Still don't get forward declaration in this

[yungman]

I read about forward declaration on line, it's pretty straight forward. But I don't understand this program from the book:

Line#3 in Auxil.h is Forward Declaration. Line#11 in Budget3.h declare friend void AuxOffice::addBudget(double, Budget3&);

This is Auxil.h

#ifndef Auxil_H
#define Auxil_H
class Budget3;//Forward declaration of Budget class
class AuxOffice
{ private: double auxBudget;
  public:   
    AuxOffice() { auxBudget = 0; }//Constructor set auxBudget = 0.
    double getDivBudget() const { return auxBudget; }//Something wrong, no return
    void addBudget(double, Budget3&);
        //{auxBudget += fromA; div.corpBudget += fromA;}
};
#endif // !Auxil_H

This is Budget3.h

#ifndef Budget3_H
#define Budget3_H
#include <iostream>
#include "Auxil.h"
class Budget3
{ private: //corpBudget is static, cannot be declared in Budget3.h, use Budget3.cpp
           static double corpBudget; double divBudget;
  public:  Budget3() { divBudget = 0; }//Constructor starting divBudget = 0
.
.
           friend void AuxOffice::addBudget(double, Budget3&);
};
#endif

1) What I don't quite get is there is no declaration to #include Budget3.h in Auxil.h at the beginning. How does the compiler knows to go to Budget3.h? Is it because Budget3.h declare FRIEND in line 11 that give the permission to the compiler to go out of Auxil.h to search for class Budget3 in budget3.h?

2) Is it true that when Budget3.h declares friend void AuxOffice::addBudget(double, Budget3&); in line 11, the member function b has to pass class Budget3 as reference so there is only one copy of Budget3 in the program?

I am still not comfortable with the FRIEND thing.

Thanks

 

[jedishrfu]

I think the budget3 reference is telling the compiler it’s a class to allow its use in defining a function that takes a Budget3 instance as an argument.

the compiler doesn’t need to know anything else about Budget3 to know what to do.

more on friend functions

https://www.geeksforgeeks.org/friend-class-function-cpp/

https://www.tutorialspoint.com/cplusplus/cpp_friend_functions.htm

 

[yungman]

I think the budget3 reference is telling the compiler it’s a class to allow its use in defining a function that takes a Budget3 instance as an argument.

the compiler doesn’t need to know anything else about Budget3 to know what to do.

more on friend functions

https://www.geeksforgeeks.org/friend-class-function-cpp/

https://www.tutorialspoint.com/cplusplus/cpp_friend_functions.htm

Thanks

I typed the program of the first link in and play with it. I'll be back.

I think this is exactly what I needed in my other Directory program that I want to have two header files, one for vector of struct management and sorting, one class for file Management to read and write files.

thanks

 

[jedishrfu]

Don’t forget that little header trick in your files it prevents files from being included multiple times should you need construct your headers in a more complicated way.

#ifndef filename_h
#define filename_h

...

#endif

 

[Filip Larsen]

Don’t forget that little header trick in your files it prevents files from being included multiple times should you need construct your headers in a more complicated way.

If one can live with the code not being 100% standardized, #pragma once is a well-supported simple alternative that frees you from having to worry about symbol names.

 

[yungman]

Don’t forget that little header trick in your files it prevents files from being included multiple times should you need construct your headers in a more complicated way.

#ifndef filename_h
#define filename_h
...
#endif

Thanks, I notice the examples don't have any of these, I have to add them in.

I spent some times on the first link and I actually typed two of them in. I still have a question. Regarding the friend function, it is different if I declare a class as friend or just that particular function as friend. I put this in my notes. You can see Fig.2 to Fig.4 are declaring class A as friend, Fig.5 to Fig.6(forgot to label the last one) only declare function showA() as friend. For function showA() as friend, I cannot have the function body inside any of the header file(see Fig.5). Where as for declaring class A as friend, I can put the body of the function inside class B.in Fig.3.

What is the difference and why? Or it's just is and just remember it? I circled the two lines in question in RED.

Thanks

 

[Vanadium 50]

Why are you using friend? It is used rarely, and when it is used, it is usually misused. It's a red flag, because it is often used as a band-aid over bad class design, breaking encapsulation rather than fixing the problem.

If it's to allow you to keep your fileClass and vector Class design, you are much better off reworking them as one class as @pbuk suggested.

 

[jedishrfu]

While it may be a RED flag for programmers, its described in his book and so the OP is trying to understand how it is used.

 

[Vanadium 50]

I just looked at what's in the book. It does not give a very clear use case: it would be better to create a public method. Or maybe refactor the classes entirely.

I see friend having two primary use cases. One is the Memento design pattern, which allows one to roll an object back to a previous state. The other is an object used only for testing and debugging. There might be some other use cases, but "I want to use a member object directly from another class rather than write an accessor or mutator" is probably not one of them.