CODEWITHSUNDEEP
c++classfriend-function
g0x0
g0x0

Reputation: 133

Why Friend Function cannot access private members of a class

class A{
public:
    void printer(){
        B obj;
        obj.private_data = 10; // <- fails, says member inaccessible
    }
}
class B{
friend void A::printer();
private:
    int private_data;
}

is it possible for printer function to access private members of class B? i tried to pass an obj of B as arg to printer but it still failed

Upvotes: 3

Views: 1493

Answers (3)

asmmo
asmmo

Reputation: 7100

Class A doesn't know about B to use it. Hence, postpone the definition of the function printer() until you define B, and if you need an instance of B to be a member var in A then make a forward declaration for B to declare a B* in A.

Hence, use something like what follows:

class A {
  public:
    void printer();
};

class B {
    friend void A::printer();

  private:
    int private_data;
};

void A::printer() {
    B obj;
    obj.private_data = 10; // <- No longer fails
    std::cout << obj.private_data;
}

int main() {
    A a;
    a.printer();
}

Demo

Upvotes: 5

Ted Lyngmo
Ted Lyngmo

Reputation: 117298

Why Friend Function cannot access private members of a class?

They can, but you may need to split the definition of the class up a bit.

Imaginary files added:

Define A (file a.hpp):

class A {
public:
    void printer();
};

Define B (file b.hpp):

#include "a.hpp" // B must see the definition of A to befriend a member function

class B {
    friend void A::printer();

private:
    int private_data;
};

Define A's member function (file a.cpp):

void A::printer() {
    B obj;

    obj.private_data = 10;
}

Upvotes: 3

H. Rittich
H. Rittich

Reputation: 845

To access B, you first need to define it. Thus, you can just declare the method printer and define it after you have defined the class B.

class A {
public:
  void printer();
};

class B {
private:
  friend class A;
  int private_data;
};

void A::printer() {
  B obj;
  obj.private_data = 10;
}

Note, you probably want to move your methods out of your class definition anyways and into a separate .cpp file. Methods defined inside the class are implicitly marked as inline which might not be what you expect.

Upvotes: 3

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