c++ forward declaration for a friend function in imbricated classes

Question

I am trying to define a function member of a class Extraction FRIEND with a class Descripteur, but when I compile I get the following error :

*Descripteurs.h:24:57: error: invalid use of incomplete type ‘class Extraction’ friend

void Extraction::globalSet(Descripteurs document); Descripteurs.h:19:7: error: forward declaration of ‘class Extraction’ class Extraction;*

given by the code :

//in Extraction.h
#include "Descripteurs.h"
class Extraction {
public:
    Extraction(Descripteurs document);
    void globalSet(Descripteurs document);
protected:
    int m_value;
}

// in Extraction.cpp    
#include "Extraction.h"
Extraction::Extraction(Descripteurs document){
    this->globalSet(document);
}
void Extraction::globalSet(Descripteurs document){
    this->m_value = document.m_nbMot;   //this is why I need a friend function
    cout << this->m_value << endl;
}

//in Descripteur.h
class Extraction; //forward declaration, is there a problem with this ?
class Descripteurs {
public:
    friend void Extraction::globalSet(Descripteurs document);
protected:
    int m_value;
};

I guess the trouble comes from the fact my classes are imbricated, because Extraction uses Descripteurs and Descripteurs has to know Exctraction to deal with the friend function. I thought the forward declaration was a solution, as explained in how comeforward or c++ friend namespace but I could not find documentation that deal with at the same time friend function, imbricated class and separated files. and if i remove "Class Extraction;" I get as expected the following error : ‘Extraction’ has not been declared friend void Extraction::globalSet(Descripteurs document);

friend function over accessor (get functions) is a choice : I don't want to make the attributes accessible from anywhere (in situation the function should take several complex attributes, and not just an int).

Can anyone tell me if I need to add some pieces of code or if there is no way to do this without using accessors ?

Any Help will be welcomed

Thanks

Alexis

I'm afraid I read that as 'inebriated classes'. – Martin James Mar 05 '14 at 15:29 — Martin James, Mar 05 '14 at 15:29

score 1 · Accepted Answer · edited May 23 '17 at 12:28

1

Unfortunately, you can't declare a member function of a forward-declared class as friend. See this question for possible workarounds.

edited May 23 '17 at 12:28

Community

1
1

answered Mar 05 '14 at 15:31

evadeflow

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thanks for the link that's exactly what I had unsuccessfully looked for ! I am going to work on those solutions, to try to understand the passkey but at least you gave me a really good hint, thanks again. – alexis Mar 05 '14 at 15:57
My pleasure. The best thanks you can give me would be to 'accept' this answer by clicking the green check mark next to it. (As the person asking the question, you decide which response deserves to be marked as 'correct'.) – evadeflow Mar 05 '14 at 16:19

c++ forward declaration for a friend function in imbricated classes

1 Answers1