Quick question about pointers!

**chickenlittle** · 11-14-2011

Code:

#include <iostream>
using namespace std;

struct account
{
	account();
	int accountnum;
	float balance;
	float interestrate;
};

account::account()
{
	accountnum = 50;
}

struct account2
{
	account2();
	account * data;
	void test();
};

account2::account2()
{
}

void account2::test()
{
	data->accountnum = 1000;
}

int main()
{
	account test;
	account2 test2;
	cout << test.accountnum << endl;
	test2.test(); //Error
	cout << test.accountnum << endl;
	return 0;
}

I have the following code. It contains two structs, account and account2.

I want to change the value of accountnum of account using a pointer in account2. However when I run the program it will keep on crashing. Please tell me what is wrong with this code and how I could fix it.

**laserlight** · 11-14-2011

Your account2 object's member pointer does not point to a valid account object.

**chickenlittle** · 11-14-2011

Do you mean this is the problem?

Code:

void account2::test()
{
    data->accountnum = 1000;
}

**laserlight** · 11-14-2011

No. Rather, the failure to prepare for that is the problem.

**chickenlittle** · 11-14-2011

"Your account2 object's member pointer does not point to a valid account object." I keep reading but I still don't quite understand what you are trying to say. I think account * data; correct. Please give me more hints.

**laserlight** · 11-14-2011

Do you find anything wrong with this code snippet?

Code:

account * data;
data->accountnum = 1000;
cout << data->accountnum << endl;

**chickenlittle** · 11-14-2011

Yes both

Code:

data->accountnum = 1000;
cout << data->accountnum << endl;

will not work.

Is account * data; the problem then?

**laserlight** · 11-14-2011

Why won't they work?

**chickenlittle** · 11-14-2011

I'm not really sure. I am going by this c++ pointer guide, Use -> for structure pointer : structure pointer « Structure « C++ Tutorial.

**laserlight** · 11-14-2011

Originally Posted by chickenlittle

I'm not really sure.

You seemed quite confident just now

Originally Posted by chickenlittle

I am going by this c++ pointer guide,

The example code there is wrong.

The problem is that data is a pointer to account, but data does not point to anything. It should point to an account object in order for you to dereference it, whether by writing *data or data->accountnum.

**chickenlittle** · 11-14-2011

So shouldn't data->accountnum = 1000 work? How would you change the value of accountnum from the struct account then?

**laserlight** · 11-14-2011

Originally Posted by chickenlittle

So shouldn't data->accountnum = 1000 work?

It works, if data points to a valid account object. For example:

Code:

account x;
account* data = &x;
data->accountnum = 1000;
cout << data->accountnum << endl;

**chickenlittle** · 11-14-2011

Yes that would work in main however if I put it into the account2 struct I would get an error

Code:

struct account2
{
	account2();
	account x;
	account* data = &x;
	void change();
};

**laserlight** · 11-14-2011

Originally Posted by chickenlittle

Yes that would work in main however if I put it into the account2 struct I would get an error

Indeed, so your account2 constructor needs to do some work. But before you reach for this... how familiar are you with std::vector?

**chickenlittle** · 11-14-2011

I have never worked with vector before.

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