为什么C ++方法不应该返回解引用的指针？

Question

I'm very new to C++ (and studying Java). This is from HW but I think I've answered the HW issue and I'm just trying to get a better sense of pointers.

Say I have the following method:

int cubed(int a) {
  int * ptr;
  *ptr = a * a * a;
  return *ptr;
}

This compiles and works fine with the following:

int test = cubed(3);

I'm trying to get a sense of why it's bad to return a dereferenced pointer.

This question says that memory leak is a problem but I'm not sure why. Maybe because I don't understand what happens to a dereferenced pointer.

Does the lvalue just hang around indefinitely at that point? Do you have to actually delete the pointer? Any insight would be appreciated.

Answer 1

The question you read is different. Say you had this code:

int cubed(int a) {
  int* ptr = new int;
  *ptr = a * a * a;
  return *ptr;
}

Now, you'd be leaking the dynamically-allocated int , because you'd return a copy of it and never delete the original.

Your code is actually worse because, instead of allocating with new , you're simply writing through an uninitialised pointer to memory that is not yours. In effect, you're writing to an int that does not exist.

What you should be doing is this:

constexpr int cubed(const int a)
{
   return a*a*a;
}

Or just invoke std::pow(value, 3) at the call site.

Answer 2

There is no memory link in your code, but there is another big bug.

You declare int * ptr; inside the function int cubed(int a) , so it is a local variable.

The value of ptr will be random - might be NULL, might point to garbage, might point to some perfectly valid address. You don't initialize the variable ptr , so your code will sometimes crash.