C++ Losing Template Data

Question

I don't consider myself all that knowledgeable in C++ but I'm having a hard time with this concept. So I have a class the holds some template datatype and a double. I want the m_data variable to be generic, but right now I'm only testing with an unsigned int. When I call the function SetData() with say a pointer to an unsigned int I lose the info the pointer is pointing to. This happens when I go out of scope, so I felt I need to do a deep copy of it...

I tried many different constructors and assignment operators but I still lose the info... I feel I'm missing something obvious about templates here.If anyone could point me in the right direction as to why the data is being lost I would be very grateful.

Small bit of code:

template<typename T>
class PointNode {

 public:

  PointNode(double p){ m_point = p;}
  ~PointNode();

 void SetData(T  * data);
 T * GetData() const;

  private:

   double m_point;
   T *m_data;

 };

template<typename T>
void PointNode::SetData(T * data)
{
  m_data = data;
}

template<typename T>
T * PointNode::GetData()
{
  return m_Data;
}

OK some more info. This class is being stored in a map that is a member of another class. Heres a bit of it.

 template<typename T>
class AuMathPointTreeT
{

  public:

  //Member Variables 

  double m_dTolerance;  

  unsigned int m_cPoint;         


  map<VectorKey, PointNode<T> > m_tree; /*map posing as a tree */

  typename map<VectorKey, PointNode<T> >::iterator iter;   /* iterator  */
  pair< typename map<VectorKey, PointNode<T> >::iterator, bool > return_val;

  /* Tree methods */

  //constructor
  AuMathPointTreeT(double tol);

 ...
};

In another program I'm using this class, creating node and setting the template data like so

if (node = pnttree.AddPoint(point) )
{
    unsigned int * data = new unsigned int();
    *data = pntCount;
    node->SetData(data);
    ++pntCount;
}

UPDATE: Ok discovered the culprit of what's wrong, and would like suggestions on how to approach it. When I insert a node into the map class a few functions are called in the process and im losing the original pointer to the newly allocated node class object. Here is what I'm doing.

template<typename T>
PointNode<T> * AuMathPointTreeT<T>::
AddPoint(double point)
{
  PointNode<T> * prNode = MakeNode(point);

  m_cPoint++;

  return prNode;
} 

template<typename T>
PointNode<T> *  AuMathPointTreeT<T>::
MakeNode(double point)
{
  PointNode<T> * prNode = new PointNode<T>;

    //set the contents for the node just performs a few calcs on the values

  prNode->SetNode(point, m_dTolerance);


  //Create the key class using the 
  VectorKey key(point, m_dTolerance);

  //Store the key,node as a pair for easy access 
  return_val = m_tree.insert( pair<VectorKey, PointNode<T> >(key, *prNode) );
  if (return_val.second == false)
    prNode = NULL;

  unsigned int * test = new unsigned int;
  *test = 55;
  prNode->SetData(test);  //if call this here its no longer the right pointer 

  return prNode;
}

So after looking at this... I really still want to return a pointer and use it. But maybe the iterator being held by return_val? Im open on suggestions for all aspects too.. Sorry this question has been a mess :\

I wanted it to be a pointer to the data, is that not possible? — Odub, Jun 06 '13 at 17:53
Please show a code that is using this class, because the code seems fine to me — Tomer Arazy, Jun 06 '13 at 17:54
Don't use pointers, *especially* not if you're not very knowledgeable with C++. They should be left to very advanced users only. — Kerrek SB, Jun 06 '13 at 17:58
Well say I want to store a struct in the data, without a pointer it would be a pretty large overhead to do by-value I feel.. — Odub, Jun 06 '13 at 18:09
It's easier to make a correct program fast than it is to make a fast program correct. — Casey, Jun 06 '13 at 18:12
So right now I'm getting a seg fault when I attempt to dereference the pointer, but I dont if I include this in the constructor m_pvData = new T; Any thoughts? — Odub, Jun 06 '13 at 20:09

score 0 · Answer 1 · answered Jun 06 '13 at 18:04

If you want a deep copy, you must use T and not T*, or you must do dynamic memory allocation with T* (but it's overkill) and will bring a similar result.

If you really want Nodes of pointers, it will be when you use your Node.

Exemple:

int number = 5;
Node<int*> oneNode(&number);  // number will die at end of scope
Node<int> anotherNode(number); //anotherNode can be used without risk

score 0 · Accepted Answer · answered Jun 06 '13 at 18:05

0

I don't think this has anything to do with the use of templates. Once a local variable goes out of scope, its location on the stack could be over-written by other data.

If you expect the template class instance to out-live the local variable whose address is passed to SetData, you should consider allocating the data on the heap not the stack. Either way, I'd suggest replacing the raw m_data pointer with an appropriate smart pointer. For example, the use of shared_ptr<> in the template class and its client code should reduce the amount of data copying while at the same time ensuring the data remains valid regardless of whether or not the original data variable is in scope.

answered Jun 06 '13 at 18:05

Aeham

438
4
11

I cant use smart pointers or shared_ptr :( nothing from C++ 11. – Odub Jun 06 '13 at 18:07
The last code snippet you added shows pntCount getting updated just after it was used to set the data. Do you realize that by using a pointer to the data you'll be pointing to the modified value (i.e. after pntCount++) rather than the original value? – Aeham Jun 06 '13 at 18:46
Sorry again I didnt add exactly what I had in my code, I do unsigned int * data = new unsigned int() followed by *data = pntCount. – Odub Jun 06 '13 at 18:52
@Odub: You can always use Boost or implement one yourself. So, you can use both smart pointers and `shared_ptr` ;) – Jun 06 '13 at 22:27
Ok so in the end you were the closes to the problem. I was indeed losing the right reference to my Node. I guess after the insert the pointer I originally created pointed to an old copy of the object. Thanks! – Odub Jun 07 '13 at 15:35

score 0 · Answer 3 · answered Jun 06 '13 at 18:12

0

your code will not compile because of your constructor

PointNode(double p){ m_point = p;}

m_point is const, you have to write it to the initializer list:

PointNode(double p) : m_point(p) {}

answered Jun 06 '13 at 18:12

WoJo

360
2
10

Yea sorry that was just a stub I wrote for this, everything compiles fine. It's a runtime issue – Odub Jun 06 '13 at 18:14

C++ Losing Template Data

3 Answers3