C + + friend __c++

Compiler uses mingw32-g++ c++11, editor for Codeblocks
If a class wants other classes or functions to be able to access non-public members of the class, then you can use the friend method.

friend function:
If you want a non-member function to manipulate a private member of a class, you can declare the function in this and the friend keyword in the previous table of the function. In general, it is a good idea to declare friends in a place where the class definition begins or ends.

Class A
{
    friend void fun (a &a);
Public:
    string member2;
Private:
    int member1;
};
void Fun (a &a)//This function can use the private member {a.member1=0 in class A
    ;
}

Friend class:
The simple point is that a class can use a private member of another class. If you want to use a private member of Class B in Class A, you need to declare friend Class A in class B;
At the same time, it should be noted that the friend relationship does not have a transitive nature.

Class B
{
    friend class A;
Private:
    int member1;
};
Class A
{public
:
    string member2;
    void Fun (B &b)
    {
        b.member1++;
    }
Private:
    int member1;
};

member functions in a class as friends:
When you declare a member function Chengyuan, you must explicitly indicate which class the member function belongs to. For example, in Class A another member function fun () to use private members in class B, you need to follow these steps.
1. First forward declaration Class B
2. Define Class A, declare the fun () function in Class A, but cannot define the fun () function
3. Then define Class B and declare the friend function in Class A
4. Define Fun function

Class B;
Class A
{public
:
    string member2;
    void Fun (B &b);
Private:
    int member1;
};
Class B
{
    friend void A::fun (b &b);
Private:
    int member1;
};
void A::fun (b& B)
{
    b.member1++;
}

Declaration and scope of friends:
A friend function can declare a + definition directly in a class.

Class B
{
    friend void F (B &b)
    {
        b.m2=0
    }
Public:
    void get_m2 () {Cout<<this->m2<<endl}
    string M1;
Private:
    int m2;
};
int main ()
{
    Ios::sync_with_stdio (false);
    b b;
    f (b);
    B.GET_M2 ();
    return 0;
}

To call fun in a class, you must declare it, or you will get an error.

Class b//will complain
{
    friend void F (B &b)
    {
        b.m2=0;
    }
Public:
    void get_m2 () {Cout<<this->m2<<endl}
    string M1;
    B ()
    {
        f ();
    }
Private:
    int m2;
};


void f ();//Add Declaration
class B
{
    friend void F (b &b)
    {
        b.m2=0;
    }
Public:
    void get_m2 () {Cout<<this->m2<<endl}
    string M1;
    B ()
    {
        f ();
    }
Private:
    int m2;
};

Note here that some compilers do not enforce the qualifying rules for appeals on friends. (as the book says)