Question

I have come across some code along the lines of below:

if(instance != (Class*)(0))

Could someone describe what this is doing?

Answer 1

It short: it tests if pointer is null or not.

In detail: The expression (Class*)(0) is actually performing a typecast from 0 (i.e. NULL) to a pointer of type Class, it then compares this pointer (which is a constant NULL) to the pointer variable instance.

An example:

void Check(YourClass *instance)
{
   if(instance != (YourClass*)(0))
      // do this
}

Now the imporatant question is why. Why not simply as:

 if(instance != 0)
      // do this

Well, it is just for code-portability. Some compilers may raise warning that Class* is being compared with int (since NULL is nothing but 0, which is int). Many static-analysis tool may also complain for simple NULL check with a class-type pointer.