zhu@crabcake.cs.jhu.edu (Benjamin Zhu) (01/22/90)
------------------------------------------------
When I am doing programming in C++ with AT&T release 2.0, I have some trouble
in declaring class member functions as friend functions. Usually, it is better
to define some member functions of one class, as friend functions of another
class, than to define the whole class as a friend of the other class. However,
it seems hard to use the first method, whereas the second scheme works all the
time. Here is a simple example.
-----------------------------------------------------------------------
#include <stream.h>
class B; // forward definition
B& B::operator=(B&); // error: class B undefined
class A {
friend B& B::operator=(B&); // error: class B undefined
public:
A(int x) : a(x) {}
private:
int a;
};
class B {
public:
B(int x) : b(x) {}
B& operator=(B& y)
// coding is deliberately nasty to test friend functions
{ b.a = y.a; }
private:
A b;
};
main()
{
B b1(10), b2;
b2 = b1;
}
----------------------------------------------------------------------------
I just need to give access privileges for the private members in A to one
member function, B& operator=(B&), in class B. However, this chunk of code
does not work. The C++ translator reports that class B is undefined, when B&
operator=(B&) is declared as a friend function of A. Notice, I have used
forward definition for class B.
O.K., let's give another shot. I swap the order of declarations for class A
and B. I get
----------------------------------------------------------------------------
#include <stream.h>
class A;
class B {
public:
B(int x) : b(x) {}
B& operator=(B& y)
// deliberately nasty code to test friend functions
{ b.a = y.a; }
private:
A b; // error: A size unknown
};
class A {
friend B& B::operator=(B&);
public:
A(int x) : a(x) {}
private:
int a;
};
main()
{
B b1(10), b2;
b2 = b1;
}
----------------------------------------------------------------------------
This time, it does not work either. The translator reports that the size of
class A is unknown when class B is declared. Thus, my forward definition
for class A is useless. On the other hand, when I declare class B as a
friend class of A, everything is fine.
Does anyone on the net have similar experience, and find the solution to get
around this problem? According to the description on Lipmann's ``C++ Prime'',
I believe this should be possible. Maybe someone can enlighten me.