C++ : pointers to stack-dynamic variables

Question

I am new to C++. I don't quite understand why this code does not work. What does this code have to do with stack-dynamic variable? Thanks for your help.

int twice(int x)
{
    int *y;
    *y = x * 2;
    return *y;
}

This `*y = x * 2;` calls _Undefined Behavior_, you didn't initialize `y` to point to a valid address!! — πάντα ῥεῖ, Jun 21 '14 at 20:45
[This is the list of C++ books recommended by Stack Overflow community](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — milleniumbug, Jun 21 '14 at 20:46

4pie0 · Answer 1 · 2014-06-21T20:51:57.467

int *y;
*y = x * 2;

is not correct because y points nowwhere, not into an allocated memory. It is undefined behavior. After this line of code your program is completely unpredictable and you cannot assume anything about its behavior.

You need to allocate the memory first using new or malloc and assign x*2 to it or pass an address to assign y into, i.e:

int *y = new int( x * 2);

Example:

int main() {

    int x = 4;
    int *y = new int( x * 2);
    cout << x << "," << *y;
    delete y;

    return 0;
}

Side note: this is dangerous to return a pointer to memory allocated in function because this might become unclear who and when is responsible for freeing allocated memory. In your particular case there is absolutely no apparent reason for dynamic allocation by the way.

C++ : pointers to stack-dynamic variables

1 Answers1