vectors and pointers and questions....

[Thresher]

Hi all.... I'm trying to practive & learn about STL and pointers and wrote a little dialog app to help me. It complies but gets an unhandled exception error. Here's my data class..

class MyData  
{
public:
MyData();
virtual ~MyData();

void setFirst(int a) { first = a;};
void setName(CString s) { sName = s; };

int getFirst() { return first; };
CString getName() { return sName; };

private:
   int first;
   CString sName;
 

} 

and instead of the storing MyData objects in a vector in the dialog, I'm trying to store pointers to the MyData objects. So i the MyDialog.h I have this:

   std::vector<MyData*>vPtrs;
   int count;// increments number of MyData 

Then in the dialog to add a MyData object to the vector when I hit Add Mydata Obj I do this:

void CMyPointerDlg::OnNewobj() 
{
//make a new instance of the obj and add it to the vector

   MyData *p = new MyData;

   p->setFirst(count);
   p->setName("NoName");

   vPtrs.push_back(p);

   int size = vPtrs.size();
   m_sObjCount.Format("%d", size);//format for dialog
   UpdateData(false);
   count++;
    
} 

and to remove an obj when I press Remove Mydata Obj I do this:

void CMyPointerDlg::OnRemoveobj() 
{

   vector<MyData*>::iterator itr;

   if(vPtrs.size() > 0)
   {
      itr = vPtrs.end();
      MyData *p;
      p= *itr;
      int i = p->getFirst();
      delete p;//ERROR!
      int size = vPtrs.size();
      m_sObjCount.Format("%d", size);
      UpdateData(false);


   }
    
} 

I get the error on the line marked above... am I totally missing the boat here? Thanks......

[ClemDuff]

vector::end() returns an iterator that's 1 past the end. Try a "--itr" before you dereference it.

[Philip Nicoletti]

I assume that you are trying to remove the last element ?

itr = vPtrs.end();

STL works with "half-open" ranges.

vPtrs.begin() ... points to the first element
vPtrs.end() ... point to ONE PAST the last element

so the following is incorrect:

itr = vPtrs.end(); 
MyData *p; 
p= *itr;   // de-references Vptr.end()

you can use the back() member function to get the last element
(not tested ...)

void CMyPointerDlg::OnRemoveobj()  
{ 
   if(!vPtrs.empty()) 
   { 
      MyData *p = vPtrs.back();  // back()
      int i = p->getFirst(); 
      delete p;
      vPtrs.pop_back();
      int size = vPtrs.size(); 
      m_sObjCount.Format("%d", size); 
      UpdateData(false); 
   } 
     
}

[Thresher]

Yes, I'm trying to remove the last element. Thanks... I'll give it a try.

[Andreas Masur]

Just as a reference...you might want to take a look at the following introduction to the 'vector' class...