creating a class from results

A few things you'll have to watch out for, are inheritance, constructors, destructors, assignment operators, etc.

Subrat2009, part of the idea of an assignment like this is to figure these things out for yourself. The challenge is not to implement it, but to figure out what needs to be implemented - you've just taken away that challenge.

kuntilanak, please try to figure this out for yourself - it's so much more rewarding, and you'll learn so much more from it.

so can I do this only by only having the print method as the only method that uses '\n'

What do you mean by that ?

well the constraints in making the class A and B is that I must only use one '\n' in the implementation... I am guessing that's inside the print function, correct?

Ok, if that's a constraint, then putting it in the print function would be a good approach.

I haven't use overloading assignment constructor, can you show me an example on how to do this?

Sure :

Foo& Foo::operator=(const Foo& foo) {
    if (this != &foo) {
        // copy the data, like :
        this->dataMember = foo.dataMember;
    }
    return *this;
}

                                              

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kuntilanak: try what happens when you use the default copy constructor and assignement operators.

are you saying we need an overloaded assignment because in the code we are assigning a to b? that should be handled by inheritance right

You seem to be mixing up pointers with object instances ... And even then, inheritance doesn't just allow you to cast one pointer to another as you desire. There are certain rules and limitations.

inheritance topic:

if B inherits A and then A has a function print, then does it mean that B must implement print?

>> if B inherits A

I guess you mean "if B inherits FROM A".


>> and then A has a function print, then does it mean that B must implement print?

No. B also inherits the member functions from A. You can always re-implement it to give it different behavior though.

On the topic of inheritance, please have a look at this basic tutorial :

        http://www.cplusplus.com/doc/tutorial/inheritance/

in the polygon example the Polygon it self doesn't have an area method.. but it's inherited class has it.. is it right?

Yes...

Cases in Inheritance :

1. Override existing things.

2. Add New things to derived class.

3. Use Existing features.

4.  Combination of _all_.

>> in the polygon example the Polygon it self doesn't have an area method.. but it's inherited class has it.. is it right?

Correct.
And both the Polygon class and its derived classes have the set_values method as implemented in the Polygon class (since none of the derived classes re-implemented it).

If I can break down the printing, the:

testing
hope this works
testing
hope this works

is printed at the constructor... but how can I print this out if I can only use the method print to print stuffs out.. is it saying that I should call the print method inside the constructor it self? as the print method is the only one that has a '\n'

remember: I can only use one '\n' in my code

>> is it saying that I should call the print method inside the constructor it self?

That's possible, isn't it ? :)

the reason why I am confused is because the print method doesn't take any arguments.. so how does it know to print :

testing
hope this works

>> the reason why I am confused is because the print method doesn't take any arguments..

Maybe you should let it take some arguments ?
Note that you can use default arguments to make sure that it's also possible to call the function without arguments.

Or, maybe you should have another function that does the actual write operation (with the newline character), and that is called by all others.

Or, maybe you can find another solution ;)

It's a free world, and there are plenty of options hehe

>>Note that you can use default arguments to make sure that it's also possible to call the function without >>arguments.

an example would help here

An example of default arguments is below. But maybe the second option I mentioned is more appropriate. It's certainly nicer :)

void fun(int arg = 5) {
  std::cout << arg << std::endl;
}
 
fun(10);   // will print 10
fun();     // will print 5

                                              

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oh so you mean have another function that basically just do a print new line :)

this last line:

routing route
changing route
routing route
changing route
routing route

comes from the printit methods right?

I imagine this one:

printing 2
printing 0
printing 0
printing 2
printing 0

is from the two print, but how is that possible...

>> oh so you mean have another function that basically just do a print new line :)

That, or a full fledged writeLine function, that writes whatever you pass to it.


>> is from the two print, but how is that possible...

The printit functions call print internally, don't they ?

>>The printit functions call print internally, don't they ?

yes they do, I just want to know if this line of code comes from two print statements, why is it printing 5 lines of stuff

printing 2
printing 0
printing 0
printing 2
printing 0

What do you think ?

>> The printit functions call print internally, don't they ?

the printit itself prints 2 lines of code each time it is executed, and then so the print should do the same... however it only prints an odd number of lines instead of even (5 instead of 6) this is what confuses me

>> the printit itself prints 2 lines of code each time it is executed,

two lines ? How do you figure that ?


How many times is the print function called ?

it's called 5 times...

so this might be a call at the destructor:

routing route
changing route
routing route
changing route
routing route

>> so this might be a call at the destructor:

Getting warmer ;)

hmmm...I have no clue where else it's called if it's not the destructor...

Don't get me wrong ... It IS the destructors. The "getting warmer" part simply implies that you have to go a bit further to figure it out completely ;)

I still don't get why B should inherit from A, my only guess is because of the assignment a = b1 and I still haven't figure out..

After thinking of it as well, I think the only place where the '\n' should be placed is at the print method... I still don't get it on how using the print to print something else... all I know is that the print should just do:

"printing x \n"

so how can I use a method that already have that inside the implementation to print something as:

testing
hope this works

You generally do not design classes by looking at the code.

You need to get your requirements straight and then design your classes. Printing a new line can be done in any class depending on your requirements. B should inherit from A only if it can be considered as a specialized substitute for A and can be used in all places where an A can be used.

>>You generally do not design classes by looking at the code.

I generally don't do that, this is the only time where I do it as it's the assignment and I dont' have more choice..

and maybe you're not reading the topic from the start, but I can only use one '/n' in all of my implementation.. don't ask me why

>> so how can I use a method that already have that inside the implementation to print something as:

We already went over that earlier, didn't we ? (http:#25664933)


>> I still don't get why B should inherit from A, my only guess is because of the assignment a = b1 and I still haven't figure out..

I'd say, just try it out the way you think it should be, and see if you can get the same output.

One question and maybe the last before I close this topic down:

The last few parts of the line is:

routing route
changing route
routing route
changing route
routing route

we know this is from the destructor, how am I suppose to know on which destructor is called first? Is the:

changing route

from B

and routing route from

A

Bottom point is that if B inherits from A, then is B's destructor called then A's destructor is called....

Destructors for local objects are called in reverse order of creation of those objects. ie. if foo was created before bar, then bar will be destroyed before foo :

        {
            Foo foo;
            Bar bar;

            // ...
        }

Furthermore, a derived class's destructor automatically calls the base class's destructor.

>>a derived class's destructor automatically calls the base class's destructor.

so if B inhertis from A then:

1. A's destructor is called
2. B's destructor is called then A's is called

which one is true?

oh and with the printing methods, I can't create another method just to print the new line, all I can have in class A and B are only the Big4 and a print method, nothing else.... so how can I make the print method to print several strings with a new line... the print method does't take any arguments as you can see from above.... any idea or hints?

>> so if B inhertis from A then:

B's destructor calls A's destructor, so first B's destructor, then A's destructor.

Remember : always reverse order.

so it calls from what it inherits from first the it's own constructor... I see... Now I just need to figure out how to use the print(), or otherwise I can't do anything as I need to print something to try it out... and I can only use this print()

>> so it calls from what it inherits from first the it's own constructor...

We were talking about the destructors, weren't we ?


>> Now I just need to figure out how to use the print(), or otherwise I can't do anything as I need to print something to try it out... and I can only use this print()

Well, several options were mentioned. I'm sure one of them works ;)

one of your solution is for the print to take an arguments, which doesn't work as the method signature is print() and the other one was to create another function, which we can't...

>> one of your solution is for the print to take an arguments, which doesn't work as the method signature is print()

Did you remember the comment about default arguments ?

>> are only the Big4 and a print method,

Btw, what are the Big4 ?

Okay, nevermind with the printing, I got it all sorted out...seems that I am misunderstanding the question that I can use only one '\n' in each method

Heh ... it sounded like a weird requirement :)

in the printit method the argument that it takes is an A& however we can see that B can also be passed into the  the arguments. Which print is called if we pass a B object to the print it? Is it B's print or A's print?

Depends if it's a virtual method or not.

But the more important question you should be asking is : which one do you want it to call ?

Btw, I hope you're also trying this out with your compiler to get a better idea of how things work, and how close/far you are from the required result ? Solving it in your head is fine if you can, but at some point, doing it in practice (ie. trying stuff out) is a lot more effective.

My understanding of virtual class, if print is declared as non-virtual then A's print will always be called... is this true?

I will try what I have in my head later on home when I have access to gcc

>> if print is declared as non-virtual then A's print will always be called... is this true?

If you use a reference-to-A or pointer-to-A, yes.

>> I will try what I have in my head later on home when I have access to gcc

It's so much easier to do this kind of assignment when you actually have access to a compiler. What takes you hours now could be finished in minutes.

I got access to my compiler at least now, and got the following error:

 error: new types may not be defined in a return type
 note: (perhaps a semicolon is missing after the definition of âAâ)
 error: two or more data types in declaration of âAâ
error: return type specification for constructor invalid


and the error is on the code below

A::A(){
 value = 0;
 std::cout << "Just got my paycheck!" << std::endl;
}

                                              

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okay ignore my previous post,  I got everything just as I want it to be printed, however this is where I got wrong:

printing 0
printing 0
printing 0
printing 0
printing 0

any idea why?

Here's my full code according to what it should produce and what I get along with my implementation

I wonder why :

a = b1

needs the

A& A::operator=(const A& a)

reason why I know this is because when I did not declare the constructor above and tried to compile the program with only the constructor::

A& A::operator=(const B& b)

it fails....

so weird...

After several investigation I found the following:
1. The last 4 prints will print a's print method, so none of the print method needs to be declared as virtual
2. My result does no. 1 correctly, however it got the value wrong... when we do b1 = b2, (b2 has a value of
    2 so b1's value should be assigned to 2 as well)... However when we do printit(b1) why is the value
    of b1 0 instead of 2?

I guess object slicing is not the problem here right, as the printit function uses a reference as the argument

Ok, I'm back. Could you post your complete code you have now ?

here it is, haven't improve much from the top... the results that I am supposed to be getting is uploaded on my post above

That's not all of the code ... The class definitions are missing ;)