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Gergely Korodi
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 Posted: Wed Jul 28, 2004 8:16 am Post subject: Question about friend member functions |
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Hello,
I have two classes, say A and B, defined in header files "A.hh" and
"B.hh," respectively. The definition of class B looks like
#ifndef _B_HH_
#define _B_HH_
#include "A.hh"
// other includes here
class B {
A a;
int f (/*lots of parameters*/);
};
#endif
I'd like B::f to be a friend of class A. Is it possible to do this? If
I write
#ifndef _A_HH_
#define _A_HH_
#include "B.hh"
class A {
friend int B::f (/*lots of parameters*/);
// some other stuff
};
the compiler complains when it gets to the "A a;" line included from
B.hh, saying class A is not defined there yet. Moreover, the parameters
of B::f are completely irrelevant to class A, so I would not like to
include the corresponding header files in A.hh just for them. Is there
any solution to tackle this?
Thanks,
Gergo
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Alberto Barbati
Guest
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| Posted: Wed Jul 28, 2004 12:46 pm Post subject: Re: Question about friend member functions |
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Gergely Korodi wrote:
| Quote: | Hello,
I have two classes, say A and B, defined in header files "A.hh" and
"B.hh," respectively. The definition of class B looks like
#ifndef _B_HH_
#define _B_HH_
#include "A.hh"
// other includes here
class B {
A a;
int f (/*lots of parameters*/);
};
#endif
I'd like B::f to be a friend of class A. Is it possible to do this? If
I write
#ifndef _A_HH_
#define _A_HH_
#include "B.hh"
class A {
friend int B::f (/*lots of parameters*/);
// some other stuff
};
the compiler complains when it gets to the "A a;" line included from
B.hh, saying class A is not defined there yet. Moreover, the parameters
of B::f are completely irrelevant to class A, so I would not like to
include the corresponding header files in A.hh just for them. Is there
any solution to tackle this?
|
What about this:
#ifndef _A_HH_
#define _A_HH_
class B; // it's not necessary to include all "B.hh"
class A {
friend class B;
// some other stuff
};
The forward declaration is not strictly necessary, but makes the code
more clear, IMHO.
Regards,
Alberto
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Victor Bazarov
Guest
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| Posted: Wed Jul 28, 2004 3:23 pm Post subject: Re: Question about friend member functions |
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Gergely Korodi wrote:
| Quote: | I have two classes, say A and B, defined in header files "A.hh" and
"B.hh," respectively. The definition of class B looks like
#ifndef _B_HH_
#define _B_HH_
#include "A.hh"
// other includes here
class B {
A a;
int f (/*lots of parameters*/);
};
#endif
I'd like B::f to be a friend of class A. Is it possible to do this? If
I write
#ifndef _A_HH_
#define _A_HH_
#include "B.hh"
class A {
friend int B::f (/*lots of parameters*/);
// some other stuff
};
the compiler complains when it gets to the "A a;" line included from
B.hh, saying class A is not defined there yet. Moreover, the parameters
of B::f are completely irrelevant to class A, so I would not like to
include the corresponding header files in A.hh just for them. Is there
any solution to tackle this?
|
I think, in general there is no direct solution because you're trying
to declare a member of a class without defining a class. That would
be a "forward declaration of a member", which is not allowed.
You either have to declare the whole class B a friend or work around
having to declare a friend. A global function could be declared outside
the class and then declared a friend of both classes, and you could just
call it in B::f if you need to.
Victor
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Michael D. Borghardt
Guest
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| Posted: Fri Jul 30, 2004 9:53 am Post subject: Re: Question about friend member functions |
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| Quote: | I'd like B::f to be a friend of class A. Is it possible to do this? If
I write
|
If I understand what you are saying try using an intermediate helper
function that can be a forward declaration
a.h
#ifndef _A_HH_
#define _A_HH_
class B;
int helper(B *b);
class A {
public:
A::A(B *b) : b_(b) {}
void A::doSomethingToB() {int i = helper(b_);}
private:
// some other stuff
B* b_;
};
#endif
b.h
ifndef _B_HH_
#define _B_HH_
#include <iostream>
#include "a.h"
class B
{
public:
B::B() : a(this) {}
void doSomethingWithA() {a.doSomethingToB();}
private:
friend int helper(B *b);
A a;
int f (/*lots of parameters*/) {std::cout << "B::f" << std::endl;return
0;}
};
#endif
main.cpp
#include "b.h"
int helper(B *b)
{
return b->f();
}
int main()
{
B b;
b.doSomethingWithA();
return 0;
}
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