friend member of a class operating on private members c++

Question

I am learning about friends in classes and my problem is: I want funcC3() to set and permanently change C1::a value, how to do it? I want global function gfunc() to be able to do the same, how? Could you please provide me the way i do it, because in the book they don't specify?

#include <iostream>
using namespace std;

class C2;

class C3
{
public:
    void funcC3(int const& x)
    {
        cout << "in third class...: " << x << endl;
    };
};

class C1
{
private:
    int a;
public:
    void funcC1()
    {
        cout << "inside C1 private thingie: " << a << endl;
    };
    int C1::getcha();
    friend class C2;
    friend int gfunc(int chair);
    friend void C3::funcC3(int const& a);
};

int C1::getcha()
{
    return a;
};

class C2
{
public:
    int a;
};

**int gfunc(int ch)**
{
    int chair = ch;
    return chair;
};

**int main()**
{
    C1 obj1;
    C3 obj3;

    obj3.funcC3(10);
    obj1.funcC1();

gfunc(12);
cout << C1.geta() << endl;
    system("pause");
}

Answer 1

I want funcC3() to set and permanently change C1::a value, how to do it?

Declare it as friend function in C1:

class C1; //forward declaration

class C3
{
//...
    void funcC3( C1& c);
};

class C1
{
//...
private:
    int a;
    friend void C3::funcC3( C1&);
};

void C3::funcC3( C1& c) { c.a = 100;}

I want global function gfunc() to be able to do the same, how?

Same as above:

class C1;
void gfunc( C1& c);

class C1 {
//...
private:
    int a;
    friend void gfunc( C1& c);
};

void gfunc( C1& c) { c.a = 100;}