I'm trying to make MainScheduler's method addJob a friend function of the class Job, as so:
#include "MainScheduler.h"
//forward declaration
class MainScheduler;
class Job:
{
friend void MainScheduler::addJob( Job* const job );
...
}
But I keep getting the error
error C2027: use of undefined type 'MainScheduler'
Do you know why am I getting this message, and how can I fix it?
You can only name a member function that has already been declared, even for a friendship declaration. This means the class must be defined, not just forward-declared.
Wrong:
class X;
class Y
{
friend void X::f(); // ERROR
};
Right:
class X
{
public:
void f();
};
class Y
{
friend void X::f();
};
It's not obvious why your #include "MainScheduler.h" did not make the definition of class MainScheduler visible, so something else strange is going on there.
This rule means it is not possible to do something like A::f() is a friend of B and B::g() is a friend of A, so sometimes you just have to settle for friending an entire class. (Or there are fancy ways of using helper classes to request/grant permission for certain sets of functions, but that's more helpful when you need an extensible library interface.)
The problem is quite simple: you cannot have dependency cycles.
// MainScheduler.h
#ifndef MAINSCHEDULER
#define MAINSCHEDULER
#include "Job.h"
class MainScheduler { friend class Job; };
#endif
// Job.h
#ifndef JOB
#define JOB
#include "MainScheduler.h"
class Job { friend class MainScheduler; };
#endif
What happens here when parsing MainScheduler.h is the following:
MAINSCHEDULER is not defined, thus the parsing startsMAINSCHEDULERMAINSCHEDULER is already defined, it skips the inclusion of MainScheduler.hJobMainSchedulerThis yields the following preprocessor output, which the compiler sees:
// ignored #include "MainScheduler.h"
class Job { friend class MainScheduler; };
class MainScheduler { friend class Job; };
this is why before you introduced the forward declaration the compiler complained about the unknown MainScheduler symbol in the definition of Job.
Your headers cannot include themselves in a cycle, and you cannot befriend a member function with only a forward declaration.
I propose you rewrite Job.h as:
class MainScheduler; // forward declaration
class Job {
friend class MainScheduler;
public:
// whatever
};
By befriending the whole class you get away with just a forward declaration and break the cycle.
