c++ friend function - operator overloading istream >>

问题

My question is in regards to friend functions as well as overloading the << and >>. From my understanding I thought friend functions could (and should) access private member variables directly. However in the case I have here the compiler would only accept my .cxx file when I used "get" functions to obtain each private variable.

Here is my header file

class BigNum 
public:

// CONSTRUCTORS and DESTRUCTORS
    BigNum();                            
    BigNum(int num, size_t optional_base = 10);                         
    BigNum(const char strin[], size_t optional_base = 10);

// MEMBER FUNCTIONS
    size_t get_digit(size_t index) const;
    size_t get_used() const;
    size_t get_capacity() const;
    size_t get_base() const;
    bool get_sign() const;

// FRIEND FUNCTIONS
    friend std::ostream& operator<<(std::ostream &os, const BigNum &bignum);
    friend std::istream& operator>>(std::istream &is, BigNum &bignum);

private:
    size_t base;            
    size_t *digits;          
    bool positive;          
    size_t used;

Here is my corresponding .cxx file with the implementations for the friend functions

#include "file.h"
#include <cstdlib>
#include <iostream>
#include <string>
#include <cstring>

using namespace std;

std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.get_sign() == false)
    os << '-';

for (size_t i = 0; i < bignum.get_used(); ++i)
    os << bignum.get_digit(bignum.get_used() - i - 1);

return os;
}

std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.get_used(); ++i)
    is >> bignum.digits[i];

return is;
}

So in this regard the above friend operators compiled correctly. However why is it that my operator >> can access one private variable directly (is >> bignum.digits[i]) but the rest of the private variables need to be retrieved by 'get functions'

Below, when I try to write the overload operators in this regard (how I thought friend functions should properly call private variables):

std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.positive == false)
    os << '-';

for (size_t i = 0; i < bignum.used; ++i)
    os << bignum.digits[used - i - 1];

return os;
}

std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.used); ++i)
    is >> bignum.digits[i];

return is;
}

I obtain the following errors.

BigNum2.cxx: In function `std::ostream&
   csci2270_hw1B::operator<<(std::ostream&, const csci2270_hw1B::BigNum&)':
BigNum2.cxx:201: error: `used' undeclared (first use this function)
BigNum2.cxx:201: error: (Each undeclared identifier is reported only once for
   each function it appears in.)
BigNum2.cxx: In function `std::istream&
   csci2270_hw1B::operator>>(std::istream&, csci2270_hw1B::BigNum&)':
BigNum2.cxx:208: error: syntax error before `)' token

The compiler I am using is g++ (Version 3.3.1). Any help is appreciated, thank you.

Revised:

I updated the code so the bignum object could access the private variables. I did the following to the friend operator overloading << and it compiled fine. Thanks for the comments, that was a rookie mistake.

std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.positive == false)
    os << '-';

for (size_t i = 0; i < bignum.used; ++i)
    os << bignum.digits[bignum.used - i - 1];

return os;
}

However the compiler is still producing errors for the >> operator

BigNum2.cxx: In function std::istream& csci2270_hw1B::operator>>(std::istream&, csci2270_hw1B::BigNum&)': BigNum2.cxx:208: error: syntax error before)' token

The >> is supposed to read in a number and the private member variable 'used' is supposed to record the length of the array. I am still somewhat confused on why the compiler accepts

std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.get_used()); ++i)
    is >> bignum.digits[i];

return is;
}

as opposed to:

std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.used); ++i)
    is >> bignum.digits[i];

return is;
}

Any thoughts? thanks.

回答1:

A friend function has access to the class' private data, but it does not get a this pointer to make that automatic, so every access to the class' data (private or otherwise) has to be qualified. For example this:

os << bignum.digits[used - i - 1];

needs to be:

os << bignum.digits[bignum.used - i - 1];

回答2:

You haven't qualified used in the first function - it needs to be bignum.used. The operator overloads are defined at global scope, so they don't get a this pointer. However, the friend functions do have access to the private members of the class.

std::ostream& operator <<(std::ostream &os, const BigNum &bignum) 
{ 
    if (bignum.positive == false) 
        os << '-'; 

    for (size_t i = 0; i < bignum.used; ++i) 
        // Note "bignum.used", instead of "used".
        os << bignum.digits[bignum.used - i - 1];     
    return os; 
} 

std::istream& operator >>(std::istream &is, BigNum &bignum) 
{ 
    for (size_t i = 0; i < bignum.used; ++i) 
        is >> bignum.digits[i]; 

    return is; 
}

回答3:

It seems there is an extra ')' in the following line right after bignum.used.

 for (size_t i = 0; i < bignum.used**)**; ++i)

来源：https://stackoverflow.com/questions/3312269/c-friend-function-operator-overloading-istream

标签

c++

operator-overloading

friend

operator-keyword