DWORD and DWORD_PTR on 64 bit machine

Question

There are few *_PTR types added to the Windows API in order to support Win64\'s 64bit addressing.

SetItemData(int nIndex,DWORD_PTR dwItemData)
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Answer 1

The function will not fail if you pass a DWORD, because it fits into a DWORD_PTR. A pointer, however, is guaranteed to fit into a DWORD_PTR but not into a DWORD on 64-bit platforms.

Thus, this code is correct:

int *before_ptr = new int;
yourListBox.SetItemData(index, (DWORD_PTR) before_ptr);
int *after_ptr = (int *) yourListBox.GetItemData(index);
ASSERT(before_ptr == after_ptr);  // Succeeds.
delete after_ptr;                 // Works.

But this code is wrong and will silently truncate the pointer to its lower 32 bits:

int *before_ptr = new int;
yourListBox.SetItemData(index, (DWORD) before_ptr);
int *after_ptr = (int *) yourListBox.GetItemData(index);
ASSERT(before_ptr == after_ptr);  // Fails.
delete after_ptr;                 // Undefined behavior, might corrupt the heap.