question on forward declaration

[subramanian100in]

Consider the program:

class Test;

class First
{
Test t_obj;
};

int main()
{
return 0;
}

If I compile, I get compilation error for the line
Test t_obj;
because Test class is not defined.

My doubt is the following:
Since I am not creating an object of class First, why doesn't the
compiler allow me to have an object of incomplete type(here Test) as
data member ? If an object of type First is created, then the compiler
will need to know the size of the object and in turn it needs to know
the size of each data member. Here no object is created.

I am unable to understand the reason. Where am I going wrong ? Kindly
explain

Thanks
V.Subramanian

[Amal P]

On Jun 29, 11:35 am, "[e-mail], India"
<[e-mail]:

Consider the program:

class Test;

class First
{
Test t_obj;

};

int main()
{
return 0;

}

If I compile, I get compilation error for the line
Test t_obj;
because Test class is not defined.

My doubt is the following:
Since I am not creating an object of class First, why doesn't the
compiler allow me to have an object of incomplete type(here Test) as
data member ? If an object of type First is created, then the compiler
will need to know the size of the object and in turn it needs to know
the size of each data member. Here no object is created.

I am unable to understand the reason. Where am I going wrong ? Kindly
explain

Thanks
V.Subramanian

This is not any of the purpose of forward declaration. That is why
compiler is not allowing you to do so. You can declare a pointer of
the Test anyway. But not an object. Because class Test does not exist
and no storage information is available. If it is pointer it is
possible because every pointer is same. Just points to an address. So
the forward declaration can be useful in declaring a class pointer
without including the whole class header.

Something like this is also valid.

class Test;

class First
{
Test t_obj;

};

class Test
{
};