Displaying blank values

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Here's a class to add "multi-select" to some simple grid items. It lets you display blank values when the data you are displaying have conflicting values.

class IMultiValue {
public:
 virtual bool IsMultiValue() = 0;
};
template <class GridItemClass>
class MultiSelect : public GridItemClass, public IMultiValue
{
public:
 MultiSelect(CString x):GridItemClass(x)
 {
  CXTPPropertyGridItem::SetValue("");
 }
 virtual void SetValue(CString strValue)
 {
  if (strValue.IsEmpty())
   CXTPPropertyGridItem::SetValue(strValue);
  else
   __super::SetValue(strValue);
 }
 virtual bool IsMultiValue()
 {
  return GetValue().IsEmpty();
 }
};

Usage:

CXTPPropertyGridItemBool* b = new MultiSelect<CXTPPropertyGridItemBool>(_T("BoolTest"));
myCategory->AddChildItem(b);

This class could be extended to have "addValue" and "resetValue" functions...
Is there a better way to do this? Has anybody else tried?