How could it get more memory than I wanted?(C++) [duplicate]

Question

I wanted to take a 1 integer memory, but how this program can work?

Code:

#include<iostream>
using namespace std;

int main(){

    int* k=new int[1];

    for(int i=0;i<5;i++)
    cin>>k[i];

    for(int i=0;i<5;i++)
    cout<<k[i]<<"\n";

    delete[] k;

    return 0;
}

Input:

Output:

It didn't get more memory than you wanted. You invoked undefined behavior and proceeded to memory stomp by writing to addresses which you did not allocate. — Cory Kramer
– Cory Kramer, Commented Oct 14, 2014 at 18:57
@Cyber: it's entirely possible he got more memory than he wanted, but the undefined behavior/memory stomp still applies. — Mooing Duck
– Mooing Duck, Commented Oct 14, 2014 at 18:59
You have undefined behavior, because you write and read out of bounds of the memory you allocate, that means your whole program is invalid. — Some programmer dude
– Some programmer dude, Commented Oct 14, 2014 at 18:59
Thanks for answering in advance @Cyber.But does it happen all the time? — ZeRoHuK
– ZeRoHuK, Commented Oct 14, 2014 at 19:03
@ZeRoHuK - But does it happen all the time? Does what happen all the time? — PaulMcKenzie
– PaulMcKenzie, Commented Oct 14, 2014 at 19:08

Baum mit Augen · Accepted Answer · 2014-10-14 18:59:41Z

3

You invoked undefined behavior by accessing memory you did not allocate. This works purely "by chance". Literally every behavior of you program would be legal, including the program ordering pizza, ...

This will probably work in practice most of the time because your OS will usually not just give you 4 Byte or something like this, but a whole page of memory (often 4kB) but to emphasize this: You can never rely on this behavior!

answered Oct 14, 2014 at 18:59

Baum mit Augen♦

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Comments

Jared Wadsworth · Accepted Answer · 2014-10-14 19:01:42Z

1

The way that a c++ program uses an array is that it the index that you want, multiplies it by the size of the element the array is made of, then adds it to the first memory location in the array. It just so happened that where you placed this in your program, going back an additional 4 elements didn't corrupt anything, so you were just fine. It doesn't actually care. However if you overwrite another variable, or a stack pointer, then you run into trouble. I wouldn't recommend doing this in practice, however, as the behavior can be undefined.

answered Oct 14, 2014 at 19:01

Jared Wadsworth

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