C ++中的新建，删除和子类

Question

TextItem is a subclass of XObject .

I am trying to figure out why the following works:

  TextItem *textItem = new TextItem();
  XObject *xItem = textItem;
  delete textItem;

But this does not:

  TextItem *textItem = new TextItem();
  XObject *xItem = textItem;
  delete xItem;

The second example fails on delete , with an assertion failure ( _BLOCK_TYPE_IS_VALID ).

Answer 1

XObject *xItem = textItem;
delete xItem;

This would work only if XObject has virtual destructor. Otherwise, the delete statement invokes undefined behavior.

class XObject
{
    public:
       virtual ~XObject();
     //^^^^^^ this makes virtual destructor
};

Answer 2

Make sure that XObject has a virtual destructor, or your second snippet has undefined behavior :

struct XObject
{
    // now deleting derived classes
    // through this base class is okay
    virtual ~XObject() {}
};

struct TextItem : XObject {};

Answer 3

Does XObject not provide a virtual destructor? When you don't have a virtual destructor, you will get undefined behaviour when deleting the TextItem through a base pointer.