Question

I expect the result is undefined or something, but the output is “10”, why?

I suppose the memory is destroyed after the function gets called.

#include <iostream>
void f(int *p)
{
    int l = 20;
    int *k = &l;
    p = k;
}
int main()
{
    int i = 10;
    int *j = &i;
    f(j);
    std::cout << *j;
    return 0;
}

Answer 1

The result isn’t undefined. You pass the pointer j by value, so you modify a copy of it inside the function. The original j is left unchaged, so the result is still 10.