Classes and Pointer Issues

[kief12]

I am having trouble accessing and setting data from inside classes.
if have a program set up similar to this example

---Desk.h----
#include "Pen.h"
class Desk
{
private:
  Pen myPen;
public:
  Desk();
  void setPen(Pen*);
  Pen* getPen();
}
---Desk.cpp-----
Desk::Desk(){}
Pen* Desk::getPen()
{
  return &myPen;
}
void setPen(Pen* p)
{
  myPen = p
}

Then the pen class looks like

---Pen.h---
#include <string>
class Pen
{
 private:
  string penName;
 public:
  Pen();
  string getPenName();
  void setPenName(string);
}
--Pen.cpp---
include "Pen.cpp"
Pen::Pen(){
 penName = "Bic";
}
string Pen::getPenName()
{
 return penName()
}
void Pen::setPenName(string s)
{
 penName = s
}

from the main function i have

 Desk myDesk;
 cout << myDesk.getPen()->getPenName();

This code works but if i send *myDesk to another class and try to use it like

--Main.cpp---
...
myOtherClass.setDesk(&myDesk);
...
---OtherClass.h---
...
Desk* myDesk;
...
---OtherClass.cpp---
...
void randomFunction(){
 string s = myDesk->getPen()->getPenName()
...

string s = myDesk->getPen()->getPenName() line crashes the program

how can i do this correctly?

Thank you,

[emaghero]

I am having trouble accessing and setting data from inside classes.
if have a program set up similar to this example

---Desk.h----
#include "Pen.h"
class Desk
{
private:
  Pen myPen;
public:
  Desk();
  void setPen(Pen*);
  Pen* getPen();
}
---Desk.cpp-----
Desk::Desk(){}
Pen* Desk::getPen()
{
  return &myPen;
}
void setPen(Pen* p)
{
  myPen = p
}

Then the pen class looks like

---Pen.h---
#include <string>
class Pen
{
 private:
  string penName;
 public:
  Pen();
  string getPenName();
  void setPenName(string);
}
--Pen.cpp---
include "Pen.cpp"
Pen::Pen(){
 penName = "Bic";
}
string Pen::getPenName()
{
 return penName()
}
void Pen::setPenName(string s)
{
 penName = s
}

from the main function i have

 Desk myDesk;
 cout << myDesk.getPen()->getPenName();

This code works but if i send *myDesk to another class and try to use it like

--Main.cpp---
...
myOtherClass.setDesk(&myDesk);
...
---OtherClass.h---
...
Desk* myDesk;
...
---OtherClass.cpp---
...
void randomFunction(){
 string s = myDesk->getPen()->getPenName()
...

string s = myDesk->getPen()->getPenName() line crashes the program

how can i do this correctly?

Thank you,

Declare the desk class as a friend of the myOtherClass class

That way you can access members of the desk class

[kief12]

Declare the desk class as a friend of the myOtherClass class

That way you can access members of the desk class

Sorry i don't understand how that would help.

i could do

myOtherClass.getPen();

but that would not be referring to the same pen would it?

I want to keep the same instance of Desk and have myOtherClass get the original desks values

[Darryl]

string Pen::getPenName()

{

 return penName();\\ <-ERROR
 
// return penName; < -correct

}

remove the parenthesis after penName()

On another note, your code to returnPen() returns a pointer to the actual Pen which make the fact that it's private pointless, because now anyone can get the Pen and change it. Instead you should return a const Pen&

[kief12]

Sorry i set up code that had the same problem as mine but this is not the actual code i didn't mean to put in the () and they are not there in my real code. i am changing the code to use But that still leaves me in the same place.

[kief12]

I think the problem maybe how i am passing the Desk to myOtherClass
from main.cpp
i do

 myOtherClass.setDesk(&myDesk);

and myOtherClass has

void myOtherClass::setDesk(Desk* d)
{
  myDesk = d;
}

i changed getPen to return a Pen not a pointer to one but this line gives me and exception not caught line in myOtherClass

if (myDesk != NULL)
{
 Pen myPen = Desk->getPen();
}

Thank You,

[weaknessforcats]

Try to avoid this style of coding:

string s = myDesk->getPen()->getPenName()

This is the famous spaghetti code. If the methods of those indirect classes chnage, the code in main() breaks. That means the user code breaks. If you have a lot of users, you won't be able to redesign those indirect classes because the users will block you. Your application dies at this point.

Also, if myDesk is 0 you crash.
Also, if MyDesk is not 0 but getPen() returns 0, you crash

You need to check pointers for null before using them.

I suggest you makee your Desk class protective:

[code=cpp]
string Desk::getPenNam e()
{
return this->myPen.getPenNa me();
}
[/code]

Then in main() all you do is:

[code=cpp]
string s = myDesk->getPenName() ;
[/code]

which is much cleaner and the user only needs to know about the Desk class.

Interface design is a big deal and takes practice.

[kief12]

Doing that seems like there will be a lot of repetition. Like if i also add a Coffee Mug class i would need to place it methods into Desk to. Is there a better way to go about that?

[weaknessforcats]

It's not about repetition. It's about interface design. And that means encapsulation.

If the user is going to be using Desk objects, then only Desk member functions should be called. Otherwise, your user has to learn every class the Desk uses in addition to learning the Desk. Worse, if you need to re-design any of those classes, your user won't like it. For example:

Version 1.0
[code=cpp]
string Desk::getPenNam e()
{
return this->myPen.getPenNa me();
}
[/code]

Version 2.0
[code=cpp]
string Desk::getPenNam e()
{
return this->myPen->getPenName() ;
}
[/code]

Between Version 1.0 and Version 2.0 it was decided to have a Pen* in Desk rather than an Pen. That was to allow the Pen to change values independently from the Desk. That is, the relationship between Desk and Pen was changed from HAS-A to ASSOCIATES-A.

The only change required was to Desk::getPenNam e(). No user code was impacted. Otherwise, the user woul have to change all the code using the Pen.

[kief12]

I am still having the same problem i can run myDesk.getPenNa me() but from the other file myDesk->getPenName() doesn't work