CPtrList

[Mahendra_786]

Hi, I am trying to iterate and delete the elements (assuming atleast 1 element) in a CptrList with this: void *pPt = NULL; POSITION pos = NULL; for (pos = m_points.GetHeadPosition(); !m_points.IsEmpty(); m_points.GetNext(pos)) { // TRACE("pos: %d, %d\n", pos, NULL); pPt = m_points.GetAt(pos); m_points.RemoveAt(pos); delete pPt; } TRACE shows that pos is not equal to NULL as it should be after the last element has been seen. So pos != NULL check does not work contrary to the MSDN Library documentation. THIS IS ONE PROBLEM. The OTHER is even if there are more than 1 elements the second element returned by GetAt is not "valid" (0xcdcdcdcd) and RemoveAt asserts. Any clues? Regards Mahendra

[Navin]

As soon as you remove the element, you invalidate the position. Here's a better way of doing it: { void *pPt = NULL; POSITION pos = NULL; for (pos = m_points.GetHeadPosition(); pos != NULL; m_points.GetNext(pos)) { // TRACE("pos: %d, %d\n", pos, NULL); pPt = m_points.GetAt(pos); delete pPt; } // Now we have a list full of invalid pointers, so remove them all m_points.RemoveAll(); } "Fish and guests stink in three days." - Benjamin Franlkin

[Mahendra_786]

But is this ok - (re)moving from the tail end if a sub-sequence has to be removed and RQUIRES using RemoveAt. This seems to works: for (pos = m_points.GetTailPosition(); !m_points.IsEmpty(); m_points.GetPrev(pos)) { pPt = m_points.GetAt(pos); delete pPt; m_points.RemoveAt(pos); } pos is still not NULL in the end though! Or like in the MSDNL RemoveAt help!: POSITION pos1 = NULL; POSITION pos2 = NULL; for (pos1 = m_points.GetHeadPosition(); (pos2 = pos1) != NULL; ) { m_points.GetNext(pos1); pPt = m_points.GetAt(pos2); delete pPt; m_points.RemoveAt(pos2); } Due Regards Mahendra

[Gary R Wheeler]

One problem I see in your example is the line delete pPt;. This line won't work correctly. Since pPt is a void pointer, the compiler won't know which delete operator and destructor to call. This will lead to memory leaks and possibly heap corruption. A better approach would be:

while (!m_points.IsEmpty()) {
MyType *pPt = (MyType *)m_points.RemoveHead();
delete pPt;
}

In this case, MyType is the type of the value pointed to by members of the list. If you use a CTypedPtrList<>, you can delete the contents of a list more simply as follows:

CTypedPtrList<MyType> m_points;
//...
while (!m_points.IsEmpty()) {
delete m_points.RemoveHead();
}

Using CTypedPtrList you don't need type casts.

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