What does struct object represents?

Question

#include<iostream>
using namespace std;



struct  A{

   int a;
   int s;
};
int main()
{
    A *S =new A();
    A  obj1=S[0];
    printf(" 0x%x",obj1);
}

My question is when i see the out put it seems be 0x0; and is some cases it becomes 0xFFFFFFFF

can someone explain?

Isn't it also UB if you use `printf` without including `cstdio`? You seem to have a schoolboy habit of writing those first two lines by default. — DeiDei, May 25 '17 at 13:40
@DeiDei That's not "undefined behaviour", but a reason for a compiler error. ... As you can see here, iostream is allowed to include cstdio itself too, so it works in this case. — deviantfan, May 25 '17 at 13:41
@deviantfan: "is allowed" does not imply it has to. The code might work by chance, but that is no way something one should rely on. — too honest for this site, May 25 '17 at 13:58
@Olaf Yes, that's why I wrote "is allowed". It is not guaranteed to compile. — deviantfan, May 25 '17 at 14:01
@deviantfan: You should have emphasised the "don't rely on it" part. As you commented, a beginner could get the impression if it works it is fine like it is. Which it is certainly not. — too honest for this site, May 25 '17 at 14:04

Basile Starynkevitch · Answer 1 · 2017-05-25T14:02:49.777

1

Your program has undefined behavior, so is wrong (because what you pass to printf does not match the " 0x%x" format control string, since it has the wrong type); see also this answer.

Read Lattner's blog on undefined behavior (it also applies to C++)

Notice that printf is a C library function (so you need to #include <cstdio>). In genuine C++11, you would use some operator << on std::ostream-s, probably like

std::cout << obj1 << std::endl;

and that line won't compile (unless you define such an operator, which you should do).

To explain the actual behavior (which is non reproducible in general), you need to dive into implementation details (and you don't want to: if you did, study the source and object code of your particular program and C++ library, of your compiler, of your operating system, look into calling conventions, instruction set, ABIs, etc...). The displayed garbage value (e.g. 0xfffffff) is what happens to sit in some relevant processor register or memory location (on the call stack).

BTW, if you compiled with all warnings & debug info (e.g. g++ -Wall -Wextra -g with GCC) you'll get a warning. I strongly recommend to enable all warnings.

Notice that a struct A{ is the same as class A{public: so you could define some explicit constructor (which would initialize the fields) and some output operator<<:

struct  A{
   int a;
   int s;
   A(int aa=0, int ss=0) : a(aa), s(ss) {};
};
std::ostream operator << (std::ostream&out, A a) { 
   out << "a=" << a << ",s=" << s;
   return out;
}

But you probably should read about the rule of five notably this.

edited May 25 '17 at 14:02

answered May 25 '17 at 13:38

Basile Starynkevitch

223,805
18
296
547

i understand your part. As i cannot do cout< – Mukesh May 25 '17 at 13:41
What does that values depicts? 0x0 and 0xFFFFFFFF – Mukesh May 25 '17 at 13:41
@Mukesh The second reason is that you have undefined variables. ... Your values are numbers in their hexadecimal form (0 and 4294967295 (or -1 in two's complement)). You should know that already when using %x – deviantfan May 25 '17 at 13:42
@deviantfan will you elaborate more? – Mukesh May 25 '17 at 13:44
@Mukesh About what? If you don't understand what a number is, I can't help you. – deviantfan May 25 '17 at 13:45
i mean those numbers are values for what? and why do they change? – Mukesh May 25 '17 at 13:47
For whatever the location or processor registers involved contained. You don't want to understand more (and if you do want, work for many years understanding the details of your compiler, your system libraries, your operating system) – Basile Starynkevitch May 25 '17 at 13:48
@Mukesh This will be my last comment for now: I told you twice already. If you don't understand it, start googling and/or reading books. (Sigh.) – deviantfan May 25 '17 at 13:48
if i do like this – Mukesh May 25 '17 at 13:50
int main() { A *S =new A(); S->a=5 A obj1=S[0]; printf(" 0x%x",obj1); } the output i see is 0x5 – Mukesh May 25 '17 at 13:50
1

@Mukesh That is because you set it to be 5. When you do `A *S =new A();` the members of `S`, and subsequently `obj1`, **can be any value**. There is no requirement for them to be any specific value and the value they have can change from run to run because it is **undefined behavior** (ie, there is no expected behavior from your code). – NathanOliver May 25 '17 at 13:55
How to provide a proper behaviour? – Mukesh May 25 '17 at 13:58
You can't. You need to initialize your values. – Basile Starynkevitch May 25 '17 at 13:58
@Mukesh You write a constructor to properly initialize the values. – NathanOliver May 25 '17 at 14:02

What does struct object represents?

1 Answers1