Question

void Test2()
{   
    int c=8;
    int b=7;
    int d=9;
    int *a; 

    a = &b;
    a+=sizeof(int); //I supposed that *a should points on variable d after this 

    cout << "b\t" << &b << "\t" << b << endl;
    cout << "a\t" << a  << "\t" << *a << endl;
    cout << "c\t" << &c  << "\t" << c << endl;
    cout << "d\t" << &d  << "\t" << d << endl;
}

I supposed that *a should points on variable d because b and d (as I thought) lie nearby in the stack of local variables. But *a points on another address so *a!=d
My question is why so? Is it the feature of Visual Studio 2010 or something else?

Answer 1

No, it’s a feature of C++ called undefined behavior. You can’t do pointer arithmetics outside an array (or one position over the bound of the array) you own.

You could get this to work by a += 1 because a is already a int*, so += 1 will make it point to the next integer. a+=sizeof(int) will move it sizeof(int) integers to the right.

 +------+------+------+------+------+
 |      |      |      |      |      |      
 +------+------+------+------+------+
    ^      ^                    ^
    |      |                    |
    a     a+1               a+sizeof(int) (assuming sizeof(int) == 4)

Again, technically it’s undefined.