C++ : how is a reference to the value of a pointer is updated when the pointer itself changes?

Question

Consider the following program:

include <iostream>

using namespace std;

int main()
{

    int *ptr = new int(10);
    int &ref = *ptr;

    cout << ref << endl << &ref << endl;

    delete ptr;
    ptr = new int(100);
    cout << ref << endl << &ref << endl;

    return 0;
}

Output:
10
0x80010348
100
0x80010348

Here the ref variable is a reference to the memory location pointed by ptr. However, when the ptr pointer is deleted and a new memory location is assigned, ref should now be referring to an undefined memory location as the previous does not exist. But when I run this program I get the updated value and address through ref with g++ on cygwin/linux as well as VC++2012 express.

However, if ptr is not deleted, then ref is not updated, and the output shows the old value 10 and old address. Is this behavior standard or is it compiler specific? If it's standard then why the update of ref depends upon whether ptr is deleted or not?

try outputting the second value of ptr too, it happens sometimes that coincidentally you get the same address. — PlasmaHH, Apr 02 '13 at 11:22
It's _undefined behavior_. It could work, or it could crash your program. — Some programmer dude, Apr 02 '13 at 11:22
Does it work everytime? This smells like undefined behavior... — bash.d, Apr 02 '13 at 11:22
It's how memory allocation works. It happened to give you the same address in second call to new, so it would appear 'updated'. — ActiveTrayPrntrTagDataStrDrvr, Apr 02 '13 at 11:35
Yes, I also suspected this is a non-standard behavior, although happens consistently. Thanks for clarifications! — sourav.d, Apr 02 '13 at 11:47

score 2 · Answer 1 · answered Apr 02 '13 at 11:29

2

It's just the way the memory allocator happens to work. In your code:

delete ptr;
ptr = new int(100);

you delete one int and then you immediately ask for a new one. The runtime says "Fine, have this one. It was just made available".

Simple recycling.

answered Apr 02 '13 at 11:29

Bo Persson

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C++ : how is a reference to the value of a pointer is updated when the pointer itself changes?

1 Answers1