Dynamically Allocating a mulitple Struct within a Struct

Question

I am new to C++ and need some help. I have the following code:

struct Force {
    float X[10];
    float Y[10];
    float Z[10];
};

struct Measurement{
    char serial_number[30];
    struct Force F1;
    struct Force F2;
 };

How should I properly allocate the following?

struct Measurement meas

The problem is that struct Force force works fine; however, when I try to define struct Measurement meas then I get "Unhandled exception" error!

First of all it would help us to answer your question better if you show us _how_ you use the `meas` variable when you get the "crash". Secondly, the first thing you always should to when getting a "crash", is to run your program in a debugger. It will help you locate the place of the crash, and also let you examine variables to help you see why it might have crashed. — Some programmer dude, Aug 10 '12 at 06:41
Why do you insist on changing the tags to C++ if you say you're learning C in the question? — R. Martinho Fernandes, Aug 10 '12 at 06:43
This piece of the code looks ok. Where is the "Dynamically Allocating" part? — Bo Persson, Aug 10 '12 at 06:43

score 2 · Answer 1 · answered Aug 10 '12 at 06:24

As I saw in your question, you are using C, so here is solution for C.

Wherever you want to have instance of structure Measurement, simply type:

struct Measurement meas;

and you will be able to access your structure elements as:

meas.F1.X and so on...

And if you wish to have dynamic allocation(i.e. at run time) then simply use malloc/calloc as follows

struct Measurement *meas = (struct Measurement *)malloc(sizeof(struct Measurement));

Doing so, you will have to access your structure elements as:

meas->F1.X and so on...

score 1 · Accepted Answer · answered Aug 10 '12 at 06:42

Technically it works like you wrote it, but struct word is unnecessary on members (actually generates warning but works.)

struct Force {
    float X[10];
    float Y[10];
    float Z[10];
};

struct Measurement {
    char serial_number[30];
    Force F1;
    Force F2;
};

Then in function use like this:

Measurement somevar;
somevar.F1.Y = 999;

Now the proper way to do this (and save stack) is to use pointers.

struct Measurement {
    char serial_number[30];
    Force* F1;
    Force* F2;
};

And then:

Measurement* m = new Measurement;
if (m) {
    m->F1 = new Force;
    m->F2 = new Force;
}

After using you have to delete all pointers to avoid memory leaks:

delete m->F1;
delete m->F2;
delete m;

There is another approach. Using:

struct Force {
    float X[10];
    float Y[10];
    float Z[10];
};

struct Measurement {
    char serial_number[30];
    Force F1;
    Force F2;
};

You can allocate with malloc some amount of memory and treat it as struct (did not have time to test it, but I use that approach many times).

Measurement* m = (Measurement*)malloc(sizeof( size in bytes of both structs ));
// zero memory on m pointer

// after use
free(m);

That's all.

Thanks. This works for me (the first approach). I will now go into details now. Thank you all guy. You have helped me a lot! — jankos, Aug 10 '12 at 06:51
You should really learn the concept of smart pointers, so you don't have to `delete` by hand, because you will likely forget about it, or forget to catch exceptions and cleanup properly. Most modern C++ code do not contain a single `delete` statement. — BatchyX, Aug 10 '12 at 06:56
Thank you for the advice on Smart pointers. I would be happy if you could show me the use of smart pointers on the above example. — jankos, Aug 10 '12 at 07:05

huseyin tugrul buyukisik · Answer 3 · 2012-08-10T06:29:02.223

0

C:

struct Measurement *meas;
meas=(struct Measurement *) malloc(sizeof(Measurement));
              ^                             ^                         
              |                             |                 
              |                             |                
          this is shape                  this is the space allocated

C++:

Measurement *meas;
meas=new Measurement;

edited Aug 10 '12 at 06:29

answered Aug 10 '12 at 06:23

huseyin tugrul buyukisik

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I could have swore the original q didn't have have dynamic in it. Why are you using malloc? – Luchian Grigore Aug 10 '12 at 06:28
malloc wants single parameter – huseyin tugrul buyukisik Aug 10 '12 at 06:30
lol he is changing the tag C to C++ and again to C then C++ :D – huseyin tugrul buyukisik Aug 10 '12 at 06:31
Whichever it is, it's wrong. If it's C++, you use new. If it's C, you don't cast the return type of malloc... :P – Luchian Grigore Aug 10 '12 at 06:32
Sorry guys for changing the tag from C to C++ and back. I am using VC++. – jankos Aug 10 '12 at 06:34
@LuchianGrigore there is no need to cast the return type in C? I've always did a casting, lol – Yamaneko Aug 10 '12 at 06:45
1

@VictorHugo it's actually frowned upon. In C++ the cast is necessary (for malloc). But in C, no. http://stackoverflow.com/a/605858/673730 – Luchian Grigore Aug 10 '12 at 06:47
it can suppress some warnings – huseyin tugrul buyukisik Aug 10 '12 at 06:49
Using bare pointers in C++ is also frowned upon, because nobody will free the memory if an exception is raised. Smart pointers are the norm now, especially with C++11. Also, for malloc(), if casting suppress a warning, then you should either stop compiling C code with a C++ compiler or make sure you include stdlib.h. If you still get warnings, ditch your compiler. – BatchyX Aug 10 '12 at 06:52
The c++ solution partially works: do I still need to define pointers to the sub-struct? – jankos Aug 10 '12 at 06:52

Dynamically Allocating a mulitple Struct within a Struct

3 Answers3