived d = Base::Create< Derived >();

Any other attempt to create it will cause a compiler error. It works
because the constructor that is used by Create is public, but cannot be
accessed outside the hierarchy because the parameter is a protected
member of Base.

Issues:

1) Two constructors required per class. This is not strictly true, but
if you don't provide a protected default constructor, the public
constructor must pass its parameter back to its base class' constructor.

2) If you leave out the public constructor in a class, but keep the
protected default constructor, you will have a class that cannot be
created (but classes derived from it can still be created). This is
useful when you want this functionality but can't provide a pure virtual
method.

3) Think carefully about what you want to return from Create. As it
stands the object is returned by value (which means the copy-constructor
and copy assignment operator can still be used to construct a new
object). This might be fine for you, but if not you can either make them
private or return a smart pointer instead (such as boost::shared_ptr,
from www.boost.org).

I will say this once more -- you would do well to look into abstract
factory and other design patterns/techniques before using this. Your
choice, though.

Oh, and I don't guarantee that the above code compiles, obviously. :)

-- Pete