Question

I saw this code snippet during our lab and it actually compiles in MSVC2008 and G++.

void LinkList< class T >::Insert(T n)  
{  
    if (this == NULL)  
    // some code here  
}

As far as I know the this must not be null since you cannot call a class functions in c++ if it wasn’t instantiated. Is this a valid code? if so what’s the reason behind and where it can be useful?

Answer 1

since you cannot call a class functions in c++ if it wasn’t instantiated

The thing is, you can, but it leads to undefined behavior.

Such a check should probably be an assert, though such code isn’t guaranteed to actually work by the standard. (If this is null, you’re already in undefined behavior land.)

---

The reason it’s “useful” is to detect using an object after it’s been deleted, or if it was never created:

template <typename T> // I hate this function
void safe_delete(T*& pPtr)
{
    delete pPtr;
    pPtr = 0;
}

T* p = new T;
safe_delete(p);

p->foo(); // this is null, and we've entered undefined behavior

Within foo, you could assert, “hey, we messed up :/”.

In my opinion such use is indicative of bad design. You shouldn’t have a pointer lying around that might possibly get invoked again. The last thing you do with a pointer is delete it; if it’s still around after that, change your code so it’s not.