Assignment Operator Access Violation

leeor_net · 11-26-2009, 02:34 PM

I've looked around a bit on Google and at a few different articles but I'm pretty sure I'm not looking for the right information.

In the following scenario, I have a very basic assignment operator:

Code:

class MyClass
{
public:
	MyClass(Bar *bar) : myBar(bar) {};
	MyClass(): myBar(NULL) {};
	MyClass& operator=(const MyClass &rhs)
	Bar *myBar;
};

MyClass& MyClass::operator=(const MyClass &rhs)
{
	myBar= rhs.myBar;  // explodes here, access violation at 0x00000000
}

int main()
{
	Bar *someBar = new Bar();

	MyClass *myClass = new MyClass(someBar);
	MyClass *myClass2 = new MyClass();

	*myClass2 = *myClass;

	return 0;
}

As a note, myBar is an object that is constructed outside of MyClass and is

What I expect is that the address of rhs.myBar be assigned to myBar. However, I get an access violation attempting to write to memory 0x00000000 which tells me that I'm trying to write into a null pointer.

My first thought was that I should be asking for the address of rhs.myBar like this:

Code:

myBar = &rhs.myBar;

But that only results in the error message "cannot convert from Bar *const *' to Bar*'". I'm pretty sure const casting is not applicable here and I'm pretty sure that my first thought is just plain wrong.

Obviously my code is incorrect. What am I doing wrong here? Where can I find more information about the particular type of operation I'm trying to do?

Memloop · 11-26-2009, 02:55 PM

Does this work?

Code:

MyClass& MyClass::operator=(const MyClass &rhs)
{
        if (this == &rhs)
        {
                 return *this;
        }

	myBar= rhs.myBar;
        return *this;
}

anon · 11-26-2009, 02:55 PM

I doubt the error is in that code (you are not dereferencing the pointer anywhere). You shouldn't even need to define the assignment operator yourself.

leeor_net · 11-26-2009, 02:59 PM

Quote:

Originally Posted by anon

I doubt the error is in that code (you are not dereferencing the pointer anywhere). You shouldn't even need to define the assignment operator yourself.

In this particular code snippet, no, I don't need to override the assignment operator. I tried to keep it as simple as possible with only the things that are relevant. The class that I actually have defined is far more complex and as car as I can tell the problem lies entirely with my pointer assignment.

As for the first reply, that is not the issue either as I know for sure that I'm not looking at the same object (different heap addresses) although I have a feeling that'll help to prevent silly errors later on.

Elysia · 11-26-2009, 03:00 PM

Aside from missing ; and missing return statement (and the missing delete statements!), the code compiles and runs fine.
If you encounter a problem, you would have to post the smallest possible amount of code that demonstrates the problem.

leeor_net · 11-26-2009, 03:12 PM

As it turns out I was looking for the wrong information having misdiagnosed the problem. I didn't think my assignment above was wrong and, upon further inspection I found that I had initialized MyClass2 to NULL (e.g., SomeClass() : myClass2(NULL)) and had then attempted the assignment operator.

Thank you, everyone, for your time. I always appreciate your help!

11-26-2009, 02:34 PM	#1
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	Assignment Operator Access Violation I've looked around a bit on Google and at a few different articles but I'm pretty sure I'm not looking for the right information. In the following scenario, I have a very basic assignment operator: Code: class MyClass { public: MyClass(Bar bar) : myBar(bar) {}; MyClass(): myBar(NULL) {}; MyClass& operator=(const MyClass &rhs) Bar myBar; }; MyClass& MyClass::operator=(const MyClass &rhs) { myBar= rhs.myBar; // explodes here, access violation at 0x00000000 } int main() { Bar someBar = new Bar(); MyClass myClass = new MyClass(someBar); MyClass myClass2 = new MyClass(); myClass2 = myClass; return 0; } As a note, myBar is an object that is constructed outside of MyClass and is What I expect is that the address of rhs.myBar be assigned to myBar. However, I get an access violation attempting to write to memory 0x00000000 which tells me that I'm trying to write into a null pointer. My first thought was that I should be asking for the address of rhs.myBar like this: Code: myBar = &rhs.myBar; But that only results in the error message "cannot convert from Bar const ' to Bar'". I'm pretty sure const casting is not applicable here and I'm pretty sure that my first thought is just plain wrong. Obviously my code is incorrect. What am I doing wrong here? Where can I find more information about the particular type of operation I'm trying to do? __________________ - Leeor Last edited by leeor_net; 11-26-2009 at 02:54 PM. Reason: Bit of clarification.
leeor_net is offline

11-26-2009, 02:55 PM	#2
Memloop Robot Join Date: Mar 2009 Posts: 219	Does this work? Code: MyClass& MyClass::operator=(const MyClass &rhs) { if (this == &rhs) { return this; } myBar= rhs.myBar; return this; }
Memloop is offline

11-26-2009, 03:12 PM	#6
leeor_net Professional Chef Join Date: Apr 2004 Location: Charles Town, WV Posts: 144	As it turns out I was looking for the wrong information having misdiagnosed the problem. I didn't think my assignment above was wrong and, upon further inspection I found that I had initialized MyClass2 to NULL (e.g., SomeClass() : myClass2(NULL)) and had then attempted the assignment operator. Thank you, everyone, for your time. I always appreciate your help! __________________ - Leeor
leeor_net is offline

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