Solved

Equiv of function prototype for classes?

Posted on 2003-10-30
5
218 Views
Last Modified: 2010-04-02
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
0
Comment
Question by:Sandra-24
  • 3
5 Comments
 
LVL 8

Expert Comment

by:mnashadka
ID: 9654354
You can do a forward declaration:

class B; // Forward declaration

class A
{
public:
  B b;
};

class B : public A
{
};
0
 
LVL 8

Expert Comment

by:mnashadka
ID: 9654358
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
 
LVL 8

Accepted Solution

by:
mnashadka earned 500 total points
ID: 9654375
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
 
LVL 4

Expert Comment

by:YuriPutivsky
ID: 9654392
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
 
LVL 3

Author Comment

by:Sandra-24
ID: 9654536
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

Featured Post

Best Practices: Disaster Recovery Testing

Besides backup, any IT division should have a disaster recovery plan. You will find a few tips below relating to the development of such a plan and to what issues one should pay special attention in the course of backup planning.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

When writing generic code, using template meta-programming techniques, it is sometimes useful to know if a type is convertible to another type. A good example of when this might be is if you are writing diagnostic instrumentation for code to generat…
Introduction This article is a continuation of the C/C++ Visual Studio Express debugger series. Part 1 provided a quick start guide in using the debugger. Part 2 focused on additional topics in breakpoints. As your assignments become a little more …
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

809 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question