How do you design a function to set two values preferebly passed as params?

Question

5.00/5 (1 vote)

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Update 2, refining question

How do you design a function to set two values which could start and end as null? I'm in another function where I'd like to have a couple of local pointers that would start as null, which a function would take as parameters, and set them to point to objects under certain conditions and leave them null in other conditions.

I've been trying to use pointers to pointers but I have not been able to make it work correctly. Is there a better approach? If not, what am I doing wrong?

Original Question with edits

I would like a function that takes two pointer parameters which begin as null and if the function finds the two objects it's looking for the, the two pointers would be set to the addresses of the found objects for use by the caller. I'm guessing the way to do this would be to use pointer to pointer params but I haven't gotten it to work.

void ConfigureStuff()
{
    CMyObject ** left = 0; // update, what should I be doing here?
    CMyObject ** right = 0;

    // update, recently tried this and it crashes here as well
    // so I must be doing something wrong above 
    *left = 0; // crashes
    *right = 0;

    FindLeftAndRight( left, right );


}

void FindLeftAndRight( CMyObject ** left, CMyObject ** right )
{
    CMyObject * searchObjectLeft = 0;
    CMyObject * searchObjectRight = 0;

    //...
    // code that finds left and right
    //...

    if ( bFoundObjects )
    {
        *left = searchObjectLeft;  // update, crashed here
        *right = searchObjectRight;
    }

}

What am I doing wrong? What's the simplest way to get what I want?

Thanks.

Posted 1-May-11 15:58pm

Brian Bennett

Updated 1-May-11 21:12pm

v9

Add a Solution

3 solutions

Solution 1

What am I doing wrong?

Pretty much everything. It would not even compile.

What's the simplest way to get what I want?

You don't want the "simplest way". You want the right way. It depends on what you want. From this code, it is not clear.

You also did not report your problem. How come you supply the code which does not compile? You do not consider it a problem; "I haven't gotten it to work" cannot be uses as an issue report.

Yes, your double pointer is the way to modify the inner pointer and return it to the caller, nothing wrong with it. By some reason you return null pointer (zero) for both parameters, but I guess you should calculate them under "//...".

Nevertheless, I looks like you're not ready for C coding just yet. 1) Functions lack return type (even void); 2) function FindLeftAndRight is used before declaration, so it cannot compile, ";" after declaration should be removed, because after it the code block goes; 3) function ConfigureStuff needs "()" after the name; 4) you need to define CMyObject, if you think it's a class (as the MFC-style name suggests), remember there are no classes in C; 4) variable bFoundObjects if not declared/initialized; 5) it looks like you lack important input and return parameter needed for calculations… do I need to continue?..

—SA

Posted 1-May-11 16:18pm

Sergey Alexandrovich Kryukov

Updated 1-May-11 16:22pm

v4

Comments

Brian Bennett 1-May-11 22:35pm

My explanation makes it clear what I want. The code above is basically pseudo-code. My code crashes where the first assignment is made, "Access violation writing location 0x00000000" after it finds valid objects.

Sergey Alexandrovich Kryukov 1-May-11 23:10pm

You think I does, I think it does not. Want to wait for the one who would understand it?
You're asking not about algorithm but about coding but present pseudo-code. It is not very productive.
If you present pseudo-code you need to warn about it.
It does not looks like you really trying to get help. Just think about it. Your exception does not show much. Not only you don't show in what line of code the exception is thrown but you don't show the code itself.
--SA

Brian Bennett 1-May-11 23:22pm

You seem irritated

Sergey Alexandrovich Kryukov 2-May-11 0:36am

It does not matter. But if you're so nice to take care about that, please formulate questions and reply correctly. Why did I spent time spotting your bugs? Just to hear from you that it was just a joke, pardon... pseudo-code?

Anyway, you got an answer:

"*left = 0;" crashes because "CMyObject ** left = 0;" You pointer to pointer points nowhere, so you don't have a single pointer to modify.

Now look, you have complained to the bug in the code you did not show. Do you realize that? You need to collaborate to get help; this is called cryptic.

--SA

Brian Bennett 2-May-11 1:04am

There is a lot of negativity in the way you teach and so the collaboration is not fun. Try being nicer. I will try to better my questions.

Still you haven't given a solution, only showed why it's going wrong. I want to start with null local variables and have the called function fill them if the results are found.

Sergey Alexandrovich Kryukov 2-May-11 1:14am

Please, stop it. Thanks for trying to do better.
Pointing out the problem should be enough. I have no idea what are you trying to achieve, so there cannot be a solution.
--SA

Brian Bennett 2-May-11 1:32am

What I'm really trying to do is create two local pointers and call a function that fills those locals with pointers to the data if the function finds valid data. If it does't find valid data the pointers would stay null. I'm used to passing references or returning a pointer for a single value but since there are two values and the values could start or end as null I've been trying the pointer to a pointer approach. Maybe the solution I'm looking for is in how to declare the local pointer to pointers, but I'm not sure if my approach is even correct in the first place. Do I have to allocate memory for the pointer to pointer, and if so would that be the only way possible?

Sergey Alexandrovich Kryukov 2-May-11 4:29am

To start with, you cannot return local pointers in the sense the pointers pointing to a local variable. This variable belongs to a current stack frame and is removed after a call is returned. If you return the pointer on such object, after a call the pointer point to the same point in memory which is not longer valid, as the stack frame was removed. Yes, allocation of memory is needed. Allocation of memory inside function is no good, because it should be symmetric: allocating and deleting context should be the same (otherwise it's easy to mess up). So, some caller should allocate memory, pass to a called function and later deallocate. By the way, calling function can pass a pointer to local memory inside, because outer stack frame exists when the inner stack frame is processed (but you cannot return such pointers outside -- this should be clear from the nature of stack -- "first comes, last goes").
--SA

Brian Bennett 2-May-11 2:05am

Should I be using void ** ?

Sergey Alexandrovich Kryukov 2-May-11 3:36am

No! You should have the outer pointer to point to some object -- of the type CMyObject*.
You see, you're trying to get anything with try-and-error. It cannot work. You need to understand how pointers work -- it's kind of easy. And I cannot help you based on this question. You really need to look at some elementary textbook samples, try them, run them step by step not going to the next sample until you understand 100%. After that the problems with your code sample will go.
--SA

Brian Bennett 2-May-11 4:51am

You must not realize that my question was already answered below. I comfortably admit that my communication needs work. I already know it's a weakness of mine. You can see by my stats that I spend a lot of time editing. Maybe you're having a bad day, but you've made a lot of assumptions that are wrong and the way you word things is demeaning and patronizing. You made me feel bad. I've been writing code for over ten years and you say I'm not ready for C coding just yet. You say I must not want an answer. That is obviously wrong or I wouldn't be here. I'm pretty much self-taught and I'm aware I have holes in my knowledge, but I love what I'm doing and am generally positive about it. I am confident using basic pointers and references and have used pointer to pointers and pointers to functions many times, but this little one got me. I'm really happy about what Niklas taught me, he helped fill a hole. I do appreciate the time you took. I hope that normally your approach is more gentle and that you're just having an off day. You probably know a lot more than I. I find that the more I know about something the more difficult it is to teach beginners. Everything seems so obvious to me that it's hard to understand why my students wouldn't get it.

I couldn't find my answer in an elementary textbook and I couldn't find a example online that was like what I was doing. But I did find the answer here.

Really, I'm not saying all this to make you mad or anything. I wouldn't give you the time of day if I didn't appreciate your help. So I hope you appreciate the time I took to write this as a little nudge to consider your approach on teaching if it's normally like this. I like how the other people answered, they didn't make me feel bad. People (like me) can know a lot about something and still have things to learn about it, maybe even some very basic things. An engineer who was helping me had been doing C++ for 20 years and he told me he was still learning new things about the language. I assume the same will be true for me.

I would suggest allowing people to be human and make mistakes but try and "read between the lines" if possible and that if the question or irritates you, don't even answer it. Don't assume too much or comment about the questioners' skill or knowledge level.

Sergey Alexandrovich Kryukov 2-May-11 5:44am

I'm not your psychoanalyst and cannot take responsibility of your self-esteem and other similar matter which has no value to me. Your views are maybe interesting but my interest is completely lost. If you have problems of that kind, please deal with them yourself.

I should stop this abuse you're trying to practice here. If you don't need help, don't waste time.
You constantly shift the subject from technical aspects to the personal issues. If you cannot stay focused in the matters we're trying to discuss, we should not discuss anything. You got some help; it should not be enough.

I'm just not interested in your evaluation of my qualifies. Can you imaging that? Not at all. And I don't want to care about your bad feelings or something. This is not a kindergarten. I don't care who pleased you before. Your attempts to discuss personal evaluations are funny to certain extent but extensive repetition makes it boring.

So, please, no more. Period.

Now, about reading a book. You did not understand me. You won't find the answer to your question in the book. Books are written not to answer to all question, but to help understanding things. The pointer techniques are not so easy to grasp at once. Reading, doing the simple samples step by step should give you the understanding. After you have this understanding (and don't doubt -- almost everyone overcomes this pretty soon), you problem will not be solved, it will disappear, so to speak... This is my point, and this is what you really need. Should you use this advice or not -- you chose. I helped so many people to get started (not just to get started, but that, too) so I think I know what I'm talking about...

--SA

Solution 2

Have you tried to Debug it? Where exactly it crashes?
What is the value of variable 'left' and 'right' when it enters the funtion?

How have you called this method?

Posted 1-May-11 17:33pm

Ajay Vijayvargiya

Comments

Brian Bennett 1-May-11 23:50pm

I've tried to debug this (with a break-point) at that first assignment (using .net) but when I step into it, I get the crash immediately. I call the function as shown inside ConfigureStuff(). The values of left and right are null (as is how I would like it to be) when the function is called.

Ajay Vijayvargiya 2-May-11 0:14am

HOW are you calling the function? What exact parameters?

Brian Bennett 2-May-11 0:40am

I changed the example above but I was calling it exactly like

CMyObj ** left = 0; // local
CMyObj ** right = 0;
FindLeftAndRight( left, right );

With the changes I made to the example, look where it crashes now.

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Niklas L · Accepted Answer · 2011-05-01T21:11:00

Solution 3

C++

void ConfigureStuff()
{
    CMyObject * left = 0;
    CMyObject * right = 0;
    FindLeftAndRight( &left, &right );

}

Posted 1-May-11 21:11pm

Niklas L

Comments

Brian Bennett 2-May-11 3:20am

You rock! Thanks. Thanks to others for their time, it is much appreciated.

Niklas L 2-May-11 4:06am

You're welcome

Ajay Vijayvargiya 2-May-11 3:36am

Looks like basic pointer level misunderstanding! :)
Edit: Not to answer, but to Brain!

Sergey Alexandrovich Kryukov 2-May-11 5:47am

Certainly. Did you see "discussion" over my answer?

Niklas gets 5 for the answer!
--SA