CODEWITHSUNDEEP
c++pointers
SteBaloche
SteBaloche

Reputation: 3

What does (Class*)(0) mean in C++

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

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

Could someone describe what this is doing?

Upvotes: 0

Views: 402

Answers (4)

Ajay
Ajay

Reputation: 18411

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.

Upvotes: 1

antonijn
antonijn

Reputation: 5760

It checks whether the pointer you're comparing it to points to NULL. The (Class *) cast is unnecessary. It is equivalent to the following in C++0x:

if (instance != nullptr)

Assuming that instance is a pointer, which it most certainly is.

Upvotes: 1

NPE
NPE

Reputation: 500207

If instance is a pointer, this checks whether it is NULL. The cast is redundant.

If instance is something other than a pointer (e.g. an instance of some class), the semantics depend on the class because operator overloading enters the picture.

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258558

It checks whether instance doesn't point to address 0, the cast to Class* is redundant.

If instance is an object, it calls bool operator != (const Class*) const;

Upvotes: 1

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