New to C++, pointer problem

[dougfm]

Hi everyone,

I'm heading into my second CS course this semester. It's going to cover OOP using C++. Now I know OOP rather well through teaching myself Obj-C and Cocoa, but I'm trying to get myself familiarized with C++ before the start of the semester. I decided to build an app using Objective-C++, incorporating a Cocoa/Obj-C view and C++ model. I started to get some odd output in the console window. I then created a stripped down C++ console version of the app and had the same problems. If only happens when I have an object return the pointer of another object.

int main (int argc, char * const argv[]) {
    
	ObjectOne object1;
	ObjectOne *obj1Ptr = &object1;
	
	// Console Output
        // 0
        // -1605028608
        cout << obj1Ptr->obj2Ptr->theCount << endl;
	cout << obj1Ptr->obj2Ptr->theCount << endl;
	
        // Console Output
        // 0
        // 0
	cout << obj1Ptr->theCounter << endl;
	cout << obj1Ptr->theCounter << endl;
	
    return 0;
}

ObjectOne.h
#include <stdio.h>
#include "ObjectTwo.h"
using namespace std;

class ObjectOne {
public:
	
	ObjectTwo *obj2Ptr;
	
	int theCounter;
	
	// FUNCTIONS
	ObjectTwo* getObj2Ptr();
	int getTheCounter();
	
	ObjectOne();
};

ObjectOne.cpp
#include "ObjectOne.h"

ObjectTwo* ObjectOne::getObj2Ptr()
{
	return obj2Ptr;
}

int ObjectOne::getTheCounter()
{
	return theCounter;
}

ObjectOne::ObjectOne()
{
	ObjectTwo object2;
	obj2Ptr = &object2;
	
	theCounter = 0;
}

ObjectTwo.h
#include <stdio.h>
using namespace std;

class ObjectTwo {
public:
	
	int theCount;
	
	int getTheCount();
	
	ObjectTwo();
};

ObjectTwo.cpp
#include "ObjectTwo.h"

int ObjectTwo::getTheCount()
{
	return theCount;
}

ObjectTwo::ObjectTwo()
{
	theCount = 0;
}

Thanks

Doug

[Lindley]

Using code tags would make that easier to read. However, here's your problem:

ObjectOne::ObjectOne()
{
    ObjectTwo object2;
    obj2Ptr = &object2;

    theCounter = 0;
}

When the function is called, object2 is created on the stack. You then keep a pointer to it. However, when the function returns, the stack unwinds and object2 is destroyed. At that point the pointer is directed at garbage.

[laserlight]

Please post your well indented code in [code][/code] bbcode tags.

At a glance, one possible problem is that you are assigning the address of a local variable to a member variable in the ObjectOne constructor. This local variable is destroyed at the end of the function, upon which the member pointer points to a destroyed object. Do you even need a pointer in the first place?

[Lindley]

There are several possible solutions to this issue. I'll attempt to lay out the pros and cons of a few.

1) Make object2 a class member of ObjectOne to begin with, rather than making the pointer a member. This is fine for the most part, and probably the easiest solution to the issue. The downside is that it requires ObjectTwo.h to be #included in ObjectOne.h----you can't use a forward-declare. However, here you're doing that anyway.

2) Allocate object2 on the heap using new and assign it to the pointer. The downside here is that you are required to write a destructor now to free that memory, and also a copy constructor and assignment operator (or disable those operations by making them private). Classes written this way tend to get unwieldy very fast, I don't recommend it.

3) Do the same as (2), but use a std::tr1::shared_ptr<ObjectTwo> rather than an ObjectTwo*. Then all copying and destruction will be handled automatically. This option should be preferred to (1) only if there's some reason you need the class member to be a pointer----say, for polymorphism or to enable a forward-declaration to break an include cycle.

The lesson here: Don't overuse pointers. And when you do use them, consider using a smart pointer rather than a "bare" pointer. It will make your life easier.

[dougfm]

Thanks everyone, that helped a lot.

Doug