Programming Forums > Application Development > C++

Jessehk · Oct 2nd, 2005, 3:34 PM

Sorry, this will be the last question of the day :p

say I had this code:

#include <iostream>

using namespace std;

struct Node
{
     int value;
};

int main()
{
     Node *p;

     p->value = 5;
     
     int number;

     number = p->value;

     cout << number;

     return 0;
}

This gives me a segmentation fault. How would I fix this?

Polyphemus_ · Oct 2nd, 2005, 3:40 PM

(Toggle Plain Text)

Node *p = new Node;

You can also use Node p if you want.

you get the seg fault because you write to non-allocated memory.

Jessehk · Oct 2nd, 2005, 3:43 PM

Quote:

Originally Posted by Polyphemus_

(Toggle Plain Text)

Node *p = new Node;

You can also use Node p if you want.

you get the seg fault because you write to non-allocated memory.

thanks

*slaps head* I guess that is what I get for working all day on my own version of a valarry class using a linked-list...

Jessehk · Oct 2nd, 2005, 4:15 PM

Any idea why this code:

(Toggle Plain Text)

void Numbers::print(Numbers &n1)
{
	cout << n1.firsts->value;
	//temp = n1.firsts->value;
}

gives me a seg. fault?

(where Numbers is a linked list, and firsts is the first Node, and value is an integer value )

grumpy · Oct 2nd, 2005, 4:56 PM

Three possibilities I can think of;

1) n1.firsts is not initialised correctlu, and therefore does not point at a valid object

2) you've changed the type of the value member in your class so it's a pointer, and haven't initialised that correctly;

3) you've managed to pass an invalid reference to the function, viz;

(Toggle Plain Text)

Node *x = NULL;
Node::print(*x);   // passing an invalid reference

InfoGeek · Oct 2nd, 2005, 7:11 PM

Quote:

Originally Posted by Jessehk

Any idea why this code:

(Toggle Plain Text)

void Numbers::print(Numbers &n1) { cout << n1.firsts->value; //temp = n1.firsts->value; }

gives me a seg. fault?

(where Numbers is a linked list, and firsts is the first Node, and value is an integer value )

Wasn't it solved in the other thread?

Jessehk · Oct 2nd, 2005, 8:28 PM

Quote:

Originally Posted by InfoGeek

Wasn't it solved in the other thread?

No.

everything in my number class works accept the copy constructor.

I'm trying to do it like this:

(Toggle Plain Text)

Numbers::Numbers(Numbers &num1) {
	firsts = currents = NULL;
	numberOf = 0;

	news = new Node;
	int temp = num1.firsts->value;
	news->value = temp;

	currents = news;

	firsts = news;
	firsts->nexts = NULL;

	currents = firsts;

	for(int x = 0; x < num1.numberOf; x++, num1.currents =  num1.currents->nexts) {
		news = new Node;
		currents->nexts = news;
		currents->value = num1.currents->value;
		currents = news;
	}

	currents->nexts = NULL;*/
}

and I keep getting seg. faults if I make it a copy constructor.

The really wierd thing is that if i rename it to print(Numbers &), it works fine ( which tells me that my data is fine, but my logic isn't).

I seem to get frusterated easily

grumpy · Oct 2nd, 2005, 9:45 PM

1) You are committing one of the deadly sins of copy constructors - you are modifying the object being copied. Try making the function into Numbers::Numbers(const Numbers &). That will trigger some error messages, which you will need to fix. Odds are, those error messages will be related to a cause of your problem.

2) Whenever you dereference a pointer, you probably need to check that it is non-NULL. For example, computing num1.firsts->value and num1.currents->value.

3) Check the default constructor to make sure it initialises all the members of the class correctly. Otherwise, the copy constructor cannot work correctly (if an object is not initialised correctly, a copy of it cannot be created successfully either).

4) Your print() function, if it works this way, is also incorrect. Your problem is that the function changes the object passed to it (whether that is a copy constructor or a print() function) --- so, when you repeat the operations, you get yourself into trouble. On that basis, I suspect you will probably find calling your print() method twice also gives a segmentation fault.

5) Also, keep in mind that a copy constructor is invoked in a different context from other members: when an object is being created. Even if the code works correctly in your print() method, there is no reason to assume it will work correctly when pasted into a copy constructor.

Jessehk · Oct 3rd, 2005, 7:55 PM

Thanks for all the advice grumpy.

I have made the function take a const argument and I am still having problems.

I think I am going to "stash this one in a drawer" and look again when I have become a bit more knowledgable (hopefully) from books and expirience.

Oct 2nd, 2005, 3:34 PM	#1
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Accessing a pointer-to-structure value Sorry, this will be the last question of the day :p say I had this code: (Toggle Plain Text) #include <iostream> using namespace std; struct Node { int value; }; int main() { Node p; p->value = 5; int number; number = p->value; cout << number; return 0; } #include <iostream> using namespace std; struct Node { int value; }; int main() { Node p; p->value = 5; int number; number = p->value; cout << number; return 0; } This gives me a segmentation fault. How would I fix this? __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Oct 2nd, 2005, 3:40 PM	#2
Polyphemus_ Expert Programmer Join Date: Aug 2005 Location: Rotterdam, the Netherlands Posts: 942 Rep Power: 4	(Toggle Plain Text) Node p = new Node; Node p = new Node; You can also use Node p if you want. you get the seg fault because you write to non-allocated memory.

Oct 2nd, 2005, 4:15 PM	#4
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Any idea why this code: (Toggle Plain Text) void Numbers::print(Numbers &n1) { cout << n1.firsts->value; //temp = n1.firsts->value; } void Numbers::print(Numbers &n1) { cout << n1.firsts->value; //temp = n1.firsts->value; } gives me a seg. fault? (where Numbers is a linked list, and firsts is the first Node, and value is an integer value ) __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Oct 2nd, 2005, 4:56 PM	#5
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Three possibilities I can think of; 1) n1.firsts is not initialised correctlu, and therefore does not point at a valid object 2) you've changed the type of the value member in your class so it's a pointer, and haven't initialised that correctly; 3) you've managed to pass an invalid reference to the function, viz; (Toggle Plain Text) Node x = NULL; Node::print(x); // passing an invalid reference Node x = NULL; Node::print(x); // passing an invalid reference

Oct 3rd, 2005, 7:55 PM	#9
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Thanks for all the advice grumpy. I have made the function take a const argument and I am still having problems. I think I am going to "stash this one in a drawer" and look again when I have become a bit more knowledgable (hopefully) from books and expirience. __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Oct 2nd, 2005, 9:45 PM	#8
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	1) You are committing one of the deadly sins of copy constructors - you are modifying the object being copied. Try making the function into Numbers::Numbers(const Numbers &). That will trigger some error messages, which you will need to fix. Odds are, those error messages will be related to a cause of your problem. 2) Whenever you dereference a pointer, you probably need to check that it is non-NULL. For example, computing num1.firsts->value and num1.currents->value. 3) Check the default constructor to make sure it initialises all the members of the class correctly. Otherwise, the copy constructor cannot work correctly (if an object is not initialised correctly, a copy of it cannot be created successfully either). 4) Your print() function, if it works this way, is also incorrect. Your problem is that the function changes the object passed to it (whether that is a copy constructor or a print() function) --- so, when you repeat the operations, you get yourself into trouble. On that basis, I suspect you will probably find calling your print() method twice also gives a segmentation fault. 5) Also, keep in mind that a copy constructor is invoked in a different context from other members: when an object is being created. Even if the code works correctly in your print() method, there is no reason to assume it will work correctly when pasted into a copy constructor.

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Programming Forums > Application Development > C++

Accessing a pointer-to-structure value