Quick question about pointers!

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.

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

Do you mean this is the problem?

Code:

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

---

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

"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.

Do you find anything wrong with this code snippet?

Code:

---

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

---

Yes both

Code:

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

---

will not work.

Is account * data; the problem then?

Why won't they work?

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

Quote:

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Originally Posted by chickenlittle

I'm not really sure.

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You seemed quite confident just now :)

Quote:

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Originally Posted by chickenlittle

I am going by this c++ pointer guide,

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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.

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

Quote:

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Originally Posted by chickenlittle

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

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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;

---

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();
};

---

Quote:

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Originally Posted by chickenlittle

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

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

I have never worked with vector before.

Right. What material are you using to learn C++?

C++ Programming from problem analysis to program design 5th edition by D.S. Malik

Quote:

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Originally Posted by chickenlittle

C++ Programming from problem analysis to program design 5th edition by D.S. Malik

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Eh, that book introduces std::vector in chapter 10, and pointers in chapter 14. Maybe you need to go through the material more carefully, without skipping chapters. To get this right, you need to be familiar with the idea that user defined classes such as std::vector<int> can behave like built-in types like int, e.g., they can be copied. Then you need to be familiar with defining classes, including implementing the copy constructor, copy assignment operator and destructor.