Understanding the use of Class keyword in this instance [duplicate]

Question

This question already has answers here:

What are forward declarations in C++? (8 answers)

Closed 8 years ago.

I was reading through VTK source files, I noticed statements as followed in some of the header files.

class vtkArray;
class VTKIOCORE_EXPORT vtkArrayWriter : public vtkWriter
{
    //Interface description here.
}

I was puzzled by the statement 'class vtkArray;'. What is the meaning of this statement? What is it being used for, when is a good time to use it?

This code is from a header file.

Answer 1

That is a forward declaration. It is mostly used when you need to declare the class type without specifying its internal form. This is useful when dealing with pointers or references as statements like:

type* x;
type& x;

does not require the compiler to know the size of type.

Answer 2

This looks like a forward declaration. http://en.wikipedia.org/wiki/Forward_declaration It is needed when you want to use a class before it is defined.

Answer 3

Is this in a header file? They are forward declaring the class vtkArray. This way they do not need to include the header file that contains that class. Then they can just include the header file in the .cpp file.

Edit: Link to explanation of forward declaration