Question

I saw the code snippet as follows:

class UPNumber {
public:
  UPNumber();
  UPNumber(int initValue);
  ...

  // pseudo-destructor (a const member function, because
  // even const objects may be destroyed)
  void destroy() const { delete this; } // why this line is correct???

  ...

private:
  ~UPNumber();
};

First, I am sure that above class definition is correct.
Here is my question, why we can define the function ‘destroy’ as above?
The reason being asking is that why we can modify ‘this’ in a const-member function?

Answer 1

The const qualifier applied to a method have the effect of making the this passed to it a const pointer; in particular, in your case it will be a const UPNumber *.

Still, this is not a problem for delete: actually you can use delete on a const pointer without having to cast anything, as specified at §5.3.5 ¶2:

[Note: a pointer to a const type can be the operand of a delete-expression; it is not necessary to cast away the constness (5.2.11) of the pointer expression before it is used as the operand of the delete-expression. ]

Notice that, before the standard was completed, there have been many discussion about whether this was or wasn’t a good idea, so some pre-standard compilers will issue an error when trying to delete const pointers.

The idea behind allowing this behavior is that otherwise you would have no way to delete const objects without using a const_cast; see this question for more info.