Yet another way to defeat C++ const

One of my favorite C++ features, and one I feel is terribly underutilized in many code bases, is the const mechanism.  It’s a great mechanism for defining dual roles for the same object type: a mutable and (ideally) non-mutable one.  But as useful as const is it’s also very easy to circumvent and I’m always interested in learning new ways to do so.

While navigating through a stackoverflow question yesterday I came across yet another way to defeat C++ const.  In C++ it’s perfectly legal to call delete on a const * value.  This code path though transitions from a const * to a type to a non-const pointer by means of ‘this’ in the destructor code.  From there it’s possible to call any other mutable method in the type and bypass the original const without any nasty casts. For example

 class Example {
public:
    ~Example() {
        MutableMethod();
    }
    void MutableMethod() {
        cout << "Example::MutableMethod()" << endl;
    }
    void ConstMethod() const {
        delete this;
    }
};

With this type I can easily defeat const restrictions by simply deleting it

 const Example* pLocal = new Example();
pLocal->ConstMethod();

Granted this is not terribly useful when considering the const of the immediate type.  It’s being deleted after all so messing around in it is unlikely to cause too much of a fuss.  However it can be interesting when considering the access of nested members within the type which may live longer than the container being deleted.

Comments

• Anonymous
  March 01, 2011
  Nice catch

• Anonymous
  March 30, 2011
  But constness of this only affects members contained directly in the type.  Instances accessed via indirection aren't affected by const this, because it creates top-level const on the pointer.  e.g. T* m_ptr; becomes T* const m_ptr; in a const member function, not const T*.  So even const member functions can mutate *m_ptr.