really stupid problem with "this" pointer

[Sergei Shelukhin]

Hi. I have barely written any C++ for past 3-4 years, I need to
refresh it in my memory so I decided to do some coding starting with
classic algos and data structures, using Visual Studio 9, unmanaged
code. Everything is humming, but I have the following problem I cannot
figure out. Say we have the following:

template <typename T> class SLL
{
public:
SLL<T>* AddHead(T value);
T value;
SLL<T>* next;
};

....

SLL<int>* sll = new SLL<int>;
sll->next = NULL; //yeah I know I also need a constructor
sll->value = 1;
sll = sll ->AddHead(0);
sll = sll ->AddHead(0);

If I use this implementation of AddHead:
SLL<T>* head = new SLL<T>;
head->value = value;
head->next = this;
return head;
everything is ok;

If I use this,
SLL<T> head;
head.value = value;
head.next = this;
return &head;
the first call is ok; before the second call "sll" points to memory
location X and contains the list of two elements; but inside the
second call "this" points to X that has uninitialized instance in it
with random value and next pointing into nowhere.
How come?

 

[Stuart Redmann]

Hi. I have barely written any C++ for past 3-4 years, I need to
refresh it in my memory so I decided to do some coding starting with
classic algos and data structures, using Visual Studio 9, unmanaged
code. Everything is humming, but I have the following problem I cannot
figure out. Say we have the following:

template <typename T> class SLL
{
public:
SLL<T>* AddHead(T value);
T value;
SLL<T>* next;
};

...

SLL<int>* sll = new SLL<int>;
sll->next = NULL; //yeah I know I also need a constructor
sll->value = 1;
sll = sll ->AddHead(0);
sll = sll ->AddHead(0);

If I use this implementation of AddHead:
SLL<T>* head = new SLL<T>;
head->value = value;
head->next = this;
return head;
everything is ok;

If I use this,
SLL<T> head;
head.value = value;
head.next = this;
return &head;
the first call is ok; before the second call "sll" points to memory
location X and contains the list of two elements; but inside the
second call "this" points to X that has uninitialized instance in it
with random value and next pointing into nowhere.
How come?

You should read the chapter parameter passing in C++ once more. You are
returning a pointer to a local variable (the variable head goes out of scope
when AddHead is left), so you cannot dereference this pointer anymore outside
your function or you'll get Undefined Behaviour. Surely your compiler will give
you a warning (at least VC6.0 does).

Regards,
Stuart

 

[Sergei Shelukhin]

Hmm, thanks.
I rather got too used to GC patterns, if I still point at it, it
should exist