Friendship declaration?

I've got the following code:

Code:

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#include <iostream>
#include "Fraction.h"
using namespace std;

class Fraction_dup {
    friend class Fraction;
public:
    void improper (Fraction x);
};

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With the following Fraction class:

Code:

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class Fraction {
    int top, bottom;

public:
    Fraction(int a = 0, int b = 1);
    ~Fraction(){}

    //overloading operators
    void operator= (Fraction x);
        Fraction operator+ (Fraction x);
        Fraction operator- (Fraction x);
        Fraction operator* (Fraction x);
        Fraction operator/ (Fraction x);

    //function declarations
    void displayFraction();
    double toDecimal();
    Fraction toFraction(double x) const;
    int gcd(int a, int b);
};

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But I keep getting the errors such as the following in the improper() definition:

Code:

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error C2248: 'Fraction::top' : cannot access private member declared in class 'Fraction'|

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What did I do wrong?

It is the Fraction class that should declare Fraction_dup to be a friend. Why do you need Fraction_dup anyway?

By the way, do not place using directives (and using declarations) at file scope in header files.

You made a mistake in assuming that friendship is mutual, or what direction it goes.

Fraction_dup has declared Fraction as a friend. That means any member function of Fraction is allowed to access private members of Fraction_dup. However, Fraction_dup is not allowed to access private members of Fraction unless Fraction declares Fraction_dup as a friend.

To put the logic of C++ friendship in human terms, if you declare me as a friend, that doesn't mean I will view you as a friend. It certainly doesn't mean I will grant you access to my privates. :)

Quote:

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It is the Fraction class that should declare Fraction_dup to be a friend.

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Thanks!

Quote:

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Why do you need Fraction_dup anyway?

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It's part of the assignment. EDIT: Scratch that, I was supposed to write a function, not a class.

Quote:

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By the way, do not place using directives (and using declarations) at file scope in header files.

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Why?

Quote:

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Originally Posted by 843

It's part of the assignment.

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What is Fraction_dup supposed to do? Also, why does it need to be a friend of Fraction? Why not just use the public interface provided by the Fraction class?

Quote:

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Originally Posted by 843

Why?

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Every source file that includes your header will then have the using directive in effect, even if it is not wanted.

Quote:

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Originally Posted by grumpy

You made a mistake in assuming that friendship is mutual, or what direction it goes.

Fraction_dup has declared Fraction as a friend. That means any member function of Fraction is allowed to access private members of Fraction_dup. However, Fraction_dup is not allowed to access private members of Fraction unless Fraction declares Fraction_dup as a friend.

To put the logic of C++ friendship in human terms, if you declare me as a friend, that doesn't mean I will view you as a friend. It certainly doesn't mean I will grant you access to my privates. :)

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Thanks! Got it :)

Quote:

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Originally Posted by laserlight

What is Fraction_dup supposed to do? Also, why does it need to be a friend of Fraction? Why not just use the public interface provided by the Fraction class?

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I was supposed to write a friend function, not a friend class. I just reread the assignment.

Quote:

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Every source file that includes your header will then have the using directive in effect, even if it is not wanted.

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I see.