In which case the C++ this pointer can be NULL

Question

I experienced an issue that in the class constructor while I was using this pointer, but this pointer happen to be NULL at that time. for example

MyClass::MyClass()
{
   //this pointer happen to be NULL in this case, and it crash in teh m_callbackfunc because it does not handle null input.
   m_callbackFunc(this);
}

I wonder why this pointer can be null? and in which case this pointer can be null?

Answer 1

The only way a this pointer can be NULL is if some code in the program has exhibited undefined behaviour.

It might for example be achieved by something like

  void *place = nullptr;
  MyClass *object = new (place) MyClass();

The problem is that this effectively dereferences a NULL pointer, so has undefined behaviour. So it is possible that MyClass constructor will have this as NULL. Any other effect is also possible - such is the nature of undefined behaviour.

Some debuggers also report this to be a NULL pointer. But that isn't actually a sign of this being NULL. It is a sign that this is a language keyword rather than something with a symbolic name in object files that the debugger can find. Some other debuggers, when asked to evaluate this simply report that nothing with that name exists in the current function/scope/whatever - possibly because the C++ compiler implements this by passing a value in a machine register.