Question

I’m trying to use operator<< for my class.
Here’s the code:

Definition in class:

friend std::ostream& operator<<(std::ostream& out, const longint& num);

Definition outside the class:

std::ostream& operator<<(std::ostream& out, const longint& num_) {
    if (num_.negative())
        out << '-';
    longint::const_iterator it = num_.array.end();
    --it;
    longint::const_iterator finish = num_.array.begin();
    --finish;
    while (it != finish) {
        out << num_.digtochar(*it);
        --it;
    }
}

longint::negative is public, longint::array and longint::digtochar are private. In a lot of googled examples I can see that private members can be used in friend operators, but I give errors:

../long_arithm.h:57:20: error: «std::list<signed char> long_arithm::longint::array» is private
../long_arithm.cpp:94:36: error: in this context
../long_arithm.h:57:20: error: «std::list<signed char> long_arithm::longint::array» is private
../long_arithm.cpp:96:40: error: in this context
../long_arithm.h:44:9: error: «long_arithm::schar long_arithm::longint::digtochar(long_arithm::schar) const» is protected
../long_arithm.cpp:99:28: error: in this context

Why? What I make wrong?

UPD. Minimal code:

// long_arithm.h
#ifndef LONG_ARITHM_H_
#define LONG_ARITHM_H_

#include <iostream>
#include <list>

namespace long_arithm {

    typedef signed char schar;

    class longint {
    public:
        typedef std::list<schar>::const_iterator const_iterator;

        inline bool negative() const { return minusSign; }

        friend std::ostream& operator<<(std::ostream& out, const longint& num);

        enum { error_char = 127 };

    protected:
        schar digtochar(schar num) const;

    private:
        bool minusSign;
        std::list<schar> array;
    };
};

// long_arithm.cpp
#include "long_arithm.h"
#include <iostream>

using namespace long_arithm;

schar longint::digtochar(schar num) const {
    switch (num) {
        case 0: return '0';
        case 1: return '1';
        case 2: return '2';
        case 3: return '3';
        case 4: return '4';
        case 5: return '5';
        case 6: return '6';
        case 7: return '7';
        case 8: return '8';
        case 9: return '9';
        default: return error_char;
    }
}

std::ostream& operator<<(std::ostream& out, const longint& num_) {
    if (num_.negative())
        out << '-';
    longint::const_iterator it = num_.array.end();
    --it;
    longint::const_iterator finish = num_.array.begin();
    --finish;
    while (it != finish) {
        out << num_.digtochar(*it);
        --it;
    }
    return out;
}

Thank you for answers.

Answer 1

Your friend declaration is inside a namespace, but the definition of the function isn’t. Therefore they don’t match, and friend privileges aren’t activated.

Move the definition inside the namespace, like

namespace long_arithm
{
    std::ostream& operator<<(std::ostream& out, const longint& num_)
    {
        if (num_.negative())
            out << '-';
        longint::const_iterator it = num_.array.end();
        --it;
        longint::const_iterator finish = num_.array.begin();
        --finish;
        while (it != finish) {
            out << num_.digtochar(*it);
            --it;
        }
        return out;
    }
}

and it should start working.