A class declaration can contain static object of self type, it can also have pointer to self type, but it cannot have a non-static object of self type.

For example, following program works fine.

// A class can have a static member of self type
#include<iostream>

using namespace std;

class Test {
static Test self; // works fine

/* other stuff in class*/

};

int main()
{
Test t;
getchar();
return 0;
}
And following program also works fine.

// A class can have a pointer to self type
#include<iostream>

using namespace std;

class Test {
Test * self; //works fine

/* other stuff in class*/

};

int main()
{
Test t;
getchar();
return 0;
}
But following program generates compilation error “field `self’ has incomplete type”

// A class cannot have non-static object(s) of self type.
#include<iostream>

using namespace std;

class Test {
Test self; // Error

/* other stuff in class*/

};

int main()
{
Test t;
getchar();
return 0;
}

• If a non-static object is member then declaration of class is incomplete and compiler has no way to find out size of the objects of the class.
• Static variables do not contribute to the size of objects. So no problem in calculating size with static variables of self type.