Object pointer as method argument

[_Flaviu]

I have a CData derived from CObject. In my CDocument, I have a CMap, with key as int, and values as CData*. In a CMyDocument method I have:

void CMyDocument::GetData(int nItemData, CData* pData/* = NULL*/)
{
m_MapData.Lookup(nItemData, pData);
TRACE("Inside doc method: pData is not NULL: %d\n", NULL != pData);
}

and in my CMyView:

int nItemData = 1;
CData\* pData = NULL;
GetDocument()->GetData(nItemData, pData));
TRACE("In MyView: pData is not NULL: %d\\n", NULL != pData);

and here is the result: Inside doc method: pData is not NULL: 1 In MyView: pData is not NULL: 0 Why inside CMyDocument I have a valid pData, but outside not ?

[Lost User]

In case 1 you have not shown the value of pData. In case 2 you have explicitly set its value to NULL.

[_Flaviu]

In case 1, the trace show me that pData is not NULL, and after GetData call, I set up nothing, I only display if pData is NULL or not ... but is NULL ...

[CPallini]

Because the CData pointer is passed by value. If you need to modify the pointer then you have to pass it by reference (or use a double pointer).

[Lost User]

Your test does not show what you think. It shows the values that you set in pData before the call to GetData, which does not change it. You need to reread the documentation, and understand the difference between a pointer and a reference.

[_Flaviu]

I used double pointer and goes perfectly. Kindly thank you all of you (including Richard)!