CODEWITHSUNDEEP
c++
Brian Hooper
Brian Hooper

Reputation: 22054

C++ friend operator overload doesn't compile

I have the following (cut-down) class definition, and it has compilation errors.

#include <iostream>
#include <string>
class number
{
    public:
        friend std::ostream &operator << (std::ostream &s, const number &num);
        friend std::string  &operator << (std::string,     const number &num);
        friend std::istream &operator >> (std::istream &s,       number &num);
        friend std::string  &operator >> (std::string,           number &num);
    protected:
    private:
        void write (      std::ostream &output_target = std::cout) const;
        void read  (      std::istream &input_source  = std::cin);
        void to_string (      std::string &number_text) const;
        void to_number (const std::string &number_text);
};

std::istream & operator >> (std::istream &s,       number &num)
{
    num.read (s);
    return s;
}
std::string  & operator >> (std::string &s, number &num)
{
    num.to_number (s);
    return s;
}
std::string  & operator << (std::string &s, const number &num)
{
    num.to_string (s);
    return s;
}
std::ostream & operator << (std::ostream &s, const number &num)
{
    num.write (s);
    return s;
}

When I compile it, I get the following errors...

frag.cpp: In function ‘std::string& operator>>(std::string&, number&)’:
frag.cpp:17: error: ‘void number::to_number(const std::string&)’ is private
frag.cpp:27: error: within this context
frag.cpp: In function ‘std::string& operator<<(std::string&, const number&)’:
frag.cpp:16: error: ‘void number::to_string(std::string&) const’ is private
frag.cpp:32: error: within this context

Could anyone help with this; in particular why to_number and to_string are thought private but read and write are OK?

Thank you.

Upvotes: 4

Views: 736

Answers (2)

msw
msw

Reputation: 43487

Péter Török got the answer, but in trying to read your code, I got tangled up in syntactic junk and names which were too similar, which I expect may have been part of the difficulty. So I did:

s/std:://g
s/to_string/wilma/g
s/to_number/betty/g

which made the errors far more visible.

Upvotes: 0

P&#233;ter T&#246;r&#246;k
P&#233;ter T&#246;r&#246;k

Reputation: 116266

The function signatures are different:

    friend std::string  &operator << (std::string,     const number &num);
    friend std::string  &operator >> (std::string,           number &num);

are declared with a string parameter by value, while

std::string  & operator >> (std::string &s, number &num) ...
std::string  & operator << (std::string &s, const number &num) ...

both have a string& reference parameter. So the function you actually implement is not the same as the one declared as friend - hence the error.

Try changing the friend declaration to

    friend std::string  &operator << (std::string&,     const number &num);
    friend std::string  &operator >> (std::string&,           number &num);

Upvotes: 5

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