Using pointers or references to struct

Question

Me and my friend is using structs in our code (our code is separate from each other). Lets take the following example:

struct Book {
     char title;
     int pages;
}

void setBook(struct Book *tempBook) {
     tempBook->title = "NiceTitle";
     tempBook->pages = 50;
}

The above code is pretty straight forward. The thing is, is there any difference in having these two mains:

 int main() {
     struct book obj1;
     setBook(&obj);
    }

Or

int main() {
    struct Book *obj2;
    setBook(obj2);
}

EDIT: I was not clear in my statement. I have initialized the pinter to

struct Book *obj2 = malloc(sizeof(struct obj2));

Yes there is a difference. One will work, and one will not work. — Sam Varshavchik, Nov 16 '16 at 12:00
`struct Book *obj2 = malloc(sizeof(struct obj2));` won't compile, please show the actual code. — Sourav Ghosh, Nov 16 '16 at 12:18
Are you kidding me, im aksing for the difference, this is an example. — Emil Møller Ringgaard, Nov 16 '16 at 13:06

Sourav Ghosh · Accepted Answer · 2016-11-16T12:17:08.430

6

In case of

 struct book obj1;
 setBook(&obj);

you're passing a valid address^#1 to the function, so the behaviour is defined.

On the other hand,

struct Book *obj2;
setBook(obj2);

you're passing an unitialized pointer^#2, accessing which invokes undefined behavior.

That said, the member char title; should be char *title;, as the string literal, when used as initializer, decays to the pointer to the first element, so you'll need a pointer on the LHS.

#1 -- obj1 is an automatic local variable, and address of that variable is a valid address in the scope.

#2 -- struct Book *obj2; defines a pointer and again, obj2 being an automatic local variable, it is not implicitly initialized to anything. So, the initial value (i.e., the memory address at which the pointer points) is indeterminate and pretty well invalid.

edited Nov 16 '16 at 12:17

answered Nov 16 '16 at 12:00

Sourav Ghosh

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Lets say i did initialize the pointer to an address, whjat would the difference be then? – Emil Møller Ringgaard Nov 16 '16 at 12:20
@EmilMøllerRinggaard what is `an address`? is it _valid_? How do you know? – Sourav Ghosh Nov 16 '16 at 12:21
It's on a microprocessor and it compiles, so i think it works – Emil Møller Ringgaard Nov 16 '16 at 12:22
in general, as long as you pass a valid pointer, id does not matter whether it is the address of a scalar or a pointer variable itself. – Sourav Ghosh Nov 16 '16 at 12:22
Okay, would you recommend not using a pointer to the struct, and just pass in the &structobject ? – Emil Møller Ringgaard Nov 16 '16 at 12:23
1

@EmilMøllerRinggaard That depends, both are valid. The rule of thumb is, if you can avoid using dynamic memory allocation `malloc()` and family, avoid them. Use them only when necessary. – Sourav Ghosh Nov 16 '16 at 12:24
One last question - Have it even more valid, to avoid dynamic memory allocation on a microcontroller then on a desktop for instance? – Emil Møller Ringgaard Nov 16 '16 at 12:26
1

@EmilMøllerRinggaard I'd say, yes. – Sourav Ghosh Nov 16 '16 at 12:28
@EmilMøllerRinggaard: "It's on a microprocessor and it compiles, so i think it works" - If things were **that** simple! The nasty thing which makes this a very wrong assumption is called "Halting Problem". Simply put: Even if your code compiles without warnings - let alone errors - does not guarantee anything about its correctness. – too honest for this site Nov 16 '16 at 13:15

score 4 · Answer 2 · answered Nov 16 '16 at 12:01

struct Book *obj2;

in main is just a pointer with an indeterminate value. It doesn't point to anything you allocated. You can dereference it, but that's undefined behavior.

struct Book obj1;

allocates a struct Book. You can use its address and modify its members.

Notes:

title from struct Book is a char, which can only hold one character, not a string (i.e., a pointer). Use char * or const char * instead.

score 0 · Answer 3 · answered Nov 16 '16 at 12:43

In the first case you are passing a variable of type 'struct Book' to 'setBook function. Since you declared the variable hence total memory occupied by obj1 -s sizeof(setBook) i.e. sizeof(char)+sizeof(int) which is equal to 5 bytes. hence you can access as obj1 .title(1 byte) and obj1.pages(4 bytes).

NOTE:- Assuming int is 4 byte long.

Let's see what happen in second scenario.

struct Book *obj2; obj2 size id 4 bytes since it is a general pointer.The field is not divided either and hence we cannot access the field.

Hence, In second scenario, you need to allocate is dynamically before trying to use the members.The allocation will result in proper sized and properly divided field.

Using pointers or references to struct

3 Answers3

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