Visual C++ :: Runtime Check Failure - Variable Used Without Being Initialized

Under visual studio, this is a typical run time error,

Code:
void func(int x){
x = 3;
}
int main() {
int x;
func(x);
}

When x is passed to the function func, it is not initialized. But my question is that why it should be an error? On the other hand, if I change the definition of func a little bit like this,

Code:
void func(int& x) {
*x = 3;
}
int main() {
int x;
func(&x);
}

Now in main, x is still not initialized, but this time there isn't a run time error like "the variable is being used without being initialized. Why?