循环和内存管理中的C ++指针算术？

Question

I am getting started with C++. I wanted to understand the different outputs while playing around with this snippet of code.

int main()
{
    int i = 3;
    int *ptr = &i; //stores address of the i
    while(*(ptr)--) //the same as i--
    {
        cout << *ptr << endl;
    }
}

When I run this code I understand that the deferenced value of ptr, which is "i", gets 1 subtracted from it and the loop exits when "i" equals 0. But when I use while(*ptr--) instead of while(*(ptr)--) I get a list of random integers which eventually go down to 0 and the loop breaks.

To my understanding when I use *ptr-- I am subtracting a byte(size of one int) from the initial address of &i(*ptr) with each loop. But why does the program terminate eventually? No matter what the value of "i" is, the program prints 23 random numbers with the last one being 0 and the loop exits. Should I not get an overflow error since the program runs out of memory?

However, when I use while(ptr--) the program does go into an infinite loop. What exactly is happening?

Thank you very much.

Answer 1

(ptr) is the same as ptr , thus (ptr)-- is the same as ptr-- .

*ptr-- IS NOT the same as i-- !

You are applying operator-- to the right side of the pointer. The suffix/postfix operator-- has a higher precedence than operator* . So, *ptr-- is the same as *(ptr--) , not (*ptr)-- like you are expecting.

IOW, ptr-- gets evaluated first, which returns a copy of the current pointer and then decrements the pointer , and then you are dereferencing the copied pointer to retrieve the value that was previously being pointing at.

That is why you are seeing garbage - you are decrementing the pointer into memory that does not belong to you.

To simply decrement the int and not the pointer, use (*ptr)-- instead. IOW, dereference the pointer first, then decrement the value being pointed at, eg:

int main()
{
    int i = 3;
    int *ptr = &i; //stores address of the i
    while((*ptr)--) //the same as i--
    {
        cout << *ptr << endl;
    }
}

Live demo

Parenthesis and operator precedence matter!