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c++friendforward-declaration

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

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

Solution

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