Equiv of function prototype for classes?

I got myself into a chicken and egg scenario.

With functions you can do the following:

void a();

void b()
{
    a();
}

void a()
{
   cout << "Hello World";
}

But with classes what do you do?

I have two classes, A, and B. B inherits from A. A creates an instance of B in one of it's member functions.

Obviously A has no idea that B even exists and If I were to move B above A in the source file then B will complain that A doesn't exist!

What do I do? Is it possible to create an object of B inside of A? I would think it is because you could create an A object inside of A.

Thanks,
-Sandra
LVL 3
Sandra-24Asked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

mnashadkaCommented:
You can do a forward declaration:

class B; // Forward declaration

class A
{
public:
  B b;
};

class B : public A
{
};
0
mnashadkaCommented:
This will also work in several source files:
// In A.h
class B; // Forward declaration

class A
{
public:
  B b;
};

// In B.h
#include "A.h"
class B : public A
{
};
0
mnashadkaCommented:
I'm sorry, I messed up one thing.  If you use the forward declaration, then the class using the declared object can only have a pointer.  It can't have an automatic object.

class B;

class A
{
public:
  B *b;
};

Then in A.cpp, create b in the constructor using new and delete it in the destructor.
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
YuriPutivskyCommented:
For mnashadka
class B; // Forward declaration

class A
{
public:
  B b;
};

That WON'T be even compiled because compiler don't know the size of class B to generate "B b;" and forward declaration can't help

But using pointers can help
////////////////
// A.h
class B;

class A
{
public:
B* b;
};

////////////
// B.h
#include "A.h"
class B : public A
{
};


For Sandra-24
What's problem you want to solve?

0
Sandra-24Author Commented:
The problem is a little lengthy:

Time object has a aToString() method that will create a string object with the time according to format rules you give it. (like PHP's Date function: http://ca3.php.net/manual/en/function.date.php )

Now LocalTime inherits everyhting from Time, and adds the ability to work with timezones. Instead of copy and pasting the ToString function to LocalTime and make it virtual (which means code duplication) I got around that by providing virtual methods for working with timzones that return error values in Time objects but function as expected on LocalTime objects. This enabled me to put the formatting rules for timezones into the ToString() function of Time. If the object is a Time function it will simply ignore the formatting options for timezones. And everything works as expected for LocalTime objects. But for one small issue. In a LocalTime object where the timezone is PST (-8) for example and you wish to convert it to Swatch time format, which is CET (+1) you need to convert the time to the new timezone. An elegant way of handling this is using LocalTime's functionality. A Copy constructor with two args will copy an existing LocalTime object to a new LocalTime object of a new timezone automatically converting the time if required. So LocalTime temp(*this,CET); will convert any time in any timezone to CET and then I can call the method on this new object to give me the correct Swatch time.

So now I'll just use new and delete with a LocalTime pointer, thanks!

-Sandra
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
C++

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.