Uninitialized pointer, points to zero

Question

#include <iostream>
int main() {
    int* pt;
    std::cout << pt << std::endl;
    if(pt)
        std::cout << "true" << std::endl;
    else
        std::cout << "false" << std::endl;
    return 0;
}

This code fragment evaulate to false. What is more it prints 0 in

    std::cout << pt << std::endl;

I thought that uninitialized pointers points to some random address, which may contain garbage value. In this case it points to 0 value like i would assign it to nullptr. Could someone clarify this for me?

it is possible that the random address could have been null_ptr. That meaning, the memory left over at the location that the pointer was declared could have originally been 0. — Nova Ardent, Jul 04 '18 at 11:33
It is so, ackward for me because now it always shows 0 and about one week before the pointer was really pointing to some random address. That is why i am interested why it like that. — Jack Wilkinson, Jul 04 '18 at 11:47

gsamaras · Answer 1 · 2018-07-04T11:51:08.283

1

This code invokes Undefined Behavior (UB), since the pointer is used uninitialized.

The compiler should emit a warning, when a warning flag is used, like -Wall for example:

warning: 'pt' is used uninitialized in this function [-Wuninitialized]
     std::cout << pt << std::endl;
                  ^~

It just happens, that at this run, on your system, it had the behavior of a null pointer. That means that the garbage value the pointer takes, happens to be 0. However, notice that, kernel zeroes appear relatively often.

edited Jul 04 '18 at 11:51

answered Jul 04 '18 at 11:34

gsamaras

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It is also interesting to read [this](https://stackoverflow.com/questions/6004816/kernel-zeroes-memory) which explains why you often still get 0. – Jul 04 '18 at 11:38
Agree @Pi, injected that into the question. Hope you like it now. – gsamaras Jul 04 '18 at 11:51
Actually, an answer with that link would have been more useful on the dupe target than it is here. – Baum mit Augen Jul 04 '18 at 11:53
@gsamaras Never disliked it ;) – Jul 04 '18 at 11:57
I attempted to answer there too @BaummitAugen, but I need to catch the bus now! Pi that's good, thanks! – gsamaras Jul 04 '18 at 12:01

Uninitialized pointer, points to zero

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